 Section No. 1 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 M. L. Cohen. Anatomy of the Human Body Part 4 by Henry Gray. Structure of the Nervous System Part 1. The nervous system is the most complicated and highly organized of the various systems which make up the human body. It is the mechanism concerned with the correlation and integration of various bodily processes and the reactions and adjustments of the organism to its environment. In addition, the cerebral cortex is concerned with conscious life. It may be divided into two parts, central and peripheral. The central nervous system consists of the encephalon or brain contained within the cranium and the medulla spinalis or spinal cord lodged in the vertebral canal. The two portions are continuous one another at the level of the upper border of the atlas vertebra. The peripheral nervous system consists of a series of nerves by which the central nervous system is connected with the various tissues of the body. For descriptive purposes, these nerves may be arranged in two groups, cerebrospinal and sympathetic. The arrangement, however, being an arbitrary one, since the two groups are intimately connected and closely intermingled, both the cerebrospinal and sympathetic nerves have nuclei of origin, the somatic efferent and sympathetic efferent, as well as nuclei of termination, somatic afferent and sympathetic afferent, in the central nervous system. The cerebrospinal nerves are 43 in number on either side, 12 cranial attached to the brain and 31 spinal to the medulla spinalis. They are associated with the functions of the special and general senses and with the voluntary movements of the body. The sympathetic nerves transmit the impulses which regulate the movements of the viscera, determine the caliber of the blood vessels, and control the phenomena of secretion. In relation with them are two rows of central ganglia, situated one on either side of the middle line in front of the vertebral columns. These ganglia are intimately connected with the medulla spinalis and the spinal nerves and are also joined to each other by vertical strands of nerve fibers, so as to constitute a pair of knotted cords, the sympathetic trunks, which reach from the base of the skull to the coccyx. The sympathetic nerves issuing from the ganglia form three great pre-vertebral plexuses which supply to thoracic, abdominal, and pelvic viscera. In relation to the walls of these viscera, intricate nerve plexuses and numerous peripheral ganglia are found. Structure of the nervous system The nervous tissues are composed of nerve cells and their various processes together with the supporting tissue cord neuroglia, which, however, is found only in the brain and the medulla spinalis. Certain long processes of the nerve cells are of special importance and it's convenient to consider them apart from the cells. They are known as nerve fibers. To the naked eye, a difference is obvious between certain portions of the brain and the medulla spinalis, that is the gray substance and the white substance. The gray substance is largely composed of nerve cells, while the white substance contains only their long processes, the nerve fibers. It is in the former that nervous impressions are received, stored, and transformed into efferent impulses and by the latter that they are conducted. Hence the gray substance forms the essential constituent of all the ganglionic centers, both those in the isolated ganglia and those aggregated in the brain and medulla spinalis. While the white substance forms the bulk of the commissural portions of the nerve centers and the peripheral nerves. Neuroglia Neuroglia, the particular ground substance in which are embedded the true nervous constituents of the brain and medulla spinalis, consists of cells and fibers. Some of the cells are stellate in shape with ill-defined cell body and their fine processes become neuroglial fibers, which extend radially and unbranched among the nerve cells and fibers which they aid in supporting. Other cells give off fibers which branch repeatedly. Some of the fibers start from the epithelial cells lining the ventricles of the brain and central canal of the medulla spinalis, and pass through nervous tissue branching repeatedly to end in slight enlargements on the pia mater. Thus, neuroglia is evidently a connective tissue in function, but is not so in development. It is ectodermal in origin, whereas all connective tissues are mesodermal. Nerve cells Nerve cells are largely aggregated in the gray substance of the brain and medulla spinalis. But smaller collections of these cells also form the swellings called ganglia seen on many nerves. These latter are found chiefly upon the spinal and cranial nerve roots and in connection with the sympathetic nerves. The nerve cells vary in shape and size and have one or more processes. They may be divided for purposes of description into three groups according to the number of processes which they possess. One, unipolar cells, which are found in the spinal ganglia. The single process after a short course divides in a T-shaped manner. Two, bipolar cells also found in the spinal ganglia when the cells are in an embryonic condition. They are best demonstrated in the spinal ganglia of fish. Sometimes the processes come off from opposite poles of the cell and the cell then assumes a spindle shape. In others, both processes emerge at the same point. In some cases, where two fibers are apparently connected with the cell, one of the fibers is really dry from the adjoining nerve cell and is passing to end in ramification around the ganglion cell. Or, again, it may be coiled spirally around the nerve process which is issuing from the cell. Three, multipolar cells, which are pyramidal or stellate in shape and characterized by their large size and by the numerous processes which issue from them. The processes are of two kinds. One of them is termed the axiscylinder process, or axon, because it becomes the axiscylinder of a nerve fiber. The others are termed the protoplasmic processes, or dendrons. They begin to divide and subdivide as soon as they emerge from the cell and finally end in minute twigs and become lost among the other elements of the nervous tissue. The body of the nerve cell, known as the cyton, consists of a finely fibrillated protoplasmic material of a reddish or yellowish-brown color which occasionally presents patches of a deeper tint caused by the aggregation of pigment granules at one side of the nucleus as in the substantia nigra and locus surleus of the brain. The protoplasm also contains peculiar angular granules which stain deeply with basic dyes such as methylene blue. These are known as Nissell's granules. They extend into the dendritic processes but not into the axiscylinder. The small clear area at the point of exit of the axon is termed the cone of origin. These granules disappear chromatologist during fatigue or after prolonged stimulation of the nerve fibers connected with the cells. They are supposed to represent a store of nervous energy and in various mental diseases are deficient or absent. The nucleus is, as a rule, a large well-defined spherical body often presenting an intranuclear network and containing a well-marked nucleus. In addition to the protoplasmic network described above, each nerve may be shown to have delicate neurofibrils running through its substance. These fibrils are continuous to the fibrils of the axon and are believed to convey nerve impulses. Golgi has also described an extracellular network which is probably a supporting structure. Nerve Fibers Nerve fibers are found universally in the peripheral nerves and in the white substance of the brain and medulla spinalis. They are of two kinds, that is medulated or white fibers and non-medulated or grey fibers. The medulated fibers form the white part of the brain and medulla spinalis and also the greater part of every cranial and spinal nerve and give to these structures their opaque white aspect. When perfectly fresh they appear to be homogeneous but soon after removal from the body each fiber presents, when examined by transmitted light, a double outline or contour as if consisting of two parts. The central portion is named the axis cylinder. Around this is a sheath of fatty material staining black with osmic acid named the white substance of Schwann or medullary sheath which gives to the fiber its double contour and the hole is enclosed in the delicate membrane, the neurolema primitive sheath or nucleated sheath of Schwann. The axis cylinder is the essential part of the nerve fiber and is always present. The medullary sheath and the neurolema are occasionally absent especially at the origin and termination of the nerve fiber. The axis cylinder undergoes no interruption from its origin in the nerve center to its peripheral termination and must be regarded as a direct prolongation of a nerve cell. It constitutes about one half or one third of the nerve fiber being greater in proportion in the fibers of the central organs than in those of the nerves. It is quite transparent and is therefore indistinguishable in a perfectly fresh and natural state of the nerve. It is made up of exceedingly fine fibrils which stain darkly with gold chloride and at its terminations may be seen to break up into these fibrelay. The fibrelay have been termed a primitive fibrelay of Schulze. The axis cylinder is said by some to be enveloped in a special reticular sheath which separates it from the medullary sheath and is composed of a substance called neurokeratin. The more common opinion is that this network or reticulum is contained in the white matter of Schwann and by some it is believed to be produced by the action of the reagents employed to show it. The medullary sheath or white matter of Schwann is regarded as being fatty matter in a fluid state which insulates and protects the essential part of the nerve, the axis cylinder. It varies in thickness and some forming a layer of extreme thinness so as to be scarcely distinguishable and others forming about one half of the nerve fiber. The variation in diameter of the nerve fibers from 2 to 16 microns depends mainly upon the amount of white substance so the axis cylinder also varies within certain limits. The medullary sheath undergoes interruptions in its continuity at regular intervals giving the fiber the appearance of constriction at these points. These are known as the nodes of Ramvier. The portion of nerve fiber between two nodes is called an internodal segment. The neurolemma or primitive sheath is not interrupted at the nodes but passes over them as a continuous membrane. If the fiber be treated with silver nitrate the reagent penetrates the neurolemma at the nodes and unexposure to light reduction takes place giving rise to the appearance of black crosses Ramvier's crosses on the axis cylinder. There may also be seen transverse lines beyond the nodes termed Froman's lines. The significance of these is not understood. In addition to these interruptions oblique clefs may be seen in the medullary sheath subdividing it into irregular portions which are termed medullary segments or segments of Lanterman. There is reason to believe that these clefs are artificially produced in the preparation of these specimens. Medulated nerve fibers when examined in the fresh condition frequently present a beaded or varicose appearance. This is due to manipulation and pressure causing the oily matter to collect into drops and in consequence of the extreme delicacy of the primitive sheath even slight pressure will cause the transudation of the fatty matter which collects as drops of oil outside the membrane. The neurolemma or primitive sheath presents the appearance of a delicate, structureless membrane. Here and there beneath it and situated in depression in the white matter of Schwann are nuclei surrounded by a small amount of protoplasm. The nuclei are oval and somewhat flattened and bear a definite relation to the nodes of Ramvier. One nucleus generally lying in the center of each internode. The primitive sheath is not present in all medulated nerve fibers being absent in those fibers which are found in the brain and medulla spinalis. Wallerian degeneration When nerve fibers are cut across the central ends of the fibers degenerate as far as the first node of Ramvier but the peripheral ends degenerate simultaneously throughout their whole length. The axons break up in the fragments and become surrounded by drops of fatty substance which are formed from the breaking down of the medullary sheath. The nuclei of the primitive sheath proliferate and finally absorption of the axons and fatty substance occurs. If the cut ends of the nerve be sutured together regeneration of the nerve fibers takes place by the down growth of axons from the central end of the nerve. At one time it was believed that the regeneration was peripheral in origin but this has been disproved. The proliferated nuclei in the peripheral portions taking part merely in the formation of the so-called scaffolding along which the new axons pass. Non-medulated fibers Most of the fibers in the sympathetic nervous system and some of the cerebrospinal consist of gray or gelatinous nerve fibers, fibers of ramac. Each of these contain an axis cylinder to which nuclei are applied at intervals. These nuclei are believed to be in connection with the delicate sheath corresponding with the neurolemma of the medulated nerve fiber. In external appearance, the non-medulated nerve fibers are semi-transparent and gray or yellowish gray. The individual fibers vary in size, generally averaging about half the size of the medulated fibers. End of Section 1 Section 2 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 ML Cohen. Anatomy of the Human Body Part 4 by Henry Gray. Structure of the Nervous System Part 2 Structure of the peripheral nerves in ganglia. The cerebrospinal nerves consist of numerous nerve fibers collected together and enclosed in the membranous sheaths. A small bundle of fibers enclosed in the tubular sheath is called a funiculus. If the nerve is of small size, it may consist of only a single funiculus. But if large, the funiculi are collected together into larger bundles of fisculi, which are bound together in a common membranous... In structure, the common membranous investment or sheath of the whole nerve, epineurium, as well as the septic given off from it to separate the fisculi consist of connective tissue composed of white and yellow elastic fibers, the latter existing in great abundance. The tubular sheath of the funiculi, perineurium, is a fine smooth transparent membrane which may be easily separated in the form of a tube from the fibers it encloses. In structure, it is made up of connective tissue which has a distinctly lamellar arrangement. The nerve fibers are held together and supported within the funiculus by delet connected tissue called the endonurium. It is continuous with septae which pass inward from the innermost layer of the perineurium and shows a ground substance in which are embedded fine bundles of fibers connected tissue running for the most part longitudinally. It serves to support capillary vessels arranged so as to form a network with elongated meshes. The cerebral spinal nerves consist almost exclusively of medulated nerve fibers, only a very small proportion of non-medulated being present. The blood vessels supplying a nerve end in a minute capillary plexus. The vessels composing which pierce the perineurium and run for the most part parallel with the fibers. They are connected together by short transverse vessels forming narrow oblong meshes similar to the capillary system of muscle. Fine non-medulated nerve fibers, vasomotive fibers, accompanying these capillary vessels and break up into elementary fibrils which form a network around the vessels. Horsely has demonstrated certain medulated fibers running in the epineurium and terminating in small, tactile corpuscles or end bulbs of crouse. These nerve fibers, which Marshall believes to be sensory in which he has termed nerve-eye nervorum, are considered by him to have an important bearing upon certain neurologic pains. The nerve fibers, so far as it is present known, do not coalesce, but pursue an uninterrupted course from the center to the periphery. In separating a nerve, however, into its component finiculi, it may be seen that these do not pursue a perfectly insulated course but occasionally join at very acute angle with other finiculi proceeding in the same direction. From this, branches are given off to join again in like manner with other finiculi. It must be distinctly understood, however, that in these communications the individual nerve fibers do not coalesce, but merely pass into the sheath of the adjacent nerve, become intermixed with nerve fibers, and again pass on to intermingle with the nerve fibers in some adjoining finiculus. Nerves, in their course, subdivide into branches, and these frequently communicate with branches of a neighboring nerve. The communications, which thus take place, form what is called a plexus. Sometimes a plexus is formed by the primary branches of the trunks of the nerves, as the cervical, brachial, lumbar, and sacral plexuses. And occasionally, by the terminal finiculi, as in the plexus formed at the periphery of the body. In the formation of a plexus, the component nerves divide, then join, and again subdivide in such a complex manner that the individual finiculi become interlaced most intricately, so that each branch leaving a plexus may contain filaments from all the primary nerve trunks which form the plexus. In the formation also a smaller plexus as at the periphery of the body, there is a free interchange of the finiculi and primitive fibers. In each case, however, the individual fibers remain separate and distinct. It is probable that through this interchange of fibers, every branch passing off from a plexus has a more extensive connection with the spinal cord that if it had proceeded to its distribution without forming connections with other nerves. Consequently, the parts supplied by these nerves have more extended relations with the nervous centers. By this means also, groups of muscles may be associated for combined action. The sympathetic nerves are constructed in the same manner as the cerebral spinal nerves, but consist mainly of non-medulated fibers, collected in finiculi and enclosed in sheets of connective tissue. There is, however, in these nerves a certain admixture of medulated fibers. The number of these latter varies in different nerves and may be estimated by the color of the nerve. Those branches of the sympathetic, which present a well-marked gray color, are composed chiefly of non-medulated nerve fibers intermixed with a few medulated fibers, while those of a white color contain many of the latter fibers and few of the former. The cerebral spinal and sympathetic nerve fibers convey various impressions. The sensory nerves, while also centripetal or afferent nerves, transmit to the nervous centers impressions made upon the peripheral extremities of the nerves, and in this way the mind, through the medium of the brain, becomes conscious of external objects. The centrifugal or efferent nerves transmit impressions from the nervous centers to the parts to which the nerves are distributed. These impressions, either exciting muscular contraction or enhancing the processes of nutrition, growth and secretion. Origins and terminations of nerves. By the expression, quote, the termination of nerve fibers, end quote, is signified their connections with the nerve centers and with the parts they supply. The former are sometimes called their origins or central terminations, the latter their peripheral terminations. Origins of nerves. The origins, in some cases, is single. That is to say, the whole nerve emerges from the nervous center by a single root. In other instances, the nerve arises by two or more roots which come off from different parts of the nerve center, sometimes widely apart from each other. And it often happens when a nerve arises in this way by two roots that the function of these two roots are different. As for example, in the spinal nerves, each of which arises by two roots, the anterior of which is motor and the posterior sensory. The point where the nerve root or roots emerge from the surface of the nervous center is named a superficial or apparent origin. But the fibers of the nerves can be traced for a certain distance into the substance of the nerve center to some portion of the gray matter which constitutes the deep or real origin of the nerve. The centrifugal or efferent nerve fibers originate in the nerve cells of the gray substance. The axis cylinder processes of these cells being prolonged to form fibers. In the case of the centripetal or afferent nerves, the fibers grow inward either from the nerve cells in the origins of special scents, for example the retina, or from nerve cells in the ganglia. Having entered the nerve center, they branch and send their ultimate twigs among the cells without however uniting with them. Peripheral Terminations of Nerves Nerve fibers terminate peripherally in various ways and these may be conveniently studied in the sensory and motor nerves respectively. The terminations of the sensory nerves are dealt with in the section on sense organs. Motor nerves can be traced into either unstriped or striped muscular fibers. In the unstriped or involuntary muscles, the nerves are derived from the sympathetic and are composed mainly of non-medulated fibers. Near their terminations, they divide into numerous branches which communicate and form intimate plexuses. At the junction of the branches, small triangular nuclear bodies, ganglion cells, are situated. From these plexuses, minute branches are given off which divide and break up into the ultimate fibrillae of which the nerves are composed. Plexus fibrillae, coarse between the involuntary muscle cells and, according to Elisha, terminate on the surfaces of the cells opposite the nuclei in minute swellings. In the striped or voluntary muscle, the nerves supplying the muscular fibers are derived from the cerebrospinal nerves and are composed mainly of medulated fibers. The nerve, after entering the sheath of the muscle, breaks up into fibers or bundles of fibers which form plexuses and divide until, as a rule, a single nerve fiber enters a single muscular fiber. Sometimes, however, if the muscular fiber be long, more than one nerve fiber enters it. Within the muscular fiber, the nerve terminates in a special expansor called Baikun who first accurately described it as a motor end plate. The nerve fiber, on approaching the muscular fiber, suddenly loses its medullary sheath. The neurolema becomes continuous with the sarcolemma of the muscle and only the axis cylinder enters the muscular fiber. There, it at once spreads out, ramifying like the roots of a tree immediately beneath the sarcolemma and becomes embedded in a layer of granular matter, containing a number of clear oblong nuclei, the whole constituting an end plate from which the contractile wave of the muscular fiber is said to start. Ganglia are small aggregation of nerve cells. They are found on the posterior roots of the spinal nerve, on the sensory roots of the trigeminal, facial, glossopharyngeal and vagus nerves, and on the acoustic nerves. They are also found in connection with the sympathetic nerves. On section, they are seen to consist of a reddish-gray substance traversed by numerous white fibers. They vary considerably in form and size. The largest are found in the cavity of the abdomen, the smallest, not visible to the naked eye, existing considerable numbers upon the nerves distributed to the different viscera. Each ganglion is invested by a smooth and firm, closely adhering, membranous envelope consisting of dense areolar tissue. This sheath is continuous with the perinurium of the nerves and sends numerous processes into the interior to support the blood vessels and the presence of the ganglion. In structure all ganglia are essentially similar consisting of the same structural elements, that is nerve cells and nerve fibers. Each nerve cell has a nucleated sheath which is continuous with the neurolemma of the nerve fiber with which the cell is connected. The nerve cells in the ganglia of the spinal nerves are pure form in shape and have each a single process. A short distance from the cell and while still within the ganglion, this process divides in a T-shaped manner one limb of the crossbar turning into the medulla spinalis the other limb passing outward to the periphery. In the sympathetic ganglia, the nerve cells are multipolar and each has one axis cylinder process and several dendrons. The axon emerges from the ganglion as a non-medulated nerve fiber. Similar cells are found in the ganglia connected with the trigeminal nerve and these ganglia are therefore regarded as the cranial portions of the sympathetic system. The sympathetic nervous system includes those portions of the nervous mechanism in which a medulated nerve fiber from the central nervous system passes to a ganglion's sympathetic or peripheral from which fibers usually non-medulated are distributed to such structures for instance blood vessels as are not under voluntary control. The spinal and sympathetic ganglia differ somewhat in the size and disposition of the cells and in the number of nerve fibers entering and leaving them. In the spinal ganglia, the nerve cells are much larger and for the most part collected in groups near the periphery while the fibers which are mostly medulated traverse the central portion of the ganglion. Whereas in the sympathetic ganglia, the cells are smaller and distributed in irregular groups throughout the whole ganglion. The fibers are also irregularly scattered. Some of the entering ones are medulated while many of the leaving the ganglia are non-medulated. Neuron theory The nerve cell and its processes collectively constitute what is termed a neuron and Waldyer formulated the theory that the nervous system is built up of numerous neurons quote, anatomically genetically independent of one another end quote. According to this theory, the neuron theory, the processes of one neuron only come into contact and are never in direct continuity with those of other neurons while impulses are transmitted from one nerve cell to another to these point of contact, the synapses. The synapse or synaptic membranes seems to allow nervous impulses to pass in one direction only namely from the terminals of the axis cylinder to the dendrons. This theory is based on the following facts that is, one embryonic nerve cells or neuroblasts are entirely distinct from one another. Two when nervous tissues are stained by the Golgi method, no continuity is seen even between neighboring neurons Three when degenerative changes occur in nervous tissue either as a result of disease or experiment they never spread from one neuron to another but are limited to the individual neurons or groups of neurons primarily affected. It must however be added that within the past few years the validity of the neuron theory had been called into question by certain eminent histologists who maintain that by the employment of a more delicate histologic method minute fibrils can be followed from one nerve cell to another their existence however in the living is open to question Mott and Maranesco made careful examination of living cells using even the ultramisoscope and agreed that neither nistle bodies nor neurofibrils are present in the living state For the present we may look upon the neurons as the units or structural elements of the nervous system all the neurons are present at birth which are present in the adult their division ceases before birth they are not all functionally active at birth but gradually assume functional activity there is no indication of any regeneration after the destruction of the cell body of any individual neurons fasciculi tracts or fiber systems are groups of axons having homologous origin and homologous distribution in regards to collateral subdivisions and terminals and are often named in accordance with their origin and termination the name of the nucleus or the location of the cell body from which the axon or fiber arises preceding that of the nucleus or location of its termination a given topographical area seldom represents a pure tract as in most cases fibers of different systems are mixed end of section 2 section 3 is 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 ML Cohen anatomy of the human body part 4 by Henry Gray development of the nervous system part 1 development of the nervous system the entire nervous system is of ectodermal origin the fluidiment is seen in the neural groove which extends along the dorsal aspect of the embryo by the elevation and ultimate fusion of the neural folds the groove is converted into the neural tube the anterior end of the neural tube becomes expanded to form the three primary brain vesicles the cavity of the tube is subsequently modified to form the ventricular cavities of the brain and the central canal of the medullus spinalis from the wall of the nervous system elements and the neuroglia of the brain and medullus spinalis are developed the medullus spinalis at first the wall of the neural tube is composed of a single layer of columnar ectodermal cells soon the sidewalls become thickened while the dorsal and ventral parts remain thin and are named the roof and floor plates a transverse section of the tube at this stage presents an oval outline while its lumen has the appearance of a slid the cells which constitute the wall of the tube proliferate rapidly lose their cell boundaries and form a syncytium this syncytium consisted first of dense protoplasm with closely plaque nuclei but later it opens out and forms a looser meshwork with the cellular strands arranged in a radiating manner from the central canal three layers may now be defined an internal or a pendymal an intermediate or mantle and an external or marginal the ependymal layer is ultimately converted into the ependymal of the central canal the processes of its cells pass outward towards the periphery of the medullus spinalis the marginal layer is devoid of nuclei and later forms a supporting framework for the white funiculi of the medullus spinalis the mantle layer represents the whole of the future gray columns in the medullus spinalis in it the cells are differentiated into two sets that is spongioblasts or young neuroglial cells and germinal cells which are the parents of the neuroblasts or young nerve cells the spongioblasts are at first connected to one another by filaments of the syncytium in these fibrils are developed so that as the neuroglial cells become defined they exhibit their characteristic mature appearance with multiple processes extending from each cell the germinal cells are large round or oval and first make their appearance between the ependymal cells on the sides of the central canal they increase rapidly in number so that by the fourth week they form an almost continuous layer on each side of the tube no germinal cells are found in the roof or floor plates the roof plate retains in certain regions of the brain its epithelial character elsewhere and its cells become spongioblasts by subdivision the germinal cells give rise to the neuroblast or young nerve cells which migrate outward from the sides of the central canal into the mantle layer and neurocrest and at the same time become pear shaped the tapering part of the cell undergoes still further elongation and forms the axis cylinder of the cell the lateral walls of the medulla spinalis continue to increase in thickness and the canal widens out near its dorsal extremity and assumes a somewhat lozen shaped appearance the widest part of the canal serves to subdivide the lateral wall of the neural tube into a dorsal or alar and eventual or basal lamina a subdivision which extends forward into the brain at a later stage the ventral part of the canal widens out while the dorsal part is first reduced to a mere slit and then becomes obliterated by the approximation infusion of its walls the ventral part of the canal persists and forms the central canal of the adult medulla spinalis the caudal end of the canal exhibits a conical expansion which is known as the terminal ventricle the ventral part of the mantle layer becomes thickened and on cross section appears as a triangular patch between the marginal and appendable layers this thickening is the rudiment of the anterior column of gray substance and contains many neuroblasts the axis cylinders of which pass out through the marginal layer and form the anterior roots of the spinal nerves the thickening of the mantle layer gradually extends in a dorsal direction and forms the posterior column of gray substance the axons of many of the neuroblast and the alar lamina run forward and cross in the floor plate to the opposite side of the medulla spinalis these form the rudiment of the anterior white commissure about the end of the fourth week nerve fibers begin to appear in the marginal layer the first to develop are the short intersegmental fibers from the neuroblast and the mantle zone and the fibers of the dorsal nerve roots which grow into the medulla spinalis from the cells of the spinal ganglia by the sixth week these dorsal root fibers form a well-defined oval bundle in the peripheral part of the alar lamina this bundle gradually increases in size and spreading towards the middle line form the rudiment of the posterior finiculus the long intersegmental fibers begin to appear about the third month and the cerebral spinal fibers about the fifth month all nerve fibers are at first destitute of medullary seeds different groups of fibers receive their seeds at different times the dorsal and ventral nerve roots about the fifth month and the cerebral spinal fibers after the ninth month by the growth of the anterior columns of gray substance in size of the anterior finiculi a fur was formed between the lateral halves of the cord anteriorly this gradually deepens to form the anterior median fissure the motor formation of the posterior septum is somewhat uncertain many believe that it is produced by the growing together of the walls of the posterior part of the central canal and by the development from its appendable cells of a septum of fibrillated tissue which separates the future finicula gracilis up to the third month of fetal life the medulla spinalis occupies the entire length of the vertebral canal and the spinal nerves pass outward at right angles to the medulla spinalis from this time onward the vertebral column grows more rapidly than the medulla spinalis and the latter being fixed above through its continuity with the brain gradually assumes a higher position within the canal by the sixth month its lower end reaches only as far as the upper end of the sacrum at birth it is on a level with the third lumbar vertebra and in the adult with the lower border of the first or upper border of the second lumbar vertebra a delicate filament the phylum terminale extends from its lower end as far as the coccyx the spinal nerves each spinal nerve is attached to the medulla spinalis an anterior or ventral and a posterior or dorsal root the fibers of the anterior roots are formed by the axons of the neuroblast which lie in the ventral part of the mantel layer these axons grow out through the overlying marginal layer and become grouped to form the anterior nerve root the fibers of the posterior roots are developed from the cells of the spinal ganglia before the neural groove is closed to form the neural tube a ridge of ectodermal cells that form the neural crest appears along the prominent margin of each neural fold when the folds meet in the middle line the two ganglion ridges fuse and form a wedge shaped area along the line of closure of the tube the cells of this area proliferate rapidly opposite the primitive segments and then migrate in a lateral and ventral direction to the sides of the neural tube where they ultimately form a series of oval shaped masses the future spinal ganglia these ganglia are arranged symmetrically on the two sides of the neural tube and, except in a region of the tail are equal in number to the primitive segments the cells of the ganglia like the cells of the mantel layer are of two kinds that is spongia blasts and neuroblasts the spongia blasts develop into the neuroglial cells of the ganglia the neuroblasts are at first round or oval in shape but soon assume the form of spindles are extremities of which gradually elongate into central and peripheral processes the central processes grow medial word and becoming connected with the neural tube constitute the fibers of the posterior nerve roots while the peripheral processes grow lateral words to mingle with the fibers of the anterior root in the spinal nerve as development proceeds the original bipolar form of the cells changes the two processes become approximated until they ultimately arise in a single stem in a T-shaped manner only in the ganglia of the acoustic nerve is the bipolar form retained more recent observers hold however that the T-form is derived from the branching of a single process which grows out from the cell the anterior or ventral and the posterior or dorsal nerve roots join immediately beyond the spinal ganglia into form the spinal nerve which divides into anterior, posterior and visceral divisions the anterior and posterior divisions proceed directly to the areas of distribution without further association with ganglion cells the visceral distributions are distributed to the thoracic, abdominal and pelvic viscera to reach which they pass through the sympathetic trunk and many of the fibers form arborizations around the ganglion cells of this trunk visceral branches are not given from all the spinal nerves they form two groups that is thoracolumbar from the first or second thoracic to the second or third lumbar nerves and pelvic from the second and third or third and fourth sacral nerves the brain the brain is developed from the anterior end of the neural tube which at an early period becomes expanded into three vesicles the primary cerebral vesicles these are marked off from each other by intervening constrictions and are named the forebrain or prosencephalon the midbrain or mesencephalon and the hindbrain or rhombencephalon the last being continuous with the medulla spinalis as the result of unequal growth of these different parts three flexures are formed and the hemorrhoic brain becomes bent on itself in a somewhat zigzag fashion the two earliest flexures are concave ventrally and are associated with corresponding flexures of the whole head the first flexure appears in the region of the midbrain and is named the ventral cephalic flexure by means of it the forebrain is bent in a ventral direction around the anterior end of the notochord and foregut with the result that the floor of the forebrain comes to lie almost parallel with that of the hindbrain this flexure causes the midbrain to become for a time the most prominent part of the brain since its doorful surface responds with the convexity of the curve the second bend appears at the junction of the hindbrain and medulla spinalis this is termed the cervical flexure and increases from the third to the end of the fifth week when the hindbrain forms nearly a right angle with the medulla spinalis after the fifth week erection of the head takes place and a cervical flexure diminishes and disappears the third bend is named the pontine flexure because it is found in the region of the future pons varioli it differs from the other two in that A its convexity is forward and B it does not affect the head the lateral walls of the brain tube like those of the medulla spinalis are divided by internal furrows into alar or dorsal and basal or ventral laminate the hindbrain or ramencephalon the cavity of the hindbrain becomes the fourth ventricle at the time when the ventral cephalic flexure makes its appearance the length of the hindbrain exceeds the combined length of the other two vesicles immediately have behind the midbrain it exhibits a marked constriction the isthmus rombincephali which is best seen when the brain is viewed from the dorsal aspect from the isthmus the anterior medullary vellum and the superior peduncle of the cerebellum are formed it is customary to divide the rest of the hindbrain that is an upper called the metencephalon and a lower the myelincephalon the cerebellum is developed by a thickening of the roof and the ponds by a thickening of the floor and lateral walls of the metencephalon the floor and lateral walls of the myelincephalon are thickened to form the medulla oblongata its roof remains thin and retaining to a great extent its epithelial nature is expanded in a lateral direction later later by the growth and backward extension of the cerebellum the roof is folded inward towards the cavity of the fourth ventricle it assists in completing the dorsal wall of this cavity and is also invaginated to form the ependymal covering of its chloride plexuses above it is continuous with the posterior medullary vellum below with the obex and lingulae the development of the medulla oblongata resembles that of the medulla spinalis but at the same time exhibits one or two interesting modifications on transverse section the myelincephalon at the early stage is seen to consist of two lateral walls connected across the midline by floor and roof plates each lateral wall consists of an alar and a basal lamina separated by an internal furrow the remains of which are represented in the adult brain by the sulcus limitans and the rhomboid fossa the contained cavity is more or less triangular in outline the base being formed by the roof plate which is thin and greatly expanded transversely pear shaped neuroblast are developed in the alar and basal lamina and their narrow stalks are elongated to form the axis cylinder of the nerve fibers opposite the furrow or boundary between the alar and basal lamina a bundle of nerve fibers attaches itself to the outer surface of the alar lamina this is named the tractus solitarius and is formed by the century fibers of the glasopharyngeal and vagus nerves it is the homolog of the oval bundle seen in the bedelus spinalis and like it is developed by an ingrowth of fibers from the ganglia of the neural crest at first it is applied to the outer surface of the alar lamina but it soon becomes buried owing to the growth over it of the neighboring parts by the fifth week the dorsal part of the alar lamina bends in lateral direction along its entire length to form what is termed the rhombic lip within a few days this lip becomes applied to and unites with the outer surface of the main part of the alar lamina and so covers in the tractus solitarius and also the spinal root of the trigeminal nerve the nodulus and focus of the cerebellum are developed from the rhombic lip neuroblast accumulate in the mantle layer those in the basal lamina correspond with the cells in the anterior gray column of the medulla spinalis and like them give origin to motor nerve fibers in the medulla oblongata they are however arranged in groups of nuclei instead of forming a continuous column from the alar lamina and its rhombic lip neuroblast migrate into the basal lamina and become aggregated to form the alivary nuclei while many send their axis cylinders through the floor plate to the opposite side and thus constitute the rudiment of the raffae of the medulla oblongata by means of this thickening of the ventral portion the motor nuclei are buried deeply in the interior and in the adult are found close to the rhomboid fossa this is still further accentuated A. by the development of the pyramids which are formed about the fourth month by the downward growth of them over fibers from the cerebral cortex and B. by the fibers which pass to and from the cerebellum on the rhomboid fossa a series of six temporary furrows appear these are termed the rhombic grooves they bear a definite relationship to certain of the cranial nerves thus from before backward the first and second grooves overlaid the nucleus of the trigeminal the third the nucleus of the facial the fourth that of the abducent the fifth that of the grasso farangile and the sixth that of the vagus the pons are developed from the ventral lateral wall and cephalon by a process similar to that which has been described for the medulla oblongata the cerebellum is developed in the roof of the anterior part of the hindbrain the alar lamina of this region become thick and to form two lateral plates which soon fuse in the middle line and produce a thick lamina which roofs the upper part of the cavity of the hindbrain vesicle this constitutes the rudiment of the cerebellum the outer sursa of which is originally smooth and convex the fissures of the cerebellum appear first in the vermis and floccular region and traces of them are found during the third month the fissures on the cerebellum hemispheres do not appear until the fifth month the primitive fissures are not developed in the order of their relative size in the adult thus the horizontal sulcus in the fifth month is merely a shallow groove the best marked of the early fissures are A. the fissure opryma between the developing Coleman and Declave and B. the fissure sucunda between the future pyramid and uvula the flocculus and nodular develop from the rhombic lip and are therefore recognizable as separate portions for any of the other cerebellum lobules the groove produced by the bending over the rhombic lip is here known as the floccula fissure when the two lateral walls fuse the right and left floccular fissures join in the middle line and their central part becomes the post-nodular fissure on the ventricular surface of the cerebellum lamina a transverse furrow the incisera festigii appears and deepens to form the tent-like recess of the roof of the fourth ventricle the rudiment of the cerebellum at first projects in a dorsal direction but by the backward growth of the cerebrum it is folded downward and somewhat flattened and the thin roof plate of the fourth ventricle originally continuous with the posterior border of the cerebellum ejected inward towards the cavity of the ventricle end of section three section four of gray's anatomy part four this is LibriVox recording all LibriVox recordings are in the public domain for more information or to volunteer please visit LibriVox.org recording by ML Cohen anatomy of the human body part four by Henry Gray development of the nervous system part two the midbrain or mesencephalon the midbrain exists for a time as a thinwalled cavity of some size and is separated from the isthmus-romin cephalide behind and from the four brain in front by slight constrictions its cavity becomes relatively reduced in diameter and forms the cerebral aqueduct of the adult brain it spays a laminate increase in thickness to form the cerebral peduncles which are at first of small size but rapidly enlarge after the fourth month the neuroblast of these laminate are grouped in relation to the sides and floor of the cerebral aqueduct and constitute the nuclei of the oculomotor and trochlear nerves and of the mesencephalic root of the trigeminal nerve by a similarly thickening process its alar laminate are developed into the quadrogeminal laminate the dorsal part of the wall for a time undergoes expansion and presents an internal median furrow and a corresponding external ridge these however disappear and the latter is replaced by a groove subsequently two oblique furrows extend medial word and backward and the thickened laminate is thus subdivided into the superior and inferior caliculi the four brain or prosencephalon a transverse section of the early four brain shows the same parts are displayed in similar sections of the medulla spinalis and medulla oblongata that is a pair of thick lateral walls connected by thin floor and roof plates moreover each lateral wall exhibits a division into dorsal or alar an eventual or basal laminate separated internally by a furrow termed the sulcus of monro this sulcus ends anteriorly at the medial end of the optic stalk then the adult brain is retained as a slight groove extending backward from the interventricular foramen to the cerebral aqueduct at a very early period in some animals before the closure of the cranial part of the neural tube two lateral diverticular the optic vesicles appear one on either side of the four brain for a time they communicate with the cavity of the four brain by relatively wide openings the peripheral parts of the vesicles expand while the proximal parts are reduced to tubular stalks the optic stalks the optic vesicles give rise to the retina and the epithelium on the back of the ciliary body and iris the optic stalk is invaded by nerve fibers to form the optic nerve the four brain then grows forward and from the alar lamina of this front portion the cerebral hemispheres originate as diverticula which rapidly expand to form two large pouches one on either side the cavities of these diverticula are rudiments of the lateral ventricles they communicate with the median part of the four brain cavity by relatively wide openings which ultimately form the interventricular foramen the median portion of the wall of the four brain vesicle consists of a thin lamina the lamina terminalis which stretches from the interventricular foramen to the recess at the base of the optic stalk the anterior part of the four brain including the rudiments of the cerebral hemispheres is named a telencephalon and its posterior portion is termed the diencephalon both of these contributes to the formation of the third ventricle the diencephalon from the alar lamina of the diencephalon the thalamus, metathalamus, and epithalamus are developed the thalamus arises as a thickening which involves the anterior two-thirds of the alar lamina the two thalami are visible for a time on the surface of the brain but are subsequently hidden by the cerebral hemispheres which grow backward over them the thalami extend medial word and gradually narrow the cavity between them into a slit-like aperture which forms the greater part of the third ventricle their medial surfaces ultimately adhere in part to each other and their intermediate mass of the ventricle is developed across the area of contact the metathalamus comprises the the lateral geniculate bodies which originate as slight outward bulging of the alar lamina in the adult the lateral geniculate body appears as an eminence on the lateral part of the posterior end of the thalamus while the medial is situated on the lateral aspect of the midbrain the epithalamus includes the pineal body the posterior commissure and the trigonhebenula the pineal body arises as an upward evagination of root plate immediately in front of the midbrain this evagination becomes solid with the exception of its proximal part which persists as the recessus pinealialis in lizards the pineal evagination is elongated into a stalk and its peripheral extremity is expanded into a vesicle in which a rudimentary lens and retina are formed the stalk becomes solid and nerve fibers make their appearance in it so that in these animals the pineal body forms a rudimentary eye the posterior commissure is formed by the encroth of fibers into the depression behind and below the pineal evagination and the trigonhebenula is developed in front of the pineal recess from the basal laminate of the dysencephalon the pars mammilaris hypothalamia is developed this comprises the corpore mammilaria and the posterior part of the tuberosinirum the corpore mammilaria arises as a single thickening which comes divided into two by a median furrow during the third month the roof plate of the diencephalon in front of the pineal body remains thin and epithelion character and is subsequently invaginated by the quarry plexus of the third ventricle the telencephalon this consists of a median portion and two lateral diverticula the median portion forms the anterior part of the cavity of the third ventricle and is closed below and in front by the lamina terminalis the lateral diverticula consists of outward pouchings of the alar laminate the cavities represent the lateral ventricles and their walls become thickened to form the nervous matter of the cerebral hemispheres the roof plate of the telencephalon remains thin and is continuous in front with the lamina terminalis and behind with the roof plate of the diencephalon in the basal laminate the car's optica hypothalamii is developed this comprises the anterior part of the tuberosinearum the infundibulum and the posterior lobe the hypothesis and the optic chiasma the anterior part of the tuberosinearum is developed from the posterior part of the floor of the telencephalon the infundibulum and posterior lobe of the hypothesis arises as a downward diverticulum from the floor the diverticulum becomes solid and forms the posterior lobe of the hypothesis the anterior lobe of the hypothesis is developed from a diverticulum of the ectodermal lining of the stomadium the optic chiasma is formed by the meeting and partial decassation of the optic nerves which subsequently grow backward as the optic tracks and end in the diencephalon the cerebral hemispheres arise as diverticula of the alar lamina of the telencephalon they increase rapidly in size and ultimately overlap the structures developed from the mid and hind brains this great expansion of the hemispheres is a characteristic feature of the brains of mammals and attains its maximum development in the brain of man Elliott Smith divides each cerebral hemispheres into three fundamental parts that is the rhinencephalon the corpus striatum and the neopallium the rhinencephalon the rhinencephalon represents the oldest part of the telencephalon and forms almost the whole of the hemisphere in fishes, amphibians, and reptiles in man it is feebly developed in comparison with the rest of the hemisphere and comprises the following parts that is the olfactory lobe consisting of the olfactory tractan bulb and trigonum olifactorium the anterior perforated substance the septum pelucidum the subcolosal supercolosal the dente gyri the fornix, the hippocampus, and the uncus the rhinencephalon appears as longitudinal elevation with a corresponding internal furrow on the undersurface of the hemisphere close to the lamina terminalis it is separated from the lateral surface of the hemisphere by a furrow the external rhinal fissure and is continuous behind with that part of the hemisphere which will ultimately form the anterior end of the temporal lobe separated by a groove into an anterior and a posterior part the anterior grows forward as a hollow stalk, the lumen of which is continuous with the anterior part of the ventricular cavity during the third month the stalk becomes solid and forms the rudiment of the olfactory bulb and tract a strand of gelatinous tissue in the interior of the bulb indicates the position of the original cavity from the posterior part the anterior perforated substance will form lobe or developed at the beginning of the fourth month the latter forms a curved elevation continuous behind with the medial surface of the temporal lobe and consisting from before backwards of the gyrus olifactorius lateralis gyrus ambiens and gyrus semilunaris parts which in the adult brain are represented by the lateral root to the olifactory tract and the uncus the position and connections of the remaining portions of the rhinencephalon are described with the anatomy of the brain the corpus traetum appears in the fourth week as a triangular thickening of the floor of the telencephalon between the optic recess and the interventricular foramen and continues behind with the thalamic part of the diencephalon it increases in size and by the second month is seen as a swelling in the floor of the fertia lateral ventricle this swelling reaches as far as the posterior end of the primitive hemisphere part of the hemisphere grows backward and downward to form the temporal lobe the posterior part of the corpus traetum is carried into the roof of the inferior horn of the ventricle where it is seen as a tail of the caudate nucleus in the adult brain during the fourth and fifth month the corpus traetum becomes incompletely subdivided by the fibers of the internal capsule into two masses an inner, the caudate nucleus and an outer, the lentiform nucleus in front the corpus traetum is continuous with the anterior perforated substance laterally it is confluent for a time with that portion of the wall of the vesicle which is developed into the insula but this continuity is subsequently interrupted by the fibers of the external capsule the neopallium forms the remaining and by far the greater part of the cerebral hemisphere it consists as an early stage of a relatively large more or less hemispherical cavity the primitive lateral ventricle enclosed by a thin wall from which the cortex of the hemisphere is developed the vesicle expands in all directions but more especially upward and backward so that by the third month the hemispheres cover the diencephalon by the sixth they overlap the midbrain and by the eighth the hindbrain the median lamina uniting the two hemispheres does not share in their expansion and thus the hemispheres are separated by a deep cleft the forerunner of the longitudinal fissure and this cleft is occupied by a septum of mesodermal tissue which constitutes the primitive falc cerebrae coincidentally with the expansion of the vesicle its cavity is drawn out into three prolongations which represent the horn of the future lateral ventricle the hindarend of the vesicle is carried downward and forward and forms the inferior horn the posterior horn is produced somewhat later in associated with the backward growth of the occipital lobe of the hemisphere the roof plate of the primitive the primitive forebrain remains thin and of an epithelial character it is invaginated into the lateral ventricle along the median wall of the hemisphere this invagination constitutes the coroidal fissure and extends from the interventricular foramen to the posterior end of the vesicle mesodermal tissue continuous to that of the primitive falc cerebrae and carrying blood vessels with it spreads between the two layers of the invaginated fold and forms the rudiment of the tila coroidia the margins of the tila become highly vascular and form the coroid plexuses which for some months almost completely fill the ventricular cavities the tila at the same time invaginates the epithelial roof of the diencephalon to form the coroid plexuses of the third ventricle by the downward and forward growth of the posterior end of the vesicle to form the temporal lobe the coroidal fissure finally reaches the interventricular foramen to the extremity of the inferior horn of the ventricle parallel width but above and in front of the coroidal fissure the median wall of the cerebral vesicle becomes folded outward and gives rise to the hippocampal fissure on the median surface and to a corresponding elevation the hippocampus with the interventricular cavity the gray or ganglionic covering of the wall of the vesicle ends at the inferior margin of the fissure with this the marginal reticular layer future white substance is exposed and its lower thinned edge is continuous with the epithelial invagination covering the coroid plexus as a result of the later downward and forward growth of the temporal lobe the hippocampal fissure and the parts associated with it extend from the interventricular foramen to the end of the inferior horn of the ventricle the thickened edge of gray substance becomes the gyros dentatus the fasciola sinaria and the supra and sub-colosal gyri while the free edge of the white substance forms the fimbria hippocampi and the body and cruise of the fornix the corpus callosum is developed within the arch of the hippocampal fissure and the upper part of the fissure forms in the adult brain the colossal fissure on the medial surface of the hemisphere the commissures the development of the posterior commissure has already been referred to the gray commissures of the hemisphere that is the corpus callosum the fornix and the anterior commissures arise from the lamina terminalis about the fourth month a small thickening appears in this lamina immediately in front of the interventricular foramen the lower part of the thickening is soon constricted off and fibers appear in it to form the anterior commissure the upper part continues to grow with the hemispheres and is evaded by two sets of fibers transverse fibers extending between the hemispheres pass into its dorsal part which is now differentiated as the corpus callosum in rare cases the corpus callosum is not developed into the ventral part longitudinal fibers from the hippocampus pass to the lamina terminalis and through that structure to the corporeal mammalaria these fibers constitute the fornix a small portion lying anterior inferiorly between the corpus callosum and the fornix is not invaded by the commissural fibers it remains thin and later a cavity the cavity of the septum callosum forms in its interior fissures and sulci the outer surface of the cerebral hemispheres is at first smooth but later it exhibits a number of elevations or convolutions separated from each other by fissures and sulci most of which make their appearance during the sixth or seventh month of fetal life the term fissure is applied to such grooves as involve the entire thickness of the cerebral wall and thus produce corresponding eminences in the ventricular cavity while the sulci affect only the superficial part of the wall and therefore leave no impression in the ventricle the fissures comprise the caroidal and hippocampal already described and two others that is the calcorine and collateral which produce the swellings known respectively as the calcar avis and the collateral eminence in the ventricular cavity of the sulci the following may be referred to that is the central sulcus fissure of Rolando which is developed in two parts the intraparital sulcus in four parts and the cingulate sulcus in two or three parts the lateral cerebral or sylvian fissures differs from all the other fissures in its motor development it appears about the third month as a depression the sylvian fossa on the lateral surface of the hemisphere this fossa corresponds with the position of the corpus striatum and its floor is molded to form the insula the intimate connection which exists between the cortex of the insula and the subjacent corpus striatum prevents this part of the hemisphere wall from expanding at the same rate as the portions which surround it the neighboring parts of the hemisphere therefore gradually grow over and cover in the insula and constitute the temporal, parietal, frontal and orbital or curricula of the adult brain the frontal and orbital or curricula are the last to form but by the end of the first year after birth the insula is completely submerged by the approximation of the opercula the fissures separating the opposed margins of the opercula constitute the composite lateral cerebral fissure if a section across the wall of the hemisphere about the sixth wreath be examined microscopically it will be found to consist of a thin marginal or reticular layer a thick ependymal layer and a thin intervening mantle layer neuroblasts from the ependymal and mantle layers migrate into the deep part of the marginal layer and form the cells of the cerebral cortex the nerve fibers which form the underlying white substance of the hemispheres consist at first of outgrowth from the cells of the corpore striata and the thalamide later the fibers from the cells of the cortex are added medulation of these fibers begins about the time of birth and is continuous until puberty a summary of these parts derived from the brain vesicles is given in the following table the hindbrain or ramencephalon give rise to the myelincephalon medencephalon and isthmus ramencephali the myelincephalon gives rise to the medulla oblongata and lower part of the fourth ventricle the medencephalon to the pond cerebellum and intermediate part of the fourth ventricle and the isthmus ramencephali the anterior medullary vellum brachium congenitivus cerebelli and upper part of the fourth ventricle the midbrain or mesencephalon gives rise to the cerebral peduncles lamina quadragemina and cerebral aqueduct the forebrain or prosencephalon gives rise to the diencephalon and telencephalon the diencephalon gives rise to the thalamus metathalamus epithalamus pars mammillaris hypothalamii and the posterior part of the third ventricle the telencephalon gives rise to the anterior part of the third ventricle pars optical hypothalamii cerebral hemispheres lateral ventricles and interventricular foramina the cranial nerves with the exception of the olofactory optic and acoustic nerves which will be especially considered developed in a similar manner to the spinal nerves the sensory or afferent nerves are derived from the cells of the ganglion rudiments of the neural crest the central processes of these cells grow into the brain and form the roots of the nerves while the peripheral processes extend outward and constitute their fibers of distribution it has been seen in considering the development of the medulla oblongata that detractus solitarius derived from the fibers which grow inward of the ganglion rudiments of the glossopharyngeal and vagus nerves is the homologue of the oval bundle in the cord which had its origin in the posterior nerve roots the motor or afferent nerves arises out gross of the neuroblast situated in the basal lamina of the mid and hind brain while however the spinal motor nerve roots arise in one series from the basal lamina the cranial motor nerves are grouped into two sets according as they spring from the medial or lateral parts of the basal lamina to the former set belong the ocular motor trochlear, abducent and hypoglossal nerves to the latter the accessory and motor fibers of the trigeminal facial glossopharyngeal vagus nerves end of section 4 section 5 of gray's anatomy part 4 this is the LibriVox recording all LibriVox recordings are in the public domain for more information or to volunteer please visit LibriVox.org recording by ML Cohen anatomy to human body part 4 by Henry Gray the spinal cord part 1 the medulla spinalis or spinal cord the medulla spinalis the spinal cord forms the elongated nearly cylindrical part of the central nervous system which occupies the upper two thirds of the vertebral canal its average length in the male is about 45 cm in the female from 42 to 43 cm while its weight amounts to about 30 g it extends from the level of the upper border of the atlas to that of the lower border of the first or upper border of the second lumbar vertebra above it is continuous with the brain below it ends in a conical extremity the conus medularis from the apex of which a delicate filament the phylum terminali descends as far as the first segment of the coccyx the position of the medulla spinalis varies with the movements of the vertebral column its lower extremity being drawn slightly upward when the column is flexed it also varies at different periods of life up to the third month of fetal life the medulla spinalis is as long as the vertebral canal but from this stage onward the vertebral column elongates more rapidly than the medulla spinalis so that by the end of the fifth month the medulla spinalis terminates at the base of the sacrum of the vertebra the medulla spinalis does not fill the part of the vertebral canal in which it lies it is in sheath by three protective membranes separated from each other by two concentric spaces the three membranes are named from without inward the duramodder the arachnoid and napiamodder the duramodder is a strong fibrous membrane which forms a wide tubular sheath this sheath extends below the termination of the medulla spinalis so that it would point cul-de-sac at the level of the lower border of the second sacral vertebra the duramodder is separated from the wall of the vertebral canal by the epidural cavity which contains a quantity of loose areolar tissue and a plexus of veins between the duramodder and the subjacent arachnoid is a capillary interval the subdural cavity which contains a small quantity of fluid probably of the nature of length the arachnoid is a thin transparent sheath of the duramodder by a comparatively wide interval the subracnoid cavity which is filled with cerebral spinal fluid the apiamodder closely invests the medulla spinalis and sends delicate septae into its substance a narrow band the ligamentum denticulatum extends along each of the lateral surfaces and is attached by a series of pointed processes to the inner surfaces of the duramodder 31 pairs of spinal nerve spring consist of a pair of medulla spinalis each nerve having an anterior or ventral and a posterior or dorsal root the latter being distinguished by the presence of an oval swelling the spinal ganglion which contains numerous nerve cells each root consists of several bundles of nerve fibers and at its attachment extends for some distance along the side of the medulla spinalis the pairs of spinal nerves are grouped as follows thoracic 12 lumbar 5 sacral 5 and for convenience of description the medulla spinalis is divided into cervical, thoracic, lumbar and sacral regions corresponding with the attachments of the different groups of nerves although no trace of transverse segmentation is visible on the surface of the medulla spinalis it is convenient to regard it as being built up in a series of superimposed spinal segments or neuromers each of which has a length equivalent to the extent of attachment of a pair of spinal nerves since the extent of attachment of the successive pairs of nerves varies in different parts it follows that the spinal segments are of varying lengths thus in the cervical region they average about 13 millimeters in the mid thoracic region about 26 millimeters while in the lumbar and sacral regions they diminish rapidly from about 15 millimeters at the level of the first pair of lumbar nerves to about 4 millimeters opposite the attachment of the lower sacral nerves as a consequence of the relevant inequality in the rates of growth of the medulla spinalis and vertebra column the nerve roots, which in the early embryo passed transversely outward to reach their respective inter vertebra foramen become more and more oblique in direction from above downward so that the lumbar and sacral nerves descend almost vertically to reach their points of exit from the appearance these nerves present at the attachment to the medulla spinalis and from their great length they are collectively termed the cauda equina the phylum terminale is the delicate filament about 20 centimeters in length prolonged downward from the apex of the conus medularis it consists of two parts an upper and a lower the upper part or phylum terminale internum measures about 15 centimeters in length and reaches as far as the lower border of the second sacral vertebra it is contained within the tubular sheath of dura mater and is surrounded by the nerve forming the cauda equina from which it can be readily recognized by its bluish-white color the lower part or phylum terminale externum is closely invested by and is adhered to the dura mater it extends downward from the apex of the tubular sheath and is attached to the back of the first segment of the coccyx the phylum terminale consists mainly of fibrous tissue continuous above with that of the pia mater adhering to its outer surface however are a few strands of nerve fibers which probably represent rudimentary second and third coccygeal nerves further the central canal of the medulla spinalis extends downward into it for five or six centimeters enlargements the medulla spinalis is not quite cylindrical being slightly flattened from before backward it presents two swellings or enlargements an upper or cervical and a lower or lumbar the cervical enlargement is the more pronounced and corresponds with the attachment of the large nerves which supply the upper limbs it extends from about the third cervical to the second thoracic vertebra it's maximal circumference about 38 millimeters being on a level with the attachment of the sixth pair of cervical nerves the lumbar enlargement gives attachment to the nerves which supply the lower limbs it commences about the level of the ninth vertebra and reaches its maximal circumference of about 33 millimeters opposite the last thoracic vertebra below which it tapers rapidly into the conus medularis fissures and sulci an anterior median fissure in a posterior median sulcus incompletely divide the medulla spinalis into two symmetrical parts which are joined across the midline by a commissural band of nervous matter the anterior median fissure fissure mediana anterior has an average depth of about 3 millimeters but this is increased in the lower part of the medulla spinalis contains a double fold of pia mater and its floor is formed by a transverse band of white substance the anterior white commissure which is perforated by blood vessels on their way to or from the central part of the medulla spinalis the posterior median sulcus sulcus medianus posterior is very shallow the septum of neuroglia reaches rather more than halfway into the substance of the medulla spinalis this septum varies in depth from 4 to 6 millimeters but diminishes considerably in the lower part of the medulla spinalis on either side of the posterior median sulcus and at a short distance from it the posterior nerve roots are attached along a vertical furrow named the posterior lateral sulcus the portion of the medulla spinalis which lies between this sulcus is named the posterior finiculus in the cervical and upper thoracic regions this finiculus presents a longitudinal furrow the posterior intermediate sulcus this marks the position of a septum which extends into the posterior finiculus and subdivides it into two fasciculi a medial named the fasciculus gracilis tract of gall and a lateral the fasciculus cuneatus tract of burdach the portion of the medulla spinalis which lies in front of the posterior lateral sulcus is termed the anterior lateral region the anterior nerve roots unlike the posterior are not attached in linear series and their position of exit is not marked by a sulcus they arise by separate bundles which spring from the anterior column of the gray substance and passing forward through the white substance emerge over an area of some slight width the most lateral of these bundles is generally taken as a dividing line which separates the anterior lateral region into two parts that is an anterior finiculus between the anterior median fissure and the most lateral of the anterior nerve roots and a lateral finiculus between the exit of these roots and the posterior lateral sulcus in the upper part of the cervical region a series of nerve roots passes outward through the lateral finiculus of the medulla spinalis these unite to form the spinal portion of the accessory nerve which runs upward through the upper cavity through the pharynemum magnum the internal structure of the medulla spinalis on examining a transverse section of the medulla spinalis it is seen to consist of gray and white nervous substance the form are being enclosed within the latter gray substance substancia grigio centralis the gray substance consists of two symmetrical portions one in each half of the medulla spinalis these are joined across the middle line of the transverse commissure of gray substance through which runs a minute canal the central canal just visible to the naked eye in the transverse section each half of the gray substance is shaped like a comma or crescent the concavity of which is directed laterally and these together with the intervening gray commissure present the appearance of the letter H an imaginary coronal playing through the central canal serves to divide each crescent into an anterior or ventral posterior or dorsal column the anterior column column anterior anterior corneau directed forward is broad end of a rounded or quadrangular shape its posterior part is termed the base and its anterior part the head but these are not differentiated from each other by any well-defined constriction it is separated from the surface of the medulla spinalis by a layer of white substance which is traversed by the bundles of the anterior nerve roots in the thoracic region the posterior lateral part of the anterior column projects lateral words as a triangular field which is named the lateral column column the lateral corneau the posterior column column the posterior posterior corneau is long and slender and is directed backwards and lateral words it reaches almost as far as the posterior lateral sulcus from which it is separated by a thin layer of white substance of lissauer it consists of a base directly continuous with the base of the anterior horn and a neck or slightly constricted portion which is succeeded by an oval or fusiform area termed the head of which the apex approaches the posterior lateral sulcus the apex is capped by a V-shape or crescentic mass of translucent gelatinous neuroglia termed the substantial gelatinosa of Rolando which contains both neuroglial cells and small nerve cells between the anterior and posterior columns the gray substance extends as a series of processes into the lateral finiculus to form a network called the formatio reticularis the quantity of gray substance as well as the form which it presents on transverse section varies markedly at different levels in the thoracic region it is small not only in amount but relatively to the surrounding white substance in the cervical and laminar enlargements it is greatly increased in the latter and especially in the conus medularis its proportion to the white substance is greatest in the cervical region its posterior column is comparatively narrow while its anterior is broad and expanded in the thoracic region both columns are attenuated and the lateral column is evident in the conus medularis the gray substance assumes the form of two oval masses one in each half of the cord connected together by a broad gray commissure the central canal canalis centralis runs throughout the entire length of medulla spinalis the portion of gray substance in front of the canal is named the anterior gray commissure that behind it the posterior gray commissure the form is thin and is in contact anteriorly with the anterior white commissure it contains a couple of longitudinal veins one on either side of the middle line the posterior gray commissure it reaches from the central canal to the posterior median septum and is thinnest in the thoracic region and thickest in the conus medularis the central canal is continued upward through the lower part of the medulla oblongata and opens into the fourth ventricle of the brain below it reaches for a short distance into the phylum terminale in the lower part of the conus medularis it exhibits a fusiform dilatation the terminal ventricle this has a vertical measurement of from 8 to 10 millimeters triangular on cross-section with its base directed forward and tends to undergo obliteration after the age of 40 years throughout the cervical and thoracic regions the central canal is situated in the anterior third of the medulla spinalis in the lumbar enlargement it is near the middle and in the conus medularis it approaches the posterior surface it is filled with cerebrospinal fluid and lined by ciliated columnar epithelium outside of which there is a strand of gelatinous substance the substantial gelatinosus centralis this gelatinous substance consists mainly of neuroglia but contains a few nerve cells and fibers it is traversed by processes from the deep ends of the columnar ciliated cells which line the central canal end of section 5 section number 6 of Grey's Anatomy part 4 this is LibriVox recording all LibriVox recordings are in the public domain for more information on the volunteer please visit LibriVox.org recording by ML Cohen anatomy of the human body part 4 by Henry Gray the spinal cord part 2 structure of the gray substance the gray substance consists of numerous nerve cells and nerve fibers held together by neuroglia throughout the greater part of the gray substance the neuroglia presents the appearance of a sponge-like network but around the central canal and on the apices of the posterior columns it consists of the gelatinous substance already referred to the nerve cells are multipolar and vary greatly in size and shape they consist of 1 motor cells of large size which are situated in the anterior horn and are especially numerous in the cervical and lumbar enlargements the axons of most of these cells pass out from the anterior nerve roots but before leaving the white substance they frequently give off collaterals which re-enter and ramify in the gray substance 2 cells of small or medium size whose axons pass into the white matter where some pursue an ascending and others a descending course but most of them divide into t-shaped manner into descending and ascending processes they give off collaterals which enter and ramify into the gray substance and the terminations of the axons behave in a similar manner the lengths of these axons vary greatly some are short and pass only between the joining spinal segments while others are longer and connect more distance segments these cells and their processes constitute a series of association or intersegmental neurons which link together to different parts of the medulla spinalis the axons of most of these cells are confined to the side of the medulla spinalis in which the nerve cells are situated but some cross to the opposite side through the anterior commissures and are termed crossed commissural fibers some of these ladder end directly in the gray substance while others enter the white substance and ascend or descend in it for varying distances before finally terminating in the gray substance 3 cells of the type 2 of Golgi limited for the most part of the column are found also in the substantial gelatinosa of Rolando their axons are short and entirely confined to the gray substance in which they break up into numerous fine filaments most of the nerve cells are arranged in longitudinal columns and appear as groups on transverse section nerve cells in the anterior column the nerve cells in the anterior column are arranged in columns of varying length the longest occupies the medial part of the anterior column and is named the anterior medial column it is well marked in C4, C5 again from C8 to L4 it disappears in L5 and S1 but is well marked in S2, S3 and S4 open prens bruce closed prens C footnote topical atlas of the spinal cord 1901 and footnote behind it is a dorsal medial column of small cells which is not represented in L5, S1, S2 nor below S4 its axons probably pass into the dorsal ramia of the spinal nerves to supply the dorsal musculature of the spinal column in the cervical and lumbar enlargements where the anterior columns expanded in lateral direction the following additional columns are present that is a anterior lateral which consists of two groups one in C4, C5, C6 the other in C6, C7, C8 in the cervical enlargement and of a group from L2 to S2 in the lumbosacral enlargement B posterior lateral in the lower 5 cervical lower 4 lumbar in upper 3 sacral segments C post posterior lateral in the last cervical furth thoracic in upper 3 sacral segments and D a central in the lower 4 lumbar and upper 2 sacral segments these cell groups are evidently related to the nerve roots of the brachial and sacral plexuses and supply fibers to the muscles of the arm and leg throughout the base of the anterior column are scattered solitary cells the axons of some of which form cross commissural fibers while others constitute the motor fibers of the posterior nerve roots footnote Lenhossik and Kahal found that in the chick embryo some of these nerve cells pass backwards through the posterior column and emerged as the motor fibers of the posterior nerve roots these fibers are said to control the peristaltic movement of the intestine their presence in man has not yet been determined and footnote nerve cells in the lateral column these form a column which is best marked where the lateral gray column is differentiated that is in the thoracic region but it can be traced throughout the entire length of the medullus binalis in the form of groups of small cells which are situated in the anterior part of the formaceo reticularis in the upper part of the cervical region and lower part of the medulla oblongata as well as in the third and fourth sacral segments this column is again differentiated in the medulla it is known as the lateral nucleus the cells of this column are fusiform or star shaped and of a medium size the axons of some of them pass into the anterior nerve roots by which they are carried to the sympathetic nerve they constitute the white rami and are sympathetic or visceral efferent fibers they are also known as preganglionic fibers of the sympathetic system the axons of others pass into the anterior and lateral funiculi where they become longitudinal nerve cells in the posterior column one the dorsal nucleus nucleus dalsalis column of clark occupies the medial part of the base of the posterior column and appears on the transverse section as a well-defined oval area it begins below at the level of the second or third lumbar nerve and reaches its maximum size opposite the 12th thoracic nerve above the level of the 9th thoracic nerve its size diminishes and ends opposite the last cervical or first thoracic nerve it represented, however in the other regions by scattered cells which become aggregated to form a cervical nucleus opposite the third cervical nerve and a sacral nucleus in the middle and lower part of the sacral region its cells are a medium size and of an oval or piriform shape their axons pass into the peripheral part of the lateral funiculus of the same side probably in the dorsospinocerebellar direct cerebellar vesiculus two the nerve cells in the substantial gelatinosa of roulando are arranged in three zones a posterior or marginal of large angular or fusiform cells an intermediate of small fusiform cells and an anterior of star-shaped cells the axons of these cells pass into the lateral and posterior funiculi and they're a sum of vertical cores in the anterior zone some Golgi cells are found whose short axons ramify in the gray substance three solitary cells of varying forms and size are scattered throughout the posterior column some of these are grouped to form the posterior basal column in the base of the posterior column lateral to the dorsal nucleus the posterior basal column is well marked in the gorilla but it's ill-defined in man the axons of its cells pass partly to the posterior and lateral funiculi of the same side and partly through the anterior white commissure to the lateral funiculus of the opposite side Golgi cells type 2 located in this region send axons to the lateral and ventral columns a few star-shaped or fusiform nerve cells of varying size are found in the substantial gelatinosa centralis their axons pass into the lateral funiculus of the same or of the opposite side the nerve fibers in the gray substance form a dense interlacement of minute fibrils among the nerve cells this interlacement is formed partly of axons which pass from the cells in the gray substance to enter the white funiculi or nerve roots partly of the axons of Golgi cells which ramify only in the gray substance and partly of collateral from the nerve fibers the anterior funiculi which, as already stated enter the gray substance and ramify within it white substance substancia alba the white substance of the medulla spinalis consists of medulated nerve fibers embedded in a sponge-like network of neuroglia and is arranged in three funiculi anterior, lateral and posterior the anterior funiculus lies between the anterior median fissure or most lateral of the anterior nerve roots the lateral funiculus between these nerve roots and the posterior lateral sulcus and the posterior funiculus between the posterior lateral and the posterior median sulci the fibers vary greatly in thickness the smaller being found than fisciculus crazylis the tractive lisaur and inner part of the lateral funiculus while the largest are situated in the anterior funiculus and in the peripheral part of the lateral funiculus Some of the nerve fibers assume a more or less transverse direction, as for example those with cross from side to side in the anterior white commissure, but the majority pursue a longitudinal course and are divisible into one, those connecting the medulla spinalis with the brain and conveying impulses to or from the latter, and two, those which are confined to the medulla spinalis and link together its different segments. That is, intersegmental or association fibers, nerve fasciculi. The longitudinal fibers are grouped into more or less definite bundles or fasciculi. These are not recognizable from each other in the normal state, and their existence has been determined by the following methods. One, a wallard discovered that if a bundle of nerve fibers be cut, the portions of the fibers which are separated from their cells rapidly degenerate and become atrophied, while the cells and the parts of the fibers connected with them undergo little alteration. See footnote. Somewhat later a change term chromatologist takes place in the nerve cells and consists of a breaking down and an ultimate disappearance of the nistle bodies. Further, the body of the cell is swollen, the nucleus displaced towards the periphery, and the part of the axon still attached to the altered cell is diminished in size and somewhat atrophied. Under favorable conditions, the cell is capable of re-assuming its normal appearance and its axon may grow again. End footnote. This is known as wallaring degeneration. Similarly, if a group of nerve cells be destroyed, the fibers arising from them undergo degeneration. Thus, if the motor cells of the cerebral cortex be destroyed, or if the fibers arising from these cells be severed, a descending degeneration from the seat of injury takes place in the fibers. In the same manner, if a spinal ganglion be destroyed, or the fibers which pass from it into the medullus by now let's be cut, an ascending degeneration will extend along these fibers. Two. Pathological changes, especially in man, have given important information by causing ascending and descending degeneration. Three. By tracing the development of the nervous system, it has been observed that at first the nerve fibers are merely naked axis cylinders, and that they do not all acquire their medullary sheaves at the same time. Hence, the fibers can be grouped into different bundles according to the dates at which they receive their medullary sheaves. Four. Various methods of staining nervous tissue are of great value in tracing the course and mode of termination of the axis cylinder processes. Faciculi in the anterior funiculus. Descending fasciculi. The anterior cerebrospinal, fasciculus cerebrospinalis anterior, direct pyramidal tract, which is usually small but varies inversely in size with the lateral cerebrospinal funiculus. It lies close to the anterior median fissure and is present only in the upper part of the medullus spinalis, gradually diminishing in size as it descends. It ends about the middle of the thoracic region. It consists of descending fibers which arise from cells in the motor area of the cerebral hemisphere of the same side and which, as they run downward in the medullus spinalis, cross in succession through the anterior white commissure to the opposite side where they end either directly or indirectly by arborizing around the motor cells in the anterior column. A few of its fibers are said to pass to lateral columns of the same side and to the gray matter at the base of the posterior column. They conduct voluntary motor impulses from the precentral gyrus to the motor centers of the cord. The vestibular spinal fissiculus situated chiefly in the marginal part of the funiculus and mainly derived from the cells of Dieter's nucleus of the same and opposite side, that is, the chief terminal nucleus of the vestibular nerve. Fibers are also contributed to this fissiculus from scattered cells of the articular formation of the medulla oblongata, the pons, and the midbrain, tegmentum. The other terminal nuclei of the vestibular nerve also contributes fibers. In the brain stem, these fibers form part of the median longitudinal bundle. The fissiculus can be traced to the sacral region. Its terminals and collaterals end either directly or indirectly among the motor cells of the anterior column. This fissiculus is probably concerned with equilibratory reflexes. The tectospinal fissiculus, situated partly in the anterior and partly in the lateral funiculus, is mainly derived from the opposite superior colliculus of the midbrain. The fibers from the superior colliculus cross the median raffae in the fountain decausation of minert and descend as the ventral longitudinal bundle in the reticular formation of the brain stem. Its collaterals and terminals end either directly or indirectly among the motor cells of the anterior column of the same side. Since the superior colliculus is an important visual reflex center, the tectospinal fissiculus is probably concerned with visual reflexes. Ascending fissiculi. The ventral spinal thalamic fissiculus, situated in the marginal part of the funiculus and intermingled more or less with the vestibular spinal fissiculus, is derived from cells in the posterior column or intermediate gray matter of the opposite side. Their axons cross in the anterior commissure. This is a somewhat doubtful fissiculus and its fibers are supposed to end in the thalamus and to conduct certain of the touch impulses. The remaining fibers of the anterior funiculus constitute what is termed the anterior proper fissiculus. Fissiculus anterior proprious anterior basis bundle. It consists of a longitudinal intersegmental fibers which arise from cells in the gray substance more especially from those in the medial group of the anterior column and after a longer or shorter course re-enter the gray substance. B fibers which cross in the anterior white commissure from the gray substance of the opposite side. End of section six.