 Section 18 of Grey's Anatomy Part 4. This is a LibriVox recording. All LibriVox recordings are in the public domain. For more information or to volunteer, please visit LibriVox.org. Recording by Veronica Jenkins. Anatomy of the Human Body Part 4 by Henry Gray. Composition and central connections of the spinal nerves. The typical spinal nerve consists of at least four types of fibers, the somatic sensory, sympathetic afferent or sensory, somatic motor and sympathetic afferent or pre-ganglionic. The somatic sensory fibers, afferent fibers, arise from cells in the spinal ganglia and are found in all the spinal nerves, except occasionally the first cervical, and conduct impulses of pain, touch and temperature from the surface of the body through the posterior roots to the spinal cord and impulses of muscle sense, tendon sense and joint sense from the deeper structures. The sympathetic afferent fibers conduct sensory impulses from the viscera through the rami communicantes and posterior roots to the spinal cord. They are probably limited to the white rami connected with the spinal nerves in two groups, that is the first thoracic to the second lumbar and the second sacral to the fourth sacral nerves. The somatic motor fibers, afferent fibers, arise from cells in the interior column of the spinal cord and pass out through the anterior roots to the voluntary muscles. The sympathetic afferent fibers probably arise from cells in the lateral column or the base of the anterior column and emerge through the anterior roots and white rami communicantes. These are pre-ganglionic fibers which end in various sympathetic ganglia from which post-ganglionic fibers conduct the motor impulses to the smooth muscles of the viscera and vessels and secretory impulses to the glands. These fibers are also limited to two regions, the first thoracic to the second lumbar and the second sacral to the fourth sacral nerves. The afferent fibers, which pass into the spinal cord, establish various types of connections, some within the cord itself for spinal reflexes, others for reflexes connected with higher centuries in the brain, while still others conduct impulses of conscious sensation by a series of neurons to the cerebral cortex. The intrinsic spinal reflex paths, the collaterals and terminals of the ascending and descending branches of the posterior root fibers, which leave the fasciculus cuneatus to enter the gray matter of the spinal cord and in various ways. Many end in the dorsal column, some near its apex, others in the substance of Rolando, others in the intermediate region between the dorsal and ventral columns, others traverse the whole thickness of the gray matter to reach the ventral column, others end in the dorsal nucleus and others pass through the gray commissure to the dorsal column of the opposite side. All of these collaterals and terminals end in connection with cells or dendrites of cells in the gray columns. The axons of these cells have various destinations. Some pass out into the lateral and ventral funiculi and turn upward to reach the brain. Those concerned with the intrinsic spinal reflexes come into relation either directly or indirectly with motor cells in the anterior column. It is very unlikely that either the terminals or collaterals of the dorsal root fibers affect simple direct connections with the motor cells of the ventral column. There is at least one if not several intercalated neurons in the path. These intercalated or correlation neurons may have short axons that do not pass out of the gray matter or the axons may pass out into the proper fasciculi and extend for varying distances up and down or in both directions, giving off collaterals and finally terminating in the gray matter of the same or the opposite side. The shortest fibers of the proper fasciculi lie close to the gray matter. The longest ones are nearer the periphery of the proper fasciculi and are more or less intermingled with the long ascending and descending fasciculi which occupy the more marginal regions of the spinal cord. Each sensory neuron with its ascending and descending branches giving off as it does many collaterals into the gray matter, each one of which may form a synapse with one or several correlation neurons, is thus brought into relation with many correlation neurons and each one of these in turn with its ascending and descending branches and their numerous collaterals is brought into relation either directly or through the intercalation of additional correlation neurons with great numbers of motor cells in the anterior column. The great complexity of these so-called simple reflex mechanisms in the least complex portion of the nervous system, the spinal cord, renders them extremely difficult of exact analysis. The association or correlation neurons are concerned not only with the reflex mechanism of the spinal cord but play an equally important role in the transmission of impulses from the higher centers in the brain to the motor neurons of the spinal cord. The complex mechanisms just described are probably concerned not so much in the contraction of individual muscles as in the complicated action of groups of muscles concerned in the enormous number of movements which the limbs and trunk exhibit in the course of our daily life. Sensory pathways from the spinal cord to the brain. The posterior root fibers conducting the impulses of conscious muscle sense, tendon sense and joint sense, those impulses which have to do with the coordination and adjustment of muscular movements ascend in the fasciculus gracilis and fasciculus cuneatus to the nucleus gracilis and the nucleus cuneatus in the medulla oblongata. In the nucleus gracilis and nucleus cuneatus, synaptic relations are found with neurons whose cell bodies are located in these nuclei and whose axons pass by way of the internal arcuate fibers cross in the raffae to the opposite side in the region between the olives and turn abruptly upward to form the medial limbniscus or medial fillet. The medial fillet passes upward in the ventral part of the formatio reticularis through the medulla oblongata pons and midbrain to the principal sensory nucleus of the ventrolateral region of the thalamus. Here the terminals form synapses with neurons of the third order whose axons pass through the internal capsule and corona radiata to the somatic sensory area of the cortex in the post-central gyrus. Fibers conducting the impulses of unconscious muscle sense pass to the cerebellum partly by way of the fasciculus gracilis and fasciculus cuneatus to the nucleus gracilis and nucleus cuneatus. Thence, neurons of the second order convey the impulses either via the dorsal external arcuate fibers directly into the inferior peduncle of the cerebellum or via the ventral external arcuate fibers which are continued from the internal arcuate fibers through the ventral part of the raffae and after crossing the midline emerge on the surface of the medulla in the ventral sulcus between the pyramids or in the groove between the pyramid and the olive. They pass over the lateral surface of the medulla and olive to reach the inferior peduncle through which they pass to the cerebellum. Other fibers conducting impulses of unconscious muscle sense pass upward in the dorsal spinocerebellar fasciculus which arises from cells in the nucleus dorsalis. The posterior root fibers conducting these impulses pass into the fasciculus cuneatus and the collateral from them to the nucleus dorsalis are said to come almost exclusively from the middle area of the fasciculus cuneatus. They form by their multiple divisions baskets about the individual cells of the nucleus dorsalis, each fiber coming in relation with the bodies and dendrites of several cells. The axons of the second order pass into the dorsal spinocerebellar fasciculus of the same side and ascend along the lateral surface of the spinal cord and medulla oblongata until they arrive at the level of the olive. They then curve backward beneath the external arcuate fibers into the inferior peduncle and pass into the cerebellum. Here they give off collaterals to the dentate nucleus and finally terminate in the cortex of the dorsal and superior portion of the vermus, partly on the same side but to a great extent by way of a large commissure to the opposite side. The fibers lose their myelin sheets as they enter the gray substance and terminate by end ramifications along the nerve cells and their processes. Some of the fibers are said to end in the nucleus dentatus and the roof nuclei of the cerebellum, the nucleus glabosus, nucleus emboliformis, and nucleus fastigius, and others pass through them to terminate in the inferior vermus. A few fibers of the dorsal spinocerebellar fasciculus are said not to enter the inferior peduncle but to pass with the ventrospinocerebellar fasciculus. The cerebellar reflex arc is supposed to be completed by the fibers of the superior peduncle which pass from the cerebellum to the red nucleus of the midbrain where some of their terminals and collaterals form synapses with neurons whose axons descend to the spinal cord in the rubro-spinal fasciculus. The terminal and collaterals of this fasciculus end either directly or indirectly about the motor cells in the anterior column. The ventrospinocerebellar fasciculus, since most of its fibers pass to the cerebellum, is also supposed to be concerned in the conduction of unconscious muscle sense. The location of its cells of origin is uncertain. They are probably in or near the dorsal nucleus of the same and the opposite side. Various other locations are given. The dorsal column, the intermediate zone of the gray matter, and the central portion of the anterior column. The neurons of the first order whose central fibers enter the fasciculus cuneatus from the dorsal roots send collaterals and terminals to form synapses with these cells. The fibers which come from the opposite gray columns cross some in the white and some in the gray commissure and pass with fibers from the same side through the lateral finiculus to the marginal region ventral to the dorsal spinocerebellar fasciculus. The fasciculus begins about the level of the third lumbar nerve and continues upward on the lateral surface of the spinal cord and medulla oblongata until it passes under cover of the external arcuate fibers. It passes just dorsal to the olive and above this joins the lateral edge of the lateral amniscus along which it runs. Ventral to the roots of the trigeminal nerve almost to the level of the superior colliculus. It then crosses over the superior peduncle, turns abruptly backward along its medial border, enters the cerebellum with it and ends in the vermus of the same and the opposite side. Some of its fibers are said to join the dorsal spinocerebellar fasciculus in the medulla oblongata and enter the cerebellum through the inferior peduncle. A number of fibers are said to continue upward in the dorsal lateral part of the tegmentum as far as the superior colliculus and a few pass to the thalamus. They probably form part of the sensory or higher reflex path. The posterior root fibers conducting impulses of pain and temperature probably terminate in the posterior column or the intermediate region of the gray matter soon after they enter the spinal cord. The neurons of the second order are supposed to pass through the anterior commissure to the superficial anterolateral fasciculus tract of Gowers and pass upward in that portion of it known as the lateral spinothalamic fasciculus. This fasciculus lies along the medial side of the ventro spinocerebellar fasciculus. It is stated by some authors that the pain fibers pass upward in the anterolateral ground bundles and some of the lower mammals. This pathway carries the pain fibers upward by a series of neurons, some of which cross to the opposite side, so that in part there is a double path. In man, however, the lateral spinothalamic fasciculus is probably the most important pathway. On reaching the medulla, these fibers continue upward through the formatio reticularis in the neighborhood of the median fillet to the thalamus, probably its ventrolateral region. Whether higher neurons convey the pain impulses to the cortex through the internal capsule is uncertain. The pathway is probably more complex and head is of the opinion that our sensations of pain are essentially thalamic. The pain and temperature pathways in the lateral spinothalamic fasciculus are not so closely intermingled, but that one can be destroyed without injury to the other. Ransom suggests that the non-medallated fibers of the posterior roots, which turn into lassoors tract and ascend or descend for short distances, not exceeding one or two segments and finally end in the substantia gelatinosa, are in part at least pain fibers and that the fasciculus of lassoar and the substantia gelatinosa represent part of the mechanism for reflexes associated with pain conduction and reception, while the fibers to the higher centers pass up in the spinothalamic tract. The fibers of tactile discrimination, according to head and Thompson, pass up in the fasciculus cuneatus and fasciculus gracilis of the same side and follow the path of the muscle sense fibers. The axons of the second order arising in the nucleus cuneatus and gracilis cross with the internal arcuate fibers and ascend to the thalamus with the medial omniscus. Thence by neurons of higher order, the impulses are carried to the somatic sensory area of the cortex through the internal capsule. The other touch fibers shortly after entering the spinal cord terminate in the dorsal column or intermediate gray matter. Neurons of the second order send their axons through the anterior commissure to pass upward in the antrolateral finiculus probably in the ventral spinothalamic fasciculus. In the medulla, they join or pass upward in the neighborhood of the medial omniscus to the thalamus and thence by neurons of higher order to the somatic sensory area of the cortex. The remaining ascending fasciculi form a part of the complex known as the superficial antrolateral fasciculus tract of Gowers. The spinotectal fasciculus as its name indicates is supposed to have its origin in the gray matter of the cord and terminations in the superior and inferior colliculi of the midbrain serving for reflexes between the cord and the visceral and auditory centers of the midbrain. The spinal olivary fasciculus alivospinal bulbospinal helwigs bundle is likewise of unknown constitution and function. There is uncertainty even in regard to the direction of its fibers. The sympathetic afferent fibers visceral afferent visceral sensory splachnic afferent enter the spinal cord by the posterior roots of the thoracic and first two or three lumbar nerves and the second to the fourth sacral nerves. The fibers pass to these nerves from the peripheral sympathetic system through the white rami communicantes. Some of the cell bodies of these afferent fibers are located in the spinal ganglia and others are in the sympathetic ganglia. Some of the afferent sympathetic fibers end about the cell bodies of sensory neurons and visceral impulses are thus transmitted to these neurons which conduct them as well as their own special impulses to the spinal cord. Other sympathetic afferent neurons whose cell bodies are located in the spinal ganglia send collaterals to neighboring cells of somatic sensory neurons and thus have a double path of transmission to the spinal cord. Such an arrangement provides a mechanism for some of the referred pains. These sympathetic afferent fibers presumably divide on entering the spinal cord into ascending and descending branches. Their distribution and termination within the spinal cord are unknown. Some of them probably eventually came into relation with the sympathetic afferent fibers whose cell bodies are located in the lateral column. Our knowledge concerning both the termination and origin of these fibers is very unsatisfactory. The sympathetic afferent fibers splanknic motor, visceral motor, preganglionic fibers are supposed to arise from cells in the intermediate zone between the dorsal and ventral gray columns and in the intermediolateral column at the margin of the lateral column. These preganglionic sympathetic fibers are not distributed throughout the entire series of spinal nerves but are confined to two groups, the thoracolumbar from the first thoracic to the second or third lumbar nerves and the sacral group from the second to the fourth sacral nerves. They pass out with the anterior root fibers and through the rami communicantes to end in sympathetic ganglia. The impulses are distributed from cells in these ganglia through postganglionic fibers to the smooth muscles and glands. The thoracolumbar outflow and the sacral outflow form two distinct functional groups which are considered more fully under the sympathetic system. End of section 18 recorded by Veronica Jenkins in Ottawa, Illinois. Section 19 of Gray's Anatomy Part 4 This is a LibriVox recording. All LibriVox recordings are in the public domain. For more information or to volunteer please visit LibriVox.org Recording by Morgan Scorpion. Anatomy of the Human Body Part 4 by Henry Gray Composition and Connections of Cranial Nerves Part 1 4e Composition and Central Connections of the Spinal Nerves The cranial nerves are more varied in their composition than the spinal nerves. Some for example contain somatic motor fibers only. Others contain the various types of fibers found in the spinal nerves namely somatic motor, sympathetic efferent, somatic sensory and sympathetic sensory. In addition there are included the nerves of the special senses namely the nerves of smell, sight, hearing, equilibration and taste. The hypoglossal nerve 12 cranial consists of somatic motor fibers only and supplies the muscles of the tongue. Its axons arise from cells in the hypoglossal nucleus and pass forward between the white reticular formation and the gray reticular formation to emerge from the antrolateral sulcus of the medulla. The hypoglossal nuclei of the two sides are connected by many commissural fibers and also by dendrites of motor cells which extend across the midline to the opposite nucleus. The hypoglossal nucleus receives either directly or indirectly numerous collateral and terminals from the opposite pyramidal tract, corticobulba or cerebral bulbous fibers which convey voluntary motor impulses from the cerebral cortex. Many reflex collateral enter the nucleus from the secondary sensory paths of the trigeminal and vagus and probably also from the nervous intermedius and the glossopharyngeal. Collattals from the posterior longitudinal bundle and the ventral longitudinal bundle are said to pass to the nucleus. The accessory nerve 11 cranial contains somatic motor fibers. The spinal part arises from lateral cell groups in the anterior column near its dorsal lateral margin in the upper five or six segments of the cord. Its roots pass through the lateral funiculus to the lateral surface of the cord. It supplies the trapezius and sternocleidomastoid deus. The cranial part arises from the nucleus ambiguous, the continuation in the medulla oblongata of the lateral cell groups of the anterior column of the spinal cord from which the spinal part has origin. The upper part of the nucleus ambiguous gives motor fibers to the vagus and glossopharyngeal nerves. The cranial part sends its fibers through the vagus to the laryngeal nerves to supply the muscles of the larynx. The root fibers of the cranial part of the accessory nerve pass anterior to the spinal tract of the trigeminal, while those of the vagus pass through or dorsal to the trigeminal root and emerge in the line of the postural lateral sulcus. The nucleus of origin of the spinal part undoubtedly receives either directly or indirectly terminals and collattals controlling voluntary movements from the pyramidal tracts. It is probable that terminals and collattals reach the nucleus either directly or indirectly from the rubro spinal and vestibular spinal tracts. It is also connected indirectly with the spinal somatic sensory nerves by association fibers of the proper fasciculi. The cranial part receives indirectly or directly terminals and collattals from the opposite pyramidal tract and form the terminal sensory nuclei of the cranial nerves. A few fibers of the cranial part are said to arise in the dorsal nucleus of the vagus and are thus sympathetic efferent. They are said to join the vagus nerve. The vagus nerve, tense cranial, contains somatic sensory, sympathetic efferent, somatic motor, sympathetic efferent and taste fibers. The efferent fibers somatic sensory, sympathetic and taste have their cells of origin in the jugular ganglion and in the nodosal ganglion, ganglion of the trunk, and entering the medulla divide into ascending and descending branches as do the sensory fibers of the posterior roots of the spinal nerves after they enter the spinal cord. One, the somatic sensory fibers are few in number, convey impulses from a limited area of the skin on the back of the ear and posterior part of the external auditory meatus, and probably join the spinal tract of the trigeminal nerve to terminate in its nucleus. Connections are probably established through the central part of the trigeminal with the thalamus and somatic sensory area of the cortex for the conscious recognition of impulses. The descending fibers in the spinal tract of the trigeminal, terminating in the nucleus of the tract, probably establish relations through connecting neurons with motor nuclei in the anterior column of the spinal cord and with motor nuclei of the medulla. Two, the sympathetic afferent fibers are usually described as terminating in the dorsal nucleus of the vagus and glossopharyngeal. Some authors, however, believe they join the tract as solitarious and terminate in its nucleus. These afferent fibers convey impulses from the heart, the pancreas, and probably from the stomach, esophagus, and respiratory tract. Their terminals in the dorsal nucleus come into relation with neurons whose axons probably descend into the spinal cord, conveying impulses to the motor nuclei supplying fibers to the muscles of respiration, i.e. the phrenic nerve and the nerves to the intercostal and levatorious costarum muscles. Other axons probably convey vasomotor impulses to certain sympathetic efferent neurons throughout the spinal cord. The dorsal nucleus, nucleus of the allus inorea, and the posterior continuation of it into the commercial nucleus of the allus inorea, constitute probably the so-called respiratory and vasomotor center of the medulla. The shorter reflex neurons of the dorsal nucleus probably affect connections either directly or indirectly with motor cells of the vagus itself and other cranial nerves. 3. Taste fibers conducting impulses from the epiglottis and larynx are supposed to pass in the vagus and join the tract as solitarious, finally terminating in the nucleus of the tract as solitarious. It is not certain that this nucleus represents the primary terminal center for taste, and some authors maintain that the taste fibers terminate in the dorsal nucleus. The secondary ascending pathways from the primary gastatory nucleus to the cortex as well as the location of the cortical center for taste are unknown. A gastatory center has been described near the anterior end of the temporal lobe. The nucleus of the tract as solitarious is connected with motor centers of the pons, medulla, and spinal cord for the reactions of mastication and swallowing. 4. Somatic motor fibers to the cross-striated muscles of the pharynx and larynx arise in the nucleus ambiguous. This nucleus undoubtedly receives either directly or indirectly collateral or terminals from the opposite pyramidal tract controlling the voluntary movements of the pharynx and larynx. The reflex pathways conveying impulses from the terminal sensory nuclei are unknown but probably form part of the intricate maze of fibers constituting the reticular formation. 5. Sympathetic efferent fibers arise from cells in the dorsal nucleus, nucleus of the allocinerea. These are pre-ganglionic fibers of the sympathetic system and all terminate in sympathetic ganglia from which postganglionic fibers are distributed to various organs, i.e. motor fibers to the esophagus, stomach and small intestine, gallbladder and to the lungs, inhibitory fibers to the heart, secretory fibers to the stomach and pancreas. The dorsal nucleus not only receives terminals of sympathetic efferent fibers for reflexes but undoubtedly receives terminals and collateral from many other sources, but the exact pathways are at present unknown. The glossopharyngeal nerve, ninth cranial, is similar to the vagus nerve as regards its central connections and is usually described with it. It contains somatic sensory, sympathetic efferent, taste, somatic motor and sympathetic efferent fibers. The efferent sensory fibers arise from cells in the superior ganglion and in the petrosal ganglion. The same uncertainty exists concerning the nuclei of termination and nuclei of origin of the various components as for the vagus. 1. The somatic sensory fibers are few in number. Some are distributed with the auricular branch of the vagus to the external ear. Others probably pass to the pharynx and foresees. They are supposed to join the spinal tract of the trigeminal and terminate in its nucleus. The connections are similar to those of the somatic sensory fibers of the vagus. 2. Sympathetic efferent fibers from the pharynx and middle ear are supposed to terminate in the dorsal nucleus. Connections are probably established with motor nuclei concerned in chewing and swallowing. Very little is known however about the connections with other parts of the brain. 3. Taste fibers from the tongue probably terminate in the nucleus of the tractus doletarius. These fibers together with similar fibers from the facial, nervous intermedius, and the vagus are supposed to form the tractus doletarius and terminate in its nucleus. The central connections have been considered under the vagus. 4. Somatic motor fibers to the stylo-pharyngeus muscle arise in the upper end of the nucleus ambiguous. The existence of these fibers in the roots of the glossopharyngeal is uncertain, as there are other paths by which such fibers might reach in the glossopharyngeal from the vagus. The sources of impulses passing to the nucleus ambiguous are considered under the vagus. 5. Sympathetic efferent fibers, motor and secretory fibers, arise from the nucleus dorsalis. Some authors believe that the secretory fibers to the parotid gland arise from a distinct nucleus, the inferior salivatory nucleus, situated near the dorsal nucleus. The pre-ganglionic fibers from this nucleus terminate in the otic ganglion. The post-ganglionic fibers from the otic ganglion pass to the parotid gland. The acoustic nerve, 8th cranial, consists of two distinct nerves, the cochlear nerve, this nerve of hearing, and the vestibular nerve, the nerve of equilibration. End of Part 19. Section 20 of Gray's Anatomy, Part 4. This is a LibriVox recording. All LibriVox recordings are in the public domain. For more information or to volunteer, please visit LibriVox.org. Recording by Morgan Scorpion. Anatomy of the Human Body, Part 4 by Henry Gray. Composition and Connections of Cranial Nerves, Part 2. The cochlear nerve arises from bipolar cells in the spiral ganglion of the cochlea. The peripheral fibers end in the organ of corti. The central fibers bifurcate as they enter the cochlear nucleus. The short ascending branches end in the ventral portion of the nucleus. The longer descending branches terminate in the dorsal portion of the nucleus. From the dorsal portion of the cochlea nucleus axons arise which pass across the dorsal aspect of the inferior peduncle and the floor of the fourth ventricle, the striae medullaris, to the median sulcus. Here they dip into the substance of the pons, cross the median plane, and join the lateral lemniscus. Some of the fibers terminate in the superior olivary nucleus. The fibers of the striae medullaris are not always visible on the floor of the rhomboid fossa. From the ventral portion of the cochlea nucleus axons pass into the trapezoid body. Here some of them end in the superior olivary nucleus of the same size. Others cross the midline and end in the superior olivary nucleus of the opposite side or pass by these nuclei giving off collateral to them and join the lateral lemniscus. Other fibers either terminate in or give off collateral to the nucleus of the trapezoid body of the same or the opposite side. Other fibers from the ventral portion of the cochlea nucleus pass dorsal to the inferior peduncle and then dip into the substance of the pons to join the trapezoid body or the superior olivary nucleus of the same size. From the superior olivary nucleus of the same and opposite sides axons join in the lateral lemniscus. Collatals and probably terminals also pass from the lateral lemniscus to other nuclei in its path and receive in turn axons from these nuclei. These are the accessory nucleus, the medial pre-olivary nucleus, the lateral pre-olivary or serilunal nucleus and the nucleus of the lateral lemniscus. The trapezoid body consists of horizontal fibers in the ventral part of the formatio reticularis of the lower part of the pons behind its deep transverse fibers and the pyramid bundles. The axons come from the dorsal and ventral portions of the cochlea nucleus. After crossing the rafae where they decorate with those from the opposite side they turn upward to form the lateral lemniscus. Fibers from the striae medialaris contribute to the trapezoid body. In addition it sends terminals or collatals to and receive axons from the superior olivary nucleus, the nucleus of the trapezoid body, the lateral pre-olivary or serilunal nucleus and the mesial pre-olivary nucleus. The cochlea nucleus, the terminal nucleus for the nerve of hearing is usually described as consisting of a larger dorsal nucleus on the dorsal and lateral aspect of the inferior peduncle, forming a prominent projection, the acoustic tubercle, and a ventral or accessory cochlea nucleus more ventral to the inferior peduncle. The two nuclei are continuous and are merely portions of one large nucleus. The axons from cells of the spiral ganglion of the cochlea nerve on reaching the nucleus divide into ascending and descending branches which enter the ventral and dorsal nuclei respectively. Axons from the large fusiform cells of the dorsal nucleus pass partly by way of the striae medialaris to the trapezoid body and lateral lemniscus and the nuclei associated with the former and partly transversely beneath the inferior peduncle and spinal tract of the trigeminal to the trapezoid body. Axons from the ventral cochlea nucleus pass partly by the striae medialaris but for the most part horizontally to the trapezoid body. The superior olivary nucleus is a small mass of grey matter situated on the dorsal surface of the lateral part of the trapezoid body. Some of its axons pass backward to the adjacent nucleus. This bundle is known as the peduncle of the superior olivary nucleus. Other fibres from the nucleus join the posterior longitudinal bundle and terminate in the nuclei of the trochlea and oculomotor nerves. The majority of axons, after giving off collatils to the nuclei itself, join the lateral lemniscus of the same side. Other axons pass in the trapezoid body toward the ventral portion of the cochlea nucleus. The nucleus of the trapezoid body lies between the root fibres of the adjacent nerve and the superior olivary nucleus. Its cells lie among the fibres of the trapezoid body. In it terminate fibres and collatils of the trapezoid body which come from the cochlea nucleus of the opposite and probably the same side and from the opposite trapezoid nucleus. They terminate in the nucleus of the trapezoid body in diffuse arborisations and peculiar end-plucks or acoustic cullises of yellowish colour which fuse with the cell bodies. Its cells are round and of medium size. Their axons pass into the trapezoid body, cross the median line and probably join the lateral fillet. The lateral pre-olivary or semilunar nucleus lies ventral to the superior olivary nucleus. In it end terminals and collatils of the trapezoid body and probably fibres of the opposite cochlea nucleus. Its axons mingle with the trapezoid body and join the lateral fillet. The mesial pre-olivary nucleus is in contact with the ventral side of the nucleus of the trapezoid body. It receives many collatils from the trapezoid body. Its cells are smaller than those of the trapezoid nucleus. Their axons join the lateral fillet. The lateral lemniscus, lateral fillet. The continuation upward of the central path of hearing, consists of fibres which come from the cochlea nuclei of the same and opposite side by way of the trapezoid body and from the pre-olivary nuclei. It lies in the ventral or ventral lateral part of the reticular formation of the pons. At first ventral then lateral to the median fillet. Above the pons these ascending fibres come to the surface at the side of the reticular formation in the trigonum lemnisci and are covered by a layer of ependymia. This part of the lateral lemniscus is known as the fillet of rail. On reaching the level of the inferior colliculus the dorsal fibres which override the superior peduncle, decorate in the vellum medullare anterior with similar fibres of the opposite side. Numerous small masses of cells are scattered along the path of the lateral lemniscus above the superior olivary nucleus and constitute lower and upper nuclei of the lateral lemniscus. They are supplied with many collatils and possibly terminals from the fibres of the lemniscus. The axons of the lower nucleus of the lateral lemniscus, which arise from the largest stellate or spindle-shaped cells with long, smooth, much-branch dendrites, are said by some authors to join the lateral lemniscus. But according to Cahal they pass medially toward the raffae. Their termination is unknown. The cells of the upper nucleus of the lateral lemniscus are more scattered. The same uncertainty exists in regard to their termination. The fibres of the lateral lemniscus end by terminals or collatils in the inferior colliculus and the medial geniculate body. A few of the fibres are said to pass by the inferior colliculus to terminate in the middle portion of the stratum griseum of the superior colliculus and are probably concerned with reflex movements of the eyes depending on acoustic stimuli. The inferior colliculi, lower or posterior quadrigeminal bodies, are important auditory reflex centers. Each consists of a compact nucleus of gray matter covered by a superficial white layer and separated from the central gray matter about by the aqueduct by a thin, deep white layer. Many of the axons which appear in the superficial white layer ascend through the inferior brachium to the medial geniculate body. Others, mainly from large cells in the dorsomesial part of the nucleus, pass through the deep white layer into the tegmentum of the same and the opposite side and descend. Their termination is unknown, but they probably constitute an auditory reflex path to the lower motor centers, perhaps descending into the spinal cord with a tectospinal fasciculus. Other axons are said to descend in the lateral lemniscus to the various nuclei in the auditory path, held, and probably to motor nuclei of the medulla and spinal cord. The medial geniculate body receives terminals and collatles from the lateral lemniscus, the central auditory path, and also large numbers of axons from the inferior colliculus of the same side and a few from the opposite side. It is thus a station in the central auditory path. A large proportion of its axons pass forward beneath the optic tract to join the corona radiata and then sweep backward and lateral wood as the auditory radiation to terminate in the cortex of the superior temporal gyrus. V. Monaco holds that Golgi cells type 2 are interpolated between the terminations of the incoming fibers to the medial geniculate body, and the cells located there which give rise to the fibers of the auditory radiation. The medial geniculate bodies are united by the long, slender, commissure of gooden. These fibers join the optic tract as it passes over the edge of the medial geniculate and passes through the posterior part of the optic chiasma. It is probably a commissure connected with the auditory system. The vestibular nerve, vestibular root 8th cranial, arise from the bipolar cells in the vestibular ganglion, scarper's ganglion. The peripheral fibers end in the semicircular canals, the saccule and the utricle, the end organs concerned with the mechanism for the maintenance of bodily equilibrium. The central fibers enter the medulla oblongata and pass between the inferior peduncle and the spinal tract of the trigeminal. They bifurcate into ascending and descending branches, as do the dorsal root fibers of all the spinal nerves and all afferent cranial nerves. The descending branches terminate in the dorsal, medial, vestibular nucleus, the principal nucleus of the vestibular nerve. This nucleus is prolonged downward into a descending portion in which end terminals and collateral of the descending branch. The ascending branches pass to detas nucleus, to vectoras nucleus, and through the inferior peduncle of the cerebellum to the nucleus tecti of the opposite side. The dorsal vestibular nucleus, medial or principal nucleus, is a large mass of small cells in the floor of the fourth ventricle under the area acoustica located partly in the medulla and partly in the pons. The stream medullaris cross the upper part of it. It is separated from the median plane by the nucleus intercalatus. Its axons pass into the posterior longitudinal bundle of the same and the opposite side, and descend to terminate in the nucleus abdicens of the same side, and in the trochlear nucleus and the ocular motor nucleus of the opposite side, and to the motor nuclei of the trigeminal on both sides. The descending portion, the nucleus of the descending tract, extends downward as far as the upper end of the nucleus gracilis, and the decossation of the medial lemniscus. It is sometimes called the inferior vestibular nucleus. Many of its axons cross the midline, and probably ascend with the medial lemniscus to the ventral lateral region of the salamus. The lateral vestibular nucleus, detus nucleus, is the continuation upward and lateral wood of the principal nucleus, and it need to terminate many of the ascending branches of the vestibular nerve. It consists of very large multi-polar cells whose axons form an important part of the posterior longitudinal bundle of the same and the opposite side. The axons bifurcate as they enter the posterior longitudinal bundle, the ascending branches send terminals and collattles to the motor nuclei of the abdicens, trochlear, and ocular motor nerves, and are concerned in coordinating the movements of the eyes with alterations in the position of the head. The descending branches pass down in the posterior longitudinal bundle into the anterior finiculus of the spinal cord as the vestibular spinal fasciculus, anterior marginal bundle, and are distributed to motor nuclei of the anterior column by terminals and collattles. Other fibres are said to pass directly to the vestibular spinal fasciculus without passing into the posterior longitudinal bundle. The fibres which pass into the vestibular spinal fasciculus are intimately concerned with equilibratory reflexes. Other axons from detus nucleus are supposed to cross and descend in the opposite medial lemniscus to the ventral lateral nuclei of the salamus. Still other fibres pass into the cerebellum with the inferior peduncle and are distributed to this cortex of the vermice and the roof nuclei of the cerebellum. According to Kahal, they merely pass through the nucleus vestigii on their way to the cortex of the vermice and the hemisphere. The superior vestibular nucleus, vectorus nucleus, is the dorsolateral part of the vestibular nucleus and receives collattles and terminals from the ascending branches of the vestibular nerve. Its axons terminate in much the same manner as do those from the lateral nucleus. The facial nerve, seventh cranial, consists of somatic sensory, sympathetic afferent, taste, somatic motor and sympathetic efferent fibres. The afferent or sensory fibres arise from cells in the geniculate ganglion. This portion of the nerve is often described as the nervous intermedius. One, the somatic sensory fibres are furin number and convey sensory impulses from the middle ear region. Their existence has not been fully confirmed. Their central termination is likewise uncertain. It is possible that they join in the spinal tract of the trigeminal as do the somatic sensory fibres of the vagus and glossopharyngeal. Two, the sympathetic afferent fibres are likewise furin number and of unknown termination. Three, taste fibres convey impulses from the anterior two-thirds of the tongue via the corda tympani. They are supposed to join the tractus solitarius and terminate in its nucleus. The central connections of this nucleus have already been considered. Four, somatic motor fibres, supplying the muscles derived from the hyoid arch, arise from the large multipolar cells of the nucleus of the facial nerve. This nucleus is serially homologous with the nucleus ambiguous and lateral part of the anterior column of the spinal cord. Voluntary impulses from the cerebral cortex are conveyed by terminals and collattles of the pyramidal tract of the opposite side, indirectly, that is, with the interpolation of a connecting neuron to the facial nucleus. This nucleus undoubtedly receives many reflex fibres from the various sources, i.e. from the superior colliculus via the ventral longitudinal bundle, tectospinal vesiculus, for optic reflexes, from the inferior colliculus via the auditory reflex path and indirectly from the terminal sensory nuclei of the brainstem. Through the posterior longitudinal bundle it is intimately connected with other motor nuclei of the brainstem. Five, sympathetic efferent fibres, preganglionic fibres, arise according to some authors from the small cells of the facial nucleus, or according to others from a special nucleus of cells scattered in the reticular formation, dorsal medial to the facial nucleus. This is sometimes called the superior salivatory nucleus. These preganglionic fibres are distributed partly via the caudatin-pani and lingual nerves to the submaxillary ganglion, dense by postganglionic vasodilator fibres, to the submaxillary and sublingual glands. Some of the preganglionic fibres pass to the sphenopalatine ganglion via the great superficial patrosal nerve. The abdicence nerve, sixth cranial, contains somatic motor fibres only, which supply the lateral rectus muscle of the eye. The fibres arise from the nucleus of the abdicence nerve and pass ventrally through the formatio reticularis of the pons to emerge in the transverse groove between the caudal edge of the pons and the pyramid. The nucleus is serially homologous with the nuclei of the trochlear and oculomotor above and with the hypoglossal and medial part of the anterior column of the spinal cord below. It is situated close to the floor of the fourth ventricle, just above the level of the strio medularis. Voluntary impulses from the cerebral cortex are conducted by the pyramidal tract fibres, corticopontine fibres. These fibres probably terminate in relation with association neurons, which control the coordinated action of all the eye muscles. This association and coordination mechanism is interposed between the terminals and collateral of the voluntary fibres, and the neurons within the nuclei of origin of the motor fibres to the eye muscles. The fibres of the posterior longitudinal bundle are supposed to play an important role in the coordination of the movements of the eyeball. Whether it is concerned only with coordinations between the vestibular apparatus and the eye, or with more extensive coordinations, is unknown. Many fibres of the posterior longitudinal bundle have their origin in the terminal nuclei of the vestibular nerve, and from the posterior longitudinal bundle, many collateral and terminals are given after the abducent nucleus as well as to the trochlear and oculomotor nuclei. The abducent nucleus probably receives collateral and terminals from the ventral longitudinal bundle, tectospinal fasciculus, fibres which have their origin in the superior colliculus, the primary visual center, and are concerned with visual reflexes. Others probably come from the reflex auditory center in the inferior colliculus, and from other sensory nuclei of the brain stem. End of Part 20. Section 21 of Gray's Anatomy, Part 4. This is a LibriVox recording. All LibriVox recordings are in the public domain. For more information or to volunteer, please visit LibriVox.org. Recording by Morgan Scorpion. Anatomy of the Human Body, Part 4 by Henry Gray. Composition and Connections of Cranial Nerves, Part 3. The Trigeminal Nerve, Fifth Cranial. Contains somatic motor and somatic sensory fibres. The motor fibres arise in the motor nucleus of the trigeminal and pass ventral laterally through the bones to supply the muscles of mastication. The sensory fibres arise from the unipolar cells of the semilunar ganglion. The peripheral branches of the T-shaped fibres are distributed to the face and anterior two-thirds of the head. The central fibres pass into the pons with the motor root and bifurcate into ascending and descending branches, which terminate in the sensory nuclei of the trigeminal. The motor nucleus of the trigeminal is situated in the upper part of the pons beneath the lateral angle of the fourth ventricle. It is serially homologous with the facial nucleus and the nucleus ambiguous, motor nucleus of the vagus and glossopharyngeal, which belong to the motor nuclei of the lateral somatic group. The axons arise from large pigmented multipolar cells. The motor nucleus receives reflex collatials and terminals, one from the trigeminal nucleus of the trigeminal of the same, and a few from the opposite side, via the central sensory tract, trigeminal thalamic tract. Two from the musincephalic root of the trigeminal, three from the posterior longitudinal bundle, four and probably from fibres in the formatio reticularis. It also receives collatials and terminals from the opposite pyramidal tract, corticopontine fibres, for voluntary movements. There is probably a connecting or association neuron interposed between these fibres and the motor nucleus. The terminal sensory nucleus consists of an enlarged upper end, the main sensory nucleus, and a long, more slender descending portion, which passes down through the pons and medulla to become continuous with the dorsal part of the posterior column of the gray matter, especially the substantia gelatinosa of the spinal cord. This descending portion consists mainly of substantia gelatinosa, and is called the nucleus of the spinal tract of the trigeminal nerve. The main sensory nucleus lies lateral to the motor nucleus beneath the superior peduncle. It receives the shorter sending branches of the sensory root. The descending branches, which form the tractus spinalis, pass down through the pons and the medulla on the lateral side of the nucleus of the tractus spinalis, in which they end by collatials and terminals, into the spinal cord on the level of the second cervical segment. It decreases rapidly in size as it descends. At first it is located between the emergent part of the facial nerve and the vestibular nerve, then between the nucleus of the facial nerve and the inferior peduncle. Lower down in the upper part of the medulla, it lies beneath the inferior peduncle and is broken up into bundles by the olivocerebellum fibers and the roots of the ninth and tenth cranial nerves. Finally it comes to the surface of the medulla under the tubercle of Rolando, and continues in this position lateral to the fasciculus cuneatus as far as the upper part of the cervical region where it disappears. The cells of the sensory nucleus are of large and medium size and send their axons into the formatio reticularis where they form a distinct bundle, the central part of the trigeminal, trigeminal thalamic tract, which passes upward through the formatio reticularis and tegmentum to the ventral lateral part of the thalamus. Most of the fibers cross to the trigeminal thalamic tract of the opposite side. This tract lies dorsal to the medial fillet, approaches close to it in the tegmentum and terminates in a distant part of the thalamus. From the thalamus, impulses are conveyed to the somatic sensory area of the cortex by axons of cells in the thalamus through the internal capsule and corona radiata. Many collateral are given off in the medulla and passed from the trigeminal thalamic tract to the motor nuclei, especially to the nucleus ambiguous, the facial nucleus and the motor nucleus of the trigeminal. The somatic sensory fibers of the vagus, the glossopharyngeal and the facial nerves probably end in the nucleus of the descending tract of the trigeminal and their cortical impalters are probably carried up in the central sensory path of the trigeminal. The mesencephalic root, descending root of the trigeminal, arises from unipolar cells arranged in scattered groups in a column at the lateral edge of the central gray matter surrounding the upper end of the fourth ventricle and the cerebral aqueduct. They have usually been considered as motor fibers that join the motor root, but Johnston claims that they join the sensory root of the trigeminal, that they develop in the allar, not the basal lamina, and that the pear-shaped unipolar cells are sensory in type. The trochleonerve, fourth cranial, contains somatic motor fibers only. It supplies the superior oblique muscle of the eye. Its nucleus of origin, trochleonucleus, is a small oval mass situated in the ventral part of the central gray matter of the cerebral aqueduct at the level of the upper part of the inferior colliculus. The axons from the nucleus pass downward in the tegmentum toward the ponds, but turn abruptly downward before reaching it, and pass into the superior medullary vellum in which they cross horizontally to dicasate with the nerve of the opposite side, and emerges from the surface of the vellum, immediately behind the inferior colliculus. The cells of the trochleonucleus are large, irregular, and yellowish in color. The nuclei of the two sides are separated by the raffae, through which dendrites extend from one nucleus to the other. They receive many collatels and terminals from the posterior longitudinal bundle, which lies on the ventral side of the nucleus. There are no branches from the fibers of the pyramidal tracts to these nuclei. The volitional pathway must be an indirect one, as is the case with the other motor nuclei. The oculomotor nerve, third cranial, contains somatic motor fibers to the oblicuous inferior, rectus inferior, rectus superior, levotopulpebrae superioris, and rectus medialis muscles, and sympathetic efferent fibers, preganglionic fibers, to the ciliary ganglion. The postganglionic fibers connected with these supply the ciliary muscle and the sphincter of the iris. The axons arise from the nucleus of the oculomotor nerve, and pass in bundles through the posterior longitudinal bundle, the tegmentum, the red nucleus, and the medial margin of the substantia nigra in a series of curves and finally emerge from the oculomotor stalkus on the medial side of the cerebral peduncle. The oculomotor nucleus lies in the gray substance of the floor of the cerebral aqueduct, subjacent to the superior colliculus, and extends in front of the aqueduct a short distance into the floor of the third ventricle. The inferior end is continuous with the trochlear nucleus. It is from 6 to 10 mm in length. It is intimately related to the posterior longitudinal bundle, which lies against its ventral lateral aspect, and many of its cells lie among the fibers of the posterior longitudinal bundle. The nucleus of the oculomotor nerve contains several distinct groups of cells which differ in size and appearance from each other, and are supposed to send their axons each to a separate muscle. Much uncertainty still exists as to which group supplies which muscle. There are seven of these groups on nuclei on either side of the midline, and one medial nucleus. The cells of the anterior nuclei are smaller and are supposed to give off the sympathetic efferent axons. The majority of fibers arise from the nucleus of the same side. Some, however, cross from the opposite side and are supposed to supply the rectus medialis muscle. Since oculomotor and abjacens nuclei are intimately connected by the posterior longitudinal bundle, this thicker section of fibers to the medial rectus may facilitate the conjugate movements of the eyes in which the medial and lateral recti are especially involved. Many collateral and terminals are given off to the oculomotor nucleus from the posterior longitudinal bundle, and thus connect it with the vestibular nucleus, the trochlear and abjacens nuclei, and probably with other cranial nuclei. Fibers from the visual reflex center in the superior colliculus pass to the nucleus. It is also connected with the cortex of the occipital lobe of the cerebrum by fibers which pass through the optic radiation. The pathway for voluntary motor impulses is probably similar to that for the abjacent nerve. The optic nerve, or nerve of sight, second cranial, consists chiefly of coarse fibers which arise from the ganglionic layer of the retina. They constitute the third neuron in the series composing the visual path and are supposed to convey only visual impressions. A number of fine fibers also pass in the optic nerve from the retina to the primary centers and are supposed to be concerned in the pupillary reflexes. There are in addition a few fibers which pass from the brain to the retina. They are supposed to control chemical changes in the retina and the movements of the pigment cells and cones. Each optic nerve has, according to Salsa, about 500,000 fibers. In the optic chiasma the nerves from the medial half of each retina cross to enter the opposite optic tract, while the nerves from the lateral half of each retina pass into the optic tract of the same side. The cross fibers tend to occupy the medial side of each optic nerve, but in the chiasma and in the optic tract they are more intermingled. The optic tract is attached to the tubal scenario and laminar terminalis and also to the cerebral peduncle as it crosses obliquely over its under surface. These are not functional connections. A small band of fibers from the medial geniculate body joins the optic tract as the latter passes over it and crosses to the opposite tract and medial geniculate body in the posterior part of the chiasma. This is the commissure of gooden and is probably connected with the auditory system. Most of the fibers of the optic tract terminate in the lateral geniculate body, some pass through the superior brachium to the superior colliculus, and others either pass over or through the lateral geniculate body to the pulvernau of the thalamus. These end stations are often called the primary visual centres. The lateral geniculate body consists of medium-sized pigmented nerve cells arranged in several layers by the penetrating fibers of the optic tract. Their axons pass upward beneath the longer fibers of the optic tract, the tainia semicircularis, the cordate nucleus and the posterior horn of the lateral ventricle, where they join the optic radiation of gratiolae. They pass backward and medially to terminate in the visuocensory cortex in the immediate neighbourhood of the calcarene fissure of the occipital lobe. This centre is connected with the one in the opposite side by commissural fibers which course in the optic radiation and the spleenium of the corpus callosum. Association fibers connect it with other regions of the cortex of the same side. The region of the pulvernau in which optic tract fibers terminate resembles in structure the lateral geniculate body. Its axons also have a similar course, though in a somewhat more dorsal plane. The superior colliculus receives fibers from the optic tract through the superior brachium. Some enter by the superficial white layer, stratum zonale. Others appear to dip down into the grey cap, stratum synoeum, while others probably dicaset across the midline to the opposite colliculus. Other fibers from the superior brachium pass into the stratum opticum, upper grey white layer. Some of these turn upward into the grey cap while others terminate among the cells of this layer. Since the superior colliculi appear to be the central organs concerned in the control of eye muscle movements and eye muscle reflexes, we should expect to find them receiving fibers from other sensory paths. Many fibers pass to the superior colliculus from the medial filet, as the latter passes through the tegmentum, bringing the superior colliculus into relation with the sensory fibers of the spinal cord. Fibers from the central sensory path of the trigeminal probably pass with these. Part of the ventral spinal cerebellar tract, gauze, is said to pass up through the reticular formation of the pons and midbrain toward the superior colliculus anthalamus. The superior colliculus is intimately connected with the central auditory path, the lateral lemniscus, as part of its fibers pass the inferior colliculus and terminate in the superior colliculus. They are probably concerned with reflex movements of the eye depending on auditory stimuli. The superior colliculus is said to receive fibers from the stria midolaris talamis of the opposite side, which pass through the commissura habennuli and turn back to the roof of the midbrain, especially to the superior colliculus. By this path, both the primary and cortical olfactory centers are brought into relation with the eye muscle reflex apparatus. The fibers which pass to the nuclei of the eye muscles arise from large cells in the stratum opticum and stratum lemnisci, and pass around the ventral aspect of the central grey matter, where most of them cross the midline in the phantom decossation of manor, and then turn downward to form the ventral longitudinal bundle. This bundle runs down partly through the red nucleus in the formatio reticularis, ventral to the posterior longitudinal bundle of the midbrain, pons and medulla omblongata, into the ventral funiculus of the spinal cord, where it is known as the tectospinal fasciculus. Some other fibers are said to pass down with the rubra spinal tract in the lateral funiculus. Some fibers do not decossate, but pass down in the ventral longitudinal bundle of the same side on which they arise, unless possibly they come from the opposite colliculus over the aqueduct. From the ventral longitudinal bundle, collatals are given off to the nuclei of the eye muscles, the oculomotor, the trochlea, and the abjacens. Many collatals pass to the red nucleus, and are probably concerned with the reflexes of the rubra spinal tract. The fibers of the tectospinal tract end by collatals and terminals, either directly or indirectly, among the motor cells in the anterior column of the spinal cord. The superior colliculus receives fibers from the visual sensory area of the occipital cortex. They pass in the optic radiation. Probably no fibers pass from the superior colliculus to the visual sensory cortex. End of Part 21. Section 22 of Grey's Anatomy, Part 4. This is a LibriVox recording. All LibriVox recordings are in the public domain. For more information or to volunteer, please visit LibriVox.org. Recording by Morgan Scorpion. Anatomy of the Human Body, Part 4 by Henry Gray. Composition and Connections of Cranial Nerves, Part 4. The olfactory nerves, first cranial, or nerves of smell, arise from spindle-shaped bipolar cells in the surface epithelium of the olfactory region of the nasal cavity. The non-medallated axons pass upward in groups through numerous foramina in the cribiform plate to the olfactory bulb. Here several fibers, each ending in a tuft of terminal filaments, come into relation with the brush-like ends of a single dendrite from a mitral cell. This interlacing gives rise to the olfactory glomeruli of the bulb. The termination of several or many olfactory fibers in a single glomerulus, where they form synapses with the dendrites of one or two mitral cells, provides for the summation of stimuli in the mitral cells, and accounts in part, at least, for the detection by the olfactory organs of very dilute solutions. Lateral arborisations of the dendrites of the mitral cells, and the connection of neighbouring glomeruli by the axons of small cells of the glomeruli, and the return of impulses of the mitral cells by keratols, either directly or through the interpolation of granule cells to the dendrites of the mitral cells, reinforce the discharge of the mitral cells along their axons. The axons turn abruptly backward in the deep fibre layer of the bulb to form the olfactory tract. The olfactory tract is continued into the olfactory trigone, just in front of the anterior perforated substance. The axons of the mitral cells on reaching the olfactory trigone separate into three bundles, the lateral olfactory stria, the medial olfactory stria, and the less marked intermedial olfactory stria. The lateral olfactory stria curve lateral wood. A few of the fibres end in the olfactory trigone and the antrolateral portion of the anterior perforated substance. Most of the fibres, however, pass into the uncus, the anterior end of the hippocampal gyrus, and there end in the complicated cortex of the hippocampal gyri. The lateral stria more or less disappear as they cross the antrolateral region of the anterior perforated substance. The greater mass of the fibres of the olfactory tract pass into the lateral stria. Numerous collatils are given into the plexiform layer of the subfrontal cortex, over which the stria pass on their way to the uncus, where they intermingle with the apical dendrons of the medium-sized and small pyramidal cells of the pyramidal layer of this subfrontal or frontal olfactory cortex. The axons give rise to projection fibres, which take an antroposterior direction to the subthalamic region, sending collatils and terminal branches to the stria medullaris and others toward the thalamus. Some of the fibres extend farther back and are believed to reach the pons and the medulla oblongata. Most of the fibres of the lateral olfactory stria pass to the hippocampal region of the cortex, especially to the gyros hippocampi, which may be regarded as the main ending place of the secondary olfactory path derived from axons of the mitral cells. The fibres of the medial olfactory stria terminate for the most part in the parolfactory area, brocus area, a few end in the subcolosal gyrus, and a few in the anterior perforated substance and the adjoining part of the septum perucidum. Some of the fibres pass into the anterior commissure, parolfactoria, to the olfactory tract of the opposite side, where they end partly within the granular layer and partly in the neighbourhood of the glomeruli of the olfactory bulb, thus connecting the bulbs of the two sides. The intermediate olfactory stria are as a rule scarcely visible. The fibres terminate in the anterior perforated substance. A few are said to continue to the uncus. The trigonum olfactorium, anterior perforated substance and the adjoining part of the septum perucidum, are important primary olfactory centres, especially for olfactory reflexes. In these surntas terminate many axons from the mitral cells of the olfactory bulb. In addition, the grey substance of the olfactory tract and the gyrus subcolosus receive terminals of the mitral cells. The pathways from these centres to lower centres in the brainstem and spinal cord are only partly known. The most direct path, the tractus olfactor mesencephalicus, basal olfactory bundle of Wallenberg, is supposed to arise from cells in the grey substance of the olfactory tract, the olfactory trigon, the anterior perforated substance and the adjoining part of the septum perucidum. The fibres are said to pass direct to the tubus scenario, to the corpus mamillare, to the brainstem and the spinal cord. The fibres which enter the mammillary body probably come into relation with cells whose axons give rise to the vasiculus mammillotegmentalis, mammillotegmental bundle of gudan, which is supposed to end in the grey substance of the tegmentum and of the aqueduct. Some of its fibres are said to join the posterior longitudinal bundle and others to extend as far as the reticular formation of the pons. Some of the fibres of the medial olfactory stria come into relation with cells in the power olfactory area of Broca and in the anterior perforated substance whose axons course in the medullary stria of the thalamus. As the axons pass to the lower part of the septum perucidum, they are joined by other fibres whose cells receive impulses from the mitral cells. These fibres of the medullary stria end for the most part in the herbenular nucleus of the same side, some however cross in the herbenular commissure, dorsal part of the posterior commissure, to the herbenular nucleus of the opposite side. A few fibres of the medullary stria are said to pass by the herbenular nucleus to the roof of the midbrain, especially the superior colliculus, while a few others come into relation with the posterior longitudinal bundle and association tracts of the mesencephalon. The ganglion of the herbenuli, located in the trigonome herbenuli, just in front of the superior colliculus, contain a mesial nucleus with small cells and a lateral nucleus with larger cells. The axons of these cells are grouped together in a bundle, the succulus retroflexus of manate, which passes ventrally medial to the red nucleus and terminates in a small medial ganglion in the substantia perforata posterior, immediately in front of the pons, called the interpreduncular ganglion. The interpreduncular ganglion has while the large nerve cells whose axons curve backward and downward as the tegmental bundle of gooden, to end partly in the dorsal tegmental nucleus and surrounding gray substance where they come into relation with association neurons and the dorsal longitudinal bundle of chutes. The majority of the axons that arise from the mitral cells of the olfactory bulb and course in the olfactory tract course in the lateral olfactory stria to the uncous and hippocampal gyrus and terminate in the cortex. Other fibres probably pass to the uncous and hippocampal gyrus from the primary olfactory centres in the trigonome and anterior perforated substance. The gyrus hippocampus is continued through the isthmus to the gyrus cinguli which passes over the corpus callosum to the area parolfactoria. The cortical portions of these gyri are connected together by a thick association bundle, the cingulum, that lies buried in the depth of the gyrus cinguli, extending forward to the parolfactory area and backward into the hippocampal region. The axons from the gyrus cinguli pass into the cingulum, many of them bifurcate. The anterior branches together with the axons which run in that direction are traceable as far forward as the anterior part of the septum perucidum and the anterior end of the corpus striatum, where some of them are incorporated with projection fibres passing toward the internal capsule. The branches and axons which pass backward terminate partly in the hippocampus, the dentate gyrus and the hippocampal gyrus. Shorter association fibres connect various sections of the gyrus fornicatus, cingulate gyrus, isthmus and hippocampal gyrus, and these with other regions of the cortex. These gyri constitute the cortical centre for smell. The dentate gyrus, which may be considered as a modified part of the hippocampus, is partially separated from the gyrus hippocampus by the hippocampal fissure and from the fimbria by the fimbrio dentate sulcus. It is intimately connected with the hippocampal gyrus and the hippocampus. When followed backward, the dentate gyrus separates from the fimbria at the splenium, loses its incisions and knobs and as the fasciolisinerea passes over the splenium onto the dorsal surface of the corpus callosum and spreads out into a thin layer of grey substance known as the indusium, which can be traced forward around the genu of the corpus callosum into the gyrus subcolosus. The white matter of the indusium known as the medial longitudinal strii, nerves of lancici, and the lateral longitudinal strii are related to the indusium somewhat as the singulum is to the gyrus singuli. Axons from the indusium pass into the longitudinal strii, some running forward and others running backward, while some, after entering the medial longitudinal strii, pierce the corpus callosum to join the phonics. Some of the fibres which pass forward extend around the front of the corpus callosum and the anterior commissure, then curve downward, according to Kahau, enter the corpus striatum where they join the olfactory projection path. Other fibres are said to arise in the power olfactory area, the gyrus subcolosus and the anterior perforated substance, diagonal band of broker, and course backward in the longitudinal strii to the dentate gyrus and the hippocampal region. The indusium is usually considered as a rudimentary part of the rhinencephalon. The olfactory projection fibres which arise from the pyramidal cells of the incus and hippocampus and from the polymorphic cells of the dentate gyrus form a dense stratum on the ventricular surface, especially on the hippocampus, called the alveus. These fibres pass over into the fimbria and are continued into the phonics. About one fourth of all the fibres of the fimbria are large projection fibres. The other three fourths consist of fine commissural fibres which pass from the hippocampus of one side through the fimbria and hippocampal commissure, ventral, sultarium, or lyra, to the fimbria and hippocampus of the opposite side where they penetrate the pyramidal layer and terminate in the stratum radiatum. The fibres which course in the phonics pass forward and downward into the corporate mammillary, where numerous collateral are given off and a few terminate. Most of the fibres in the phonics, however, pass through the corpore, cross the middle line and turn downward in the reticular formation in which they are said to be traceable as far as the pons and possibly farther. As the phonics passes beneath the corpus callosum, it receives fibres from the longitudinal striae of the indusium and from the singulum. These are the perforating fibres of the phonics which pass through the corpus callosum and course in the phonics toward the mammillary body. As the phonics passes the anterior end of the thalamus, a few fibres are given off to the striae medullaris of the thalamus and turn back in the striae to the herbenular ganglion of the same and the opposite side, having probably the same relation that the reflex fibres have which arise from the primary centres and course in the striae medullaris of the thalamus. Aside from the fibres of the phonics which pass through the mammillary body to decosate and descend as the mammillomason cephalic fasciculus, many fibres are said to pass into the bundle of vix zazia and one bundle of fibres is said to pass from the phonics to the tubus the nureum. The mammillary bodies receive collatils and terminals then from the cortical centres via the phonics and probably other collatils and terminals are received directly from the primary centres through the tractus olfacto-mason cephalicus. According to Cahal fibres also reach the mammillary body through the peduncle of the corpus mammillary from the arcuate fibres of the tegmentum and from the main filet. The phonics probably brings the cortical centres into relation with the reflex path that runs from the primary centres to the mammillary body and the tubus nureum. The bundle of vix zazia, the mammillostalamic fasciculus, arises from cells in both the medial and lateral nuclei of the mammillary body and by fibres that are directly continued from the phonics. There axons divide within the grey matter. The corse of branches pass into the anterior nucleus of the thalamus as the bundle of vix zazia, the finer branches pass downward as the mammillotegmental bundle of guden. The bundle of vix zazia spreads out fan-like as it terminates in the anterior or dorsal nucleus of the thalamus. A few of the fibres pass through the dorsal nucleus to the angular nucleus of the thalamus. The axons from these nuclei are supposed to form part of the thalamocortical system. The mammillotegmental bundle has already been considered under the olfactory reflex paths. The amidelloid nucleus and the tainier semicircularis, striaterminalis, probably belong to the central olfactory apparatus. The tainier semicircularis extends from the region of the anterior perforated substance to the nucleus amygdalae. Its anterior connections are not clearly understood. Fibres are said to arise from cells in the anterior perforated substance. Some of the fibres pass in front of the anterior commissure. Others join the phonics for a short distance as they pass behind the anterior commissure. The two strands ultimately join to form the tainier and pass backward in the groove between the chordate nucleus and the thalamus to the amygdalae nucleus. Other fibres are said to pass in the opposite direction from the amygdalae nucleus to the thalamus. End of Part 22. Section 23 of Gray's Anatomy Part 4. This is a LibriVox recording. All LibriVox recordings are in the public domain. For more information or to volunteer, please visit LibriVox.org. Anatomy of the Human Body Part 4 by Henry Gray. Pathways from the brain to the spinal cord. The descending fasciculi, which convey impulses from the higher centers to the spinal cord and located in the lateral and ventral funiculi. The motor tract, conveying voluntary impulses, arises from the pyramid cells situated in the motor area of the cortex, the anterior, central, and the posterior portions of the frontal gyri and the paracentral lobule. The fibres are at first somewhat widely diffused, but as they descend through the coroner radiata, they gradually approach each other and pass between the lentiform nucleus and thalamus in the genu and anterior two-thirds of the occipital part of the internal capsule. Those in the genu are named the geniculate fibres, while the remainder constitute the cerebrospinal fibres. Proceeding downward, they enter the middle three-fifths of the base of the cerebral peduncle. The genuculate fibres cross the middle line and end by arborizing around the cells of the motor nuclei of the cranial nerves. The cerebrospinal fibres are continued downward into the pyramids of the medulla oblongata, and the transit of the fibres from the medulla oblongata is affected by two paths. The fibres nearest to the anterior median fissure cross the middle line, forming the decussation of the pyramids, and descend in the opposite side of medulla spinalis as the lateral cerebrospinal fasciculus cross pyramidal tract. Throughout the length of the medulla spinalis, fibres from this column pass into the gray substance to terminate either directly or indirectly around the motor cells of the anterior column. The more laterally placed portion of the tract does not decussate in the medulla oblongata, but descends as the anterior cerebrospinal fasciculus, direct pyramidal tract. These fibres however end in the anterior gray column of the opposite side of the medulla spinalis by passing across in the anterior white commissure. There is considerable variation in the extent to which decussation takes place in the medulla oblongata. About two-thirds or three-fourths of the fibres usually decussate in the medulla oblongata and the remainder in the medulla spinalis. The axons of the motor cells in the anterior column pass out as the fibres of the anterior roots of the spinal nerves, along which the impulses are conducted to the muscles of the trunk and limbs. From this it will be seen that all the fibres of the motor tract pass to the nuclei of the motor nerves on the opposite side of the brain or medulla spinalis, a fact which explains why involving the motor area of one side causes paralysis of the muscles of the opposite side of the body. Further, it will be seen that there is a break in the continuity of the motor chain. In the case of the cranial nerves, this break occurs in the nuclei of these nerves and in the case of the spinal nerves in the anterior gray column of the medulla spinalis. For clinical purposes, it is convenient to emphasize this break and divide the motor tract into two portions. One, a series of upper motor neurons which comprises the motor cells in the cortex and their descending fibres down to the nuclei of the motor nerves. Two, a series of lower motor neurons which includes the cells of the nuclei of the motor cerebral nerves or the cells of the anterior columns of the medulla spinalis and their axis cylinder processes to the periphery. The rubro spinal fasciculus arises from the large cells of the red nucleus. The fibers cross the raffae of the midbrain in the decassation of feral and descend in the fermatio reticularis of the pons and medulla dorsal to the medial lamniscus and as they pass into the spinal cord come to lie in a position ventral to the cross permenal tracts in the lateral finiculus. The rubro spinal fibers end either directly or indirectly by terminals and collaterals about the motor cells in the anterior column on the side opposite from their origin in the red nucleus. A few are said to pass down on the same side. Since the red nucleus is intimately related to the cerebellum by terminals and collaterals of the superior peduncle which arises in the dentate nucleus of the cerebellum, the rubro spinal fasciculus is supposed to be concerned with cerebellar reflexes, complex motor coordination necessary in locomotion and equilibrium. The afferent paths concerned in these reflexes have already been partly considered, namely the dorsal and ventral spinal cerebellar fasciculi and probably some of the fibers of the posterior finiculi which reach the cerebellum by the inferior peduncle. The tectospinal fasciculus arises from the superior colliculus of the roof tectum of the midbrain. The axons come from large cells in the stratum opticum and stratum limbnicii and sweep ventrally around the central gray matter of the aqueduct, cross the raffae in the fountain decassation of Maynard and turn downward in the tegmentum in the ventral longitudinal bundle. Some of the fibers do not cross in the raffae but pass down on the same side. It is uncertain whether they come from the superior colliculus of the same side or arch over the aqueduct from the colliculus of the opposite side. The tectospinal fasciculus which comprises the major part of the ventral longitudinal bundle passes down through the tegmentum and reticular formation of the pons and medulla oblongata ventral to the medial longitudinal bundle. In the medulla the two bundles are more or less intermingled and the tectospinal portion is continued into the anterior lateral funiculus of the spinal cord ventral to the rubro spinal fasciculus with which some of its fibers are intermingled. Some of the fibers of the tectospinal fasciculus pass through the red nucleus giving off collaterals to it. Others are given off to the motor nuclei of the cranial nerves and in the spinal cord they terminate either directly or indirectly by terminals and collaterals among the nuclei of the anterior column. Since the superior colliculus is an important optic reflex center this tract is probably concerned in optic reflexes and possibly also with auditory reflexes since some of the fibers of the central auditory path the lateral lamniscus terminate in the superior colliculus. The vestibulospinal fasciculus part of the anterior marginal fasciculus or loanthals tract situated chiefly in the marginal part of the anterior funiculus is mainly derived from the cells of the terminal nuclei of the vestibular nerve probably deters and vectorus and some of its fibers are supposed to come from the nucleus fastigius roof nucleus of the cerebellum. The latter nucleus is intimately connected with deters and vectorus nuclei. The vestibulospinal fasciculus is concerned with equilibratory reflexes its terminals and collaterals end about the motor cells in the anterior column. It extends to the sacral region of the cord. Its fibers are intermingled with the ascending spinal thalamic fasciculus with the anterior proper fasciculus and laterally with the tectospinal fasciculus. Its fibers are supposed to be both crossed and uncrossed in the brainstem it is associated with the dorsal longitudinal bundle. The pontospinal fasciculus vectoru arises from the cells in the reticular formation of the pons from the same and the opposite side and is associated in the brainstem with the ventral longitudinal bundle. In the cord it is intermingled with the fibers of the vestibulospinal fasciculus in the anterior funiculus. Not much is known about this tract. There are probably other descending fasciculi such as the thalamospinal but not much is known about them. End of section 23. Section 24 of Gray's Anatomy part 4. This is a LibriVox recording. All LibriVox recordings are in the public domain. For more information or to volunteer please visit LibriVox.org. Read by Laurie Ann Walden. Anatomy of the Human Body part 4 by Henry Gray. The meninges of the brain and medulla spinalis. Part 1. The brain and medulla spinalis are enclosed within three membranes. These are named from without inward the dura mater, the arachnoid and the pia mater. The dura mater. The dura mater is a thick and dense inelastic membrane. The portion which encloses the brain differs in several essential particulars from that which surrounds the medulla spinalis and therefore it is necessary to describe them separately. But at the same time it must be distinctly understood that the two form one complete membrane and are continuous with each other at the foramen magnum. The cranial dura mater, dura mater encephaly, dura of the brain lines the interior of the skull and serves the twofold purpose of an internal periosteum to the bones and a membrane for the protection of the brain. It is composed of two layers, an inner or meningeal and an outer or endosteal closely connected together except in certain situations where, as already described, they separate to form sinuses for the passage of venous blood. Its outer surface is rough and fibrillated and adheres closely to the inner surfaces of the bones, the adhesions being most marked opposite the sutures and at the base of the skull. Its inner surface is smooth and lined by a layer of endothelium. It sends inward four processes which divide the cavity of the skull into a series of freely communicating compartments for the lodgement and protection of the different parts of the brain. And it is prolonged to the outer surface of the skull through the various foramina which exist at the base and thus becomes continuous with the perichranium. Its fibrous layer forms sheaths for the nerves which pass through these apertures. Around the margin of the foramen magnum it is closely adherent to the bone and is continuous with the spinal dura mater. Processes. The processes of the cranial dura mater which projects into the cavity of the skull are formed by reduplications of the inner or meningeal layer of the membrane and are four in number. The fox cerebrae, the tentorium cerebellae, the fox cerebellae, and the diaphragma cellae. The fox cerebrae so named from its sickle-like form is a strong arched process which descends vertically in the longitudinal fissure between the cerebral hemispheres. It is narrow in front where it is attached to the crystal gully of the ethmoid, and broad behind where it is connected with the upper surface of the tentorium cerebellae. Its upper margin is convex and attached to the inner surface of the skull in the middle line as far back as the internal occipital protuberance. It contains the superior sagittal sinus. Its lower margin is free and concave and contains the inferior sagittal sinus. The tentorium cerebellae is an arched lamina elevated in the middle and inclining downward toward the circumference. It covers the superior surface of the cerebellum and supports the occipital lobes of the brain. Its anterior border is free and concave and bounds a large oval opening, the incisura tentorii, for the transmission of the cerebral peduncles. It is attached behind by its convex border to the transverse ridges upon the inner surface of the occipital bone, and there encloses the transverse sinuses. In front to the superior angle of the petrus part of the temporal bone on either side, enclosing the superior patrosal sinuses. At the apex of the petrus part of the temporal bone the free and attached borders meet, and crossing one another are continued forward to be fixed to the anterior and posterior clenoid processes respectively. To the middle line of its upper surface the posterior border of the fox cerebrae is attached, the straight sinus being placed at their line of junction. The fox cerebellae is a small triangular process of dura mater received into the posterior cerebellar notch. Its base is attached above to the under and back part of the tentorium. Its posterior margin to the lower division of the vertical crest on the inner surface of the occipital bone. As it descends it sometimes divides into two smaller folds which are lost on the sides of the foramen magnum. The diaphragma celli is a small circular horizontal fold which roofs in the cell atyrstica and almost completely covers the hypothesis. A small central opening transmits the infundibulum. Structure. The cranial dura mater consists of white fibrous tissue and elastic fibers arranged in flattened laminae which are imperfectly separated by lacunar spaces in blood vessels into two layers endosteal and meningeal. The endosteal layer is the internal periosteum for the cranial bones and contains the blood vessels for their supply. At the margin of the foramen magnum it is continuous with the periosteum lining the vertebral canal. The meningeal or supporting layer is lined on its inner surface by a layer of nucleated flattened mesothelium similar to that found on cirrus membranes. The arteries of the dura mater are very numerous. Those in the anterior fossa are the anterior meningeal branches of the anterior and posterior ethmoidal and internal carotid and a branch from the middle meningeal. Those in the middle fossa are the middle and accessory meningeal of the internal maxillary, a branch from the ascending pharyngeal which enters the skull through the foramen lacerum, branches from the internal carotid and a recurrent branch from the lacrimal. Those in the posterior fossa are meningeal branches from the occipital, one entering the skull through the jugular foramen and another through the mastoid foramen, the posterior meningeal from the vertebral, occasional meningeal branches from the ascending pharyngeal, entering the skull through the jugular foramen and hypoglossal canal and a branch from the middle meningeal. The veins returning the blood from the cranial dura mater, anastomose with the diploic veins and N in the various sinuses. Many of the meningeal veins do not open directly into the sinuses but indirectly through a series of ampuley termed venous lacunae. These are found on either side of the superior sagittal sinus especially near its middle portion and are often invaginated by arachnoid granulations. They also exist near the transverse and straight sinuses. They communicate with the underlying cerebral veins and also with the diploic and emissary veins. The nerves of the cranial dura mater are filaments from the semilunar ganglion from the ophthalmic, maxillary, mandibular, vagus and hypoglossal nerves and from the sympathetic. The spinal dura mater, dura mater spinalis, spinal dura, forms a loose sheath around the medulla spinalis and represents only the inner or meningeal layer of the cranial dura mater. The outer or endosteal layer ceases at the foramen magnum, its place being taken by the periosteum lining the vertebral canal. The spinal dura mater is separated from the arachnoid by a potential cavity, the subdural cavity. The two membranes are, in fact, in contact with each other except where they are separated by a minute quantity of fluid which serves to moisten the opposed surfaces. It is separated from the wall of the vertebral canal by a space, the epidural space, which contains a quantity of loose areolar tissue and a plexus of veins. The situation of these veins between the dura mater and the periosteum of the vertebrae corresponds therefore to that of the cranial sinuses between the meningeal and endosteal layers of the cranial dura mater. The spinal dura mater is attached to the circumference of the foramen magnum and to the second and third cervical vertebrae. It is also connected to the posterior longitudinal ligament, especially near the lower end of the vertebral canal by fibrous slips. The subdural cavity ends at the lower border of the second sacral vertebra. Below this level the dura mater closely invests the phelan terminale and descends to the back of the coccyx, where it blends with the periosteum. The sheath of dura mater is much larger than is necessary for the accommodation of its contents, and its size is greater in the cervical and lumbar regions than in the thoracic. On each side may be seen the double openings which transmit the two roots of the corresponding spinal nerve, the dura mater being continued in the form of tubular prolongations on them as they pass through the intervertebral foramina. These prolongations are short in the upper part of the vertebral column, but gradually become longer below, forming a number of tubes of fibrous membrane which enclose the lower spinal nerves and are contained in the vertebral canal. Structure The spinal dura mater resembles in structure the meningeal or supporting layer of the cranial dura mater, consisting of white fibrous and elastic tissue arranged in bands or lamellae, which for the most part are parallel with one another and have a longitudinal arrangement. Its internal surface is smooth and covered by a layer of mesothelium. It is sparingly supplied with blood vessels and a few nerves have been traced into it. The arachnoid The arachnoid is a delicate membrane enveloping the brain and medulla spinalis and lying between the pia mater internally and the dura mater externally. It is separated from the pia mater by the subarachnoid cavity, which is filled with cerebrospinal fluid. The cranial part, arachnoidia encephaly of the arachnoid, invests the brain loosely and does not dip into the sulci between the gyri nor into the fissures with the exception of the longitudinal. On the upper surface of the brain the arachnoid is thin and transparent. At the base it is thicker and slightly opaque toward the central part where it extends across between the two temporal lobes in front of the ponds so as to leave a considerable interval between it and the brain. The spinal part, arachnoidia spinalis of the arachnoid is a thin, delicate, tubular membrane loosely investing the medulla spinalis. Above it is continuous with the cranial arachnoid. Below it widens out and invests the cauda equina and the nerves proceeding from it. It is separated from the dura mater by the subdural space but here and there this space is traversed by isolated connective tissue trabeculi which are most numerous on the posterior surface of the medulla spinalis. The arachnoid surrounds the cranial and spinal nerves and encloses them in loose sheaths as far as their points of exit from the skull and vertebral canal. Structure. The arachnoid consists of bundles of white fibrous and elastic tissue intimately blended together. Its outer surface is covered with a layer of low cuboidal mesothelium. The inner surface and the trabeculi are likewise covered by a somewhat low type of cuboidal mesothelium which in places are flattened to a pavement type. Vessels of considerable size but few in number and according to Bokdilek a rich plexus of nerves derived from the motor root of the trigeminal, the facial and the accessory nerves are found in the arachnoid. The subarachnoid cavity, cavum subarachnoidiali subarachnoid space is the interval between the arachnoid and pia mater. It is occupied by a spongy tissue consisting of trabeculi of delicate connective tissue and inner communicating channels in which the subarachnoid fluid is contained. This cavity is small on the surface of the hemispheres of the brain. On the summit of each gyrus the pia mater and the arachnoid are in close contact but in the sulci between the gyri triangular spaces are left in which the subarachnoid trabecular tissue is found for the pia mater dips into the sulci whereas the arachnoid bridges across them from gyrus to gyrus. At certain parts of the base of the brain the arachnoid is separated from the pia mater by wide intervals which communicate freely with each other and are named subarachnoid cisternae and these the subarachnoid tissue is less abundant. End of section 24. Section 25 of Grey's Anatomy Part 4. This is a LibriVox recording. All LibriVox recordings are in the public domain. For more information or to volunteer please visit LibriVox.org. Read by Laurie Ann Walden. Anatomy of the Human Body Part 4 by Henry Gray. The Meninges of the Brain and Medulla Spinalis Part 2. Subarachnoid cisternae, cisternae subarachnoid alleys. The cisterna cerebellum medularis, cisterna magna, is triangular on sagittal section and results from the arachnoid bridging over the interval between the medulla oblongata and the undersurfaces of the hemispheres of the cerebellum. It is continuous with the subarachnoid cavity of the medulla spinalis at the level of the foramen magnum. The cisterna pontis is a considerable space on the ventral aspect of the pond. It contains the basilar artery and is continuous behind with the subarachnoid cavity of the medulla spinalis and with the cisterna cerebellum medularis and in front of the ponds with the cisterna interpeduncularis. The cisterna interpeduncularis, cisterna basalis, is a wide cavity where the arachnoid extends across between the two temporal lobes. It encloses the cerebral peduncles and the structures contained in the interpeduncular fosso and contains the arterial circle of Willis. In front the cisterna interpeduncularis extends forward across the optic chiasma forming the cisterna chiasmatis and on to the upper surface of the corpus callosum for the arachnoid stretches across from one cerebral hemisphere to the other immediately beneath the free border of the false cerebri and thus leaves a space in which the anterior cerebral arteries are contained. The cisterna fosso cerebri lateralis is formed in front of either temporal lobe by the arachnoid bridging across the lateral fissure. This cavity contains the middle cerebral artery. The cisterna veni-magnis cerebri occupies the interval between the splinium of the corpus callosum and the superior surface of the cerebellum. It extends between the layers of the telakoroidia of the third ventricle and contains the great cerebral vein. The subarachnoid cavity communicates with the general ventricular cavity of the brain by three openings. One, the foramen of magindi, is in the middle line at the inferior part of the roof of the fourth ventricle. The other two are at the extremities of the lateral recesses of that ventricle behind the upper roots of the glossopharyngeal nerves and are known as the foramina of luchka. It is still somewhat uncertain whether these foramina are actual openings or merely modified areas of the inferior vellum which permit the passage of the cerebrospinal fluid from the ventricle into the subarachnoid spaces as through a permeable membrane. The spinal part of the subarachnoid cavity is a very wide interval and is the largest at the lower part of the vertebral canal where the arachnoid encloses the nerves which form the cauda equina. Above it is continuous with the cranial subarachnoid cavity. Below it ends at the level of the lower border of the second sacral vertebra. It is partially divided by a longitudinal septum, the subarachnoid septum, which connects the arachnoid with the pia mater opposite the posterior median sulcus of the medulla spinalis and forms a partition incomplete and crib reform above but more perfect in the thoracic region. The spinal subarachnoid cavity is further subdivided by the ligamentum denticulatum which will be described with the pia mater. The cerebrospinal fluid is a clear limpid fluid having a saltish taste and a slightly alkaline reaction. According to Lausanne it consists of 98.5 parts of water the remaining 1.5% being solid matters animal and saline. It varies in quantity being most abundant in old persons and is quickly secreted. The arachnoid villi granulationes arachnoidiales glanduli patchioni patchionian bodies are small fleshy-looking elevations usually collected into clusters of variable size which are present upon the outer surface of the dura mater in the vicinity of the superior sagittal sinus and in some other situations. Upon laying open the sagittal sinus and the venus lacuni on either side of it villi will be found protruding into its interior. They are not seen in infancy and very rarely until the third year. They are usually found after the seventh year and from this period they increase in number and size as age advances. They are not glandular in structure but are enlarged normal villi of the arachnoid. As they grow they push the thinned dura mater before them and cause absorption of the bone from pressure and so produce the pits or depressions on the inner wall of the calvarium. Structure. An arachnoidial villus represents an invasion of the dura by the arachnoid membrane. The latter penetrates the dura in such a manner that the arachnoid mesothelial cells come to lie directly beneath the vascular endothelium of the great dural sinuses. It consists of the following parts. One. In the interior is a core of subarachnoid tissue continuous with the mesh work of the general subarachnoid tissue through a narrow pedicle by which the villus is attached to the arachnoid. Two. Around this tissue is a layer of arachnoid membrane limiting and enclosing the subarachnoid tissue. Three. Outside this is the thinned wall of the lacuna which is separated from the arachnoid by a potential space which corresponds to and is continuous with the sub dural cavity. Four. And finally if the villus projects into the sagittal sinus it will be covered by the greatly thinned wall of the sinus which may consist merely of endothelium. It will be seen therefore that fluid injected into the subarachnoid cavity will find its way into these villi and it has been found experimentally that it passes from the villi into the venous sinuses into which they project. The pia mater. The pia mater is a vascular membrane consisting of a minute plexus of blood vessels held together by an extremely fine areolar tissue and covered by a reflection of the mesothelial cells from the arachnoid trabeculae. It is an incomplete membrane absent probably at the foramen of magindi and the two foramina of luchka and perforated in a peculiar manner by all the blood vessels as they enter or leave the nervous system. In the perivascular spaces the pia apparently enters as a mesothelial lining of the outer surface of the space. A variable distance from the exterior these cells become unrecognizable and are apparently lacking replaced by neuroglia elements. The inner walls of these perivascular spaces seem likewise covered for a certain distance by the mesothelial cells reflected with the vessels from the arachnoid covering of these vascular channels as they traverse the subarachnoid spaces. The cranial pia mater, pia mater encephaly, pia of the brain, invests the entire surface of the brain dips between the cerebral gyri and cerebellar lamini and is invaginated to form the tila coroidia of the third ventricle and the coroid plexuses of the lateral and third ventricles. As it passes over the roof of the fourth ventricle it forms the tila coroidia and the coroid plexuses of this ventricle. On the cerebellum the membrane is more delicate. The vessels from its deep surface are shorter and its relations to the cortex are not so intimate. The spinal pia mater, pia mater spinalis, pia of the cord, is thicker, firmer and less vascular than the cranial pia mater. This is due to the fact that it consists of two layers, the outer or additional one being composed of bundles of connective tissue fibers arranged for the most part longitudinally. Between the layers are cleft-like spaces which communicate with the subarachnoid cavity and a number of blood vessels which are enclosed in perivascular lymphatic sheaves. The spinal pia mater covers the entire surface of the medulla spinalis and is very intimately adherent to it. In front it sends a process backward into the anterior fissure. A longitudinal fibrous band called the linea splindens extends along the middle line of the anterior surface and a somewhat similar band, the ligamentum denticulatum, is situated on either side. Between the conus medularis the pia mater is continued as a long slender filament philum terminale which descends through the center of the massive nerves forming the cauda aquaena. It blends with the dura mater at the level of the lower border of the second sacral vertebra and extends downward as far as the base of the coccyx where it fuses with the periosteum. It assists in maintaining the medulla spinalis in its position during the movements of the trunk and is from this circumstance called the central ligament of the medulla spinalis. The pia mater forms sheaths for the cranial and spinal nerves. These sheaths are closely connected with the nerves and blend with their common membranous investments. The ligamentum denticulatum dentate ligament is a narrow fibrous band situated on either side of the medulla spinalis throughout its entire length and separating the anterior from the posterior nerve roots. Its medial border is continuous with the pia mater at the side of the medulla spinalis. Its lateral border presents a series of triangular tooth-like processes the points of which are fixed at intervals to the dura mater. These processes are 21 in number on either side. The first being attached to the dura mater opposite the margin of the foramen magnum between the vertebral artery in the hypoglossal nerve and the last near the lower end of the medulla spinalis. End of the meninges of the brain and medulla spinalis. The cerebrospinal fluid. The cerebrospinal fluid for the most part elaborated by the coroid plexuses is poured into the cerebral ventricles which are lined by smooth appenduma. That portion of the fluid formed in the lateral ventricles escapes by the foramen of Monroe into the third ventricle and thence by the aqueduct into the fourth ventricle. Likewise, an ascending current of fluid apparently occurs in the central canal of the spinal cord. This, representing a possible product of the appenduma, may be added to the intraventricular supply. From the fourth ventricle the fluid is poured into the subarachnoid spaces through the medial foramen of Magindy and the two lateral foramina of Lushka. There is no evidence that functional communications between the cerebral ventricles and the subarachnoid spaces exist in any region except from the fourth ventricle. In addition to the elaboration of the cerebrospinal fluid by the coroid plexuses, there seems fairly well established a second source of the fluid from the nervous system itself. The blood vessels that enter and leave the brain are surrounded by perivascular channels. It seems most likely that the outer wall of these channels is lined by a continuation inward of the peel mesothelium, while the inner wall is probably derived from the mesothelial covering of the vessels, which are thus protected throughout the subarachnoid spaces. These mesothelial cells continue inward only a short distance, neuroglia cells probably replacing on the outer surface the mesothelial elements. Through these perivascular channels there is probably a small amount of fluid flowing from nerve cell to subarachnoid space. The chemical differences between the subarachnoid fluid, product of coroid plexuses and perivascular system, and the ventricular fluid, product of coroid plexuses alone, indicate that the products of nerve metabolism are poured into the subarachnoid space. The absorption of the cerebrospinal fluid is a dual process, being chiefly a rapid drainage through the arachnoid villi into the great dural sinuses, and in small part a slow escape into the true lymphatic vessels by way of an abundant but indirect perineural course. In general the arachnoid channels are equipped as fluid retainers with unquestionable powers of diffusion or absorption in regard to certain elements in the normal cerebrospinal fluid, deriving in this way a cellular nutrition. The subdual space between arachnoid and dura is usually considered to be a part of the cerebrospinal channels. It is a very small space, the two limiting surfaces being separated by merely a capillary layer of fluid. Whether this fluid is exactly similar to the cerebrospinal fluid is very difficult to ascertain. Likewise our knowledge of the connections between the subdural and subarachnoid spaces is hardly definite. In some ways the subdural space may be likened to a cirrus cavity. The inner surface of the dura is covered by flattened polygonal mesothelial cells, but the outer surface of the arachnoid is covered by somewhat cuboidal mesothelium. The fluid of the subdural space has probably a local origin from the cells lining it. End of section 25