 Section 9 of Grey's Anatomy, Part 1 This is a LibriVox recording. All LibriVox recordings are in the public domain. For more information or to volunteer, please visit LibriVox.org. Recording by Morgan Scorpion. Anatomy of the Human Body, Part 1 by Henry Gray. Osteology. Introduction. Development of the skeleton. The general framework of the body is built up mainly of a series of bones, supplemented however in certain regions by pieces of cartilage. The bony part of the framework constitutes the skeleton. In the skeleton of the adult there are 206 distinct bones as follows. The axial skeleton, divided into the vertical column with 26 bones, the skull with 22 bones, the high-oed groin, 1, the ribs and sternum, 25, 74 bones in total. The appendicular skeleton, upper extremities with 64 bones, lower extremities with 62 bones, 126 altogether, and the auditory ossicles, 6. Total of 206. The patellae are included in this enumeration, but the smaller sesamoid bones are not reckoned. Bones are divisible into four classes, long, short, flat and irregular. Long bones. The long bones are found in the limbs, and each consists of a body or shaft and two extremities. The body or diaphysis is cylindrical, with a central cavity termed the medullary canal. The wall consists of dense, compact tissue of considerable thickness in the middle part of the body, but becoming thinner towards the extremities. Within the medullary canal is some cancerous tissue, scanty in the middle of the body, but greater in amount towards the ends. The extremities are generally expanded for the purpose of articulation and to afford broad surfaces for muscular attachment. They are usually developed from separate centres of ossification termed epiphyces, and consist of cancerous tissue surrounded by thin, compact bone. The medullary canal and the spaces in the cancerous tissue are filled with marrow. The long bones are not straight but curved, the curve generally taking place in two planes, thus affording greater strength to the bone. The bones belonging to this class are the clavicle, humerus, radius, ulna, femur, tibia, fibula, metacarpals, metatarsals and phalanges. Short bones. Where a part of the skeleton is intended for strength and compactness combined with limited movement, it is constructed of a number of short bones, as in the carpus and tarsus. These consist of cancerous tissue covered by a thin crust of compact substance. The patellae, together with the other sesamoid bones, are by some regarded as short bones. Flat bones. Where the principal requirement is either extensive protection or the provision of broad surfaces for muscular attachment, the bones are expanded into broad, flat plates, as in the skull and the scapula. These bones are composed of two thin layers of compact tissue enclosing between them a variable quantity of cancerous tissue. In the cranial bones, the layers of compact tissue are familiarly known as the tables of the skull. The outer one is thick and tough, the inner is thin, dense and brittle, and hence is termed the vitreous table. The intervening cancerous tissue is called the diploid, and this, in certain regions of the skull, becomes absorbed so as to leave spaces filled with air, air sinuses, between the two tables. The flat bones are the occipital, parietal, frontal, nasal, lacrimal, roma, scapula, oscocci, hip bone, sternum, ribs, and, according to some, the patellae. Irregular bones. The irregular bones are such as, from their peculiar form, cannot be grouped under the preceding heads. They consist of cancerous tissue enclosed within a thin layer of compact bone. The irregular bones are the vertebrae, sacrum, coccyx, temporal, sphenoid, ethmoid, zygomatic, maxilla, mandible, palatine, inferior nasal conca, and hyoid. Surfaces of bones. If the surface of a bone be examined, certain eminences and depressions are seen. These eminences and depressions are of two kinds, articular and non-articular. Well-marked examples of articular eminences are found in the heads of the humorous and femur, and of articular depressions in the glenoid cavity of the scapula, and the acetabulum of the hip bone. Non-articular eminences are designated according to their form. Thus, a broad, rough, uneven elevation is called a tuberosity for tuberants or process. A small, rough prominence a tubercle. A sharp, slender, pointed eminence a spine. A narrow, rough elevation running some way along the surface, a ridge, crest, or line. Non-articular depressions are also a variable form and are described as fossae, pits, depressions, grooves, furrows, fissures, notches, etc. These non-articular eminences and depressions serve to increase the extent of surface for the attachment of ligaments and muscles, and are usually well-marked in proportion to the musculapty of the subject. A short perforation is called a phoramen. A longer passage, a canal. 1. Development of the skeleton The skeleton. The skeleton is of mesodermal origin, and may be divided into A, that of the trunk, axial skeleton, comprising the vertical column, skull, ribs, and sternum, and B, that of the limbs, appendicular skeleton. 2. The vertical column The notochord is a temporary structure and forms a central axis, around which the segments of the vertical column are developed. Note 11. In the amphioxus, the notochord persists and forms the only representative of a skeleton in that animal. End of note. It is derived from the endoderm, and consists of a rod of cells, which lies on the ventral aspect of the neural tube, and reaches from the anterior end of the migraine to the extremity of the tail. On either side of it is a column of paraxial mesoderm, which becomes subdivided into a number of more or less cubical segments, the primitive segments. These are separated, from one another, by intersegmental scepter, and are arranged symmetrically on either side of the neural tube and notochord. To every segment, a spinal nerve is distributed. At first, each segment contains a central cavity, the myocoyl, but this is soon filled with a core of angular and spindle-shaped cells. The cells of the segment become differentiated into three groups, which form respectively the cutis plate or dermatome, the muscle plate or myotome, and the sclerotome. The cutis plate is placed on the lateral and dorsal aspect of the myocoyl, and from it the true skin of the corresponding segment is derived. The muscle plate is situated on the medial side of the cutis plate and furnishes the muscles of the segment. The cells of the sclerotome are largely derived from those forming the core of the myocoyl, and lie next to the notochord. Fusion of the individual sclerotomes in an antroposterior direction soon takes place, and thus a continuous strand of cells, the sclerotogenous layer, is formed along the ventrolateral aspects of the neural tube. The cells of this layer proliferate rapidly, and extending medial woods surround the notochord, at the same time they grow backward on the lateral aspects of the neural tube and eventually surround it, and thus the notochord and neural tube are enveloped by a continuous sheath of mesoderm, which is termed the membranous vertebral column. In this mesoderm the original segments are still distinguishable, but each is now differentiated into two portions, an anterior, consisting of loosely arranged cells, and a posterior, of more condensed tissue. Between the two portions the rudiment of the intervertebral fibrocartilage is laid down. Cells from the posterior mass grow into the intervals between the myotomes of the corresponding and succeeding segments, and extend both dorsally and ventrally. The dorsal extensions surround the neural tube and represent the future vertebral arch, while the ventral extend into the body wall as the costal processes. The hindre part of the posterior mass joins the anterior mass of the succeeding segment to form the vertebral body. Each vertebral body is therefore a composite of two segments, being formed from the posterior portion of one segment and the anterior part of that immediately behind it. The vertebral and costal arches are derivatives of the posterior part of the segment in front of the intersegmental septum with which they are associated. This stage is succeeded by that of the cartilaginous vertebral column. In the fourth week two cartilaginous centres make their appearance, one on either side of the notochord. These extend around the notochord and form the body of the cartilaginous vertebra. A second pair of cartilaginous foci appear in the lateral parts of the vertebral bow and grow backward on either side of the neural tube to form the cartilaginous vertebral arch, and a separate cartilaginous centre appears for each costal process. By the eighth week the cartilaginous arch has fused with the body, and in the fourth month the two halves of the arch are joined on the dorsal aspect of the neural tube. The spinous process is developed from the junction of the two halves of the vertebral arch. The transverse process grows out from the vertebral arch behind the costal process. In the upper cervical vertebrae a band of mesodermal tissue connects the ends of the vertebral arches across the ventral surfaces of the intervertebral fibrocartilages. This is termed the hypercordal bar or brace. In all except the first it is transitory and disappears by fusing with the fibrocartilages. In the atlas however the entire bow persists and undergoes quantification. It develops into the anterior arch of the bone, while the cartilage representing the body of the atlas forms the dense or odontoid process which fuses with the body of the second cervical vertebra. The portions of the notochord which are surrounded by the bodies of the vertebrae atrophy and ultimately disappear, while those which lie in the centres of the intervertebral fibrocartilages undergo enlargement and persist throughout life as the central nucleus pulposus of the fibrocartilages. The ribs. The ribs are formed from the ventral or costal processes of the primitive vertebral bowels, the process extending between the muscle plates. In the thoracic region of the vertebral column the costal processes grow lateral wood to form a series of arches, the primitive costal arches. As already described the transverse process grows out behind the vertebral end of each arch. It is at first connected to the costal process by continuous mesoderm, but this becomes differentiated later to form the costotransverse ligament. Between the costal process and the tip of the transverse process the costotransverse joint is formed by absorption. The costal process becomes separated from the vertebral bow by the development of the costocentral joint. In the cervical vertebrae the transverse process forms the posterior boundary of the forearm and transversarium, while the costal process corresponding to the head and neck of the rib fuses with the body of the vertebra and forms the antrolateral boundary of the forearm. The distal portions of the primitive costal arches remain undeveloped. Occasionally the arch of the seventh cervical vertebra undergoes greater development and by the formation of costovertebral joints is separated off as a rib. In the lumbar region the distal portions of the primitive costal arches fail. The proximal portions fuse with the transverse processes to form the transverse processes of descriptive anatomy. Occasionally a movable rib is developed in connection with the first lumbar vertebra. In the sacral region costal processes are developed only in connection with the upper three, or it may be four, vertebrae. The processes of adjacent segments fuse with one another to form the lateral part of the sacrum. The coxigill vertebrae are devoid of costal processes. The sternum. The ventral ends of the ribs become united to one another by a longitudinal bar termed the sternal plate and opposite the first seven pairs of ribs these sternal plates fuse in the middle line to form the manubrium and body of the sternum. The ziphoid process is formed by a backward extension of the sternal plates. The skull. Up to a certain age the development of the skull corresponds with that of the vertebral column, but it is modified later in association with the expansion of the brain vesicles, the formation of the organs of smell, sight and hearing, and the development of the mouth and pharynx. The notochord extends as far forward as the anterior end of the midbrain and becomes partly surrounded by mesoderm. The posterior part of this mesodermal investment corresponds with the basilar part of the occipital bone and shows a subdivision into four segments which are separated by the roots of the hypoglossal nerve. The mesoderm then extends over the brain vesicles and thus the entire brain is enclosed by a mesodermal investment which is termed the membranous cranium. From the inner layer of this the bones of the skull and the membranes of the brain are developed. From the outer layer the muscles, blood vessels, true skin and subcutaneous tissues of the scalp. In the shark and dogfish this membranous cranium undergoes complete chondrification and forms the cartilaginous skull or chondrocranium of these animals. In mammals on the other hand the process of chondrification is limited to the base of the skull, the roof and sides being covered in by membrane. Thus the bones of the base of the skull are preceded by cartilage, those of the roof and sides by membrane. The posterior part of the base of the skull is developed around the notochord and exhibits a segmented condition analogous to that of the vertical column while the anterior part arises in front of the notochord and shows no regular segmentation. The base of the skull may therefore be divided into A. a chordal or vertebral and B. a pre-chordal or pre-vertible portion. In the lower vertebrates two pairs of cartilages are developed, a pair of paracordal cartilages, one on either side of the notochord and a pair of pre-chordal cartilages, the trabeculi cranii, in front of the notochord. The paracordal cartilages unite to form a basilar plate from which the cartilaginous part of the occipital bone and the basispenoid are developed. On the lateral aspects of the paracordal cartilages the auditory vesicles are situated and the mesodermin closing them is soon converted into cartilage, forming the cartilaginous ear capsules. These cartilaginous ear capsules, which are of an oval shape, fuse with the sides of the basilar plate and from them arise the petrus and mastoid portions of the temporal bones. The trabeculi cranii are two curved bowels of cartilage which embrace the hypothesis cerebri. Their posterior ends soon unite with the basilar plate while their anterior ends join to form the ethmoidal plate, which extends forwards between the forebrain and the olfactory pits. Later the trabeculi meet and fuse below the hyperfysis, forming the flaw of the fossa hyperfisius and so cutting off the anterior lobe of the hyperfysis from the sternodium. The median part of the ethmoidal plate forms the bony and cartilaginous parts of the nasal septum. From the lateral margins of the trabeculi cranii, three processes grow out on either side. The anterior forms the ethmoidal labyrinth and the lateral and allocatilages of the nose. The middle gives rise to the small wing of the sphenoid, while from the posterior the great wing and lateral pterigoid plate of the sphenoid are developed. The bones of the vault are of membranous formation and are termed dermal or covering bones. They are partly developed on the musoderm of the membranous cranium and partly from that which lies outside the enter derm of the foregut. They comprise the upper part of the occipital squamous interperiotal, the squamous and tympanic parts of the temporals, the periotals, the frontal, the vomar, the medial pterigoid plates and the bones of the face. Some of them remain distinct throughout life, e.g. periotal and frontal, while others join with the bones of the chondrocranium, e.g. interperiotal, scromae of the temporals and medial pterigoid plates. Recent observations have shown that, in mammals, the basacranial cartilage, both in the caudal and precaudal regions of the base of the skull, is developed as a single plate which extends from behind forward. In man, however, its posterior part shows an indication of being developed from two quantifying centres which fuse rapidly in front and below. The anterior and posterior thirds of the cartilage surround the notochord, but its middle third lies on the dorsal aspect of the notochord, which in this region is placed between the cartilage and the wall of the pharynx. End of section 9 Section 10 of Grey's Anatomy Part 1 This is a Librivox recording. All Librivox recordings are in the public domain. For more information or to volunteer, please visit Librivox.org. Recording by Morgan Scorpion Anatomy of the Human Body Part 1 by Henry Gray Bone Part 1 Structure and physical properties Bone is one of the hardest structures of the animal body. It possesses also a certain degree of toughness and elasticity. Its colour, in a fresh state, is pinkish-white externally and deep red within. On examining a section of any bone, it is seen to be composed of two kinds of tissue, one of which is dense in texture, like ivory, and is termed compact tissue. The other consists of slender fibres and lamellae, which join to form a reticular structure. This, from its resemblance to latticework, is called cancerous tissue. The compact tissue is always placed on the exterior of the bone, the cancerous in the interior. The relative quality of these two kinds of tissue varies in different bones, and in different parts of the same bone, according as strength or lightness is requisite. Close examination of the compact tissue shows it to be extremely porous, so that the difference in structure between it and the cancerous tissue depends merely upon the different amounts of solid matter, and the size and number of spaces in each. The cavities are small in the compact tissue and the solid matter between them abundant, while in cancerous tissue the spaces are large and the solid matter is in smaller quantity. Bone during life is permeated by vessels, and is enclosed, except where it is coated with articular cartilage, in a fibrous membrane, the periosteum, by means of which many of these vessels reach the hard tissue. If the periosteum be stripped from the surface of the living bone, small bleeding points are seen which mark the entrance of the periosteal vessels, and on section during life every part of the bone exudes blood from the minute vessels which ramify in it. The interior of each of the long bones of the limbs presents a cylindrical cavity filled with marrow and lined by a highly vascular areola structure called the medullary membrane. The strength of bone compared with other materials Substance Medium steel Weight in pounds per cubic foot 490 Ultimate strength pounds per square inch Tension 65,000 Compression 60,000 Substance Granite Weight in pounds per cubic foot 170 Ultimate strength pounds per square inch Tension 1500 Compression 15,000 Shear 2000 Substance Oak White Weight in pounds per cubic foot 46 Ultimate strength pounds per square inch Tension 12,500 Footnote Indicates stresses with the grain, i.e. when the load is parallel to the long axis of the material or parallel to the direction of the fibres of the material. End footnote Compression 7,000 Shear 4,000 Footnote Indicates unit stresses across the grain, i.e. at right angles to the direction of the fibres of the material. End footnote Substance Compact bone Low Weight in pounds per cubic foot 119 Ultimate strength pounds per square inch Tension 13,200 Compression 18,000 Shear 11,800 Substance Compact bone High Weight in pounds per cubic foot Left blank Ultimate strength pounds per square inch Tension 17,700 Compression 24,000 Shear 7,150 Periosteum The periosteum adheres to the surface of each of the bones in nearly every part, but not to cartilaginous extremities. When strong tendons or ligaments are attached to a bone, the periosteum is incorporated with them. It consists of two layers closely united together. The outer one forms chiefly of connective tissue, containing occasionally a few fat cells, the inner one, of elastic fibres of the finer kind, forming dense membranous networks, which again can be separated into several layers. In young bones the periosteum is thick and very vascular, and is intimately connected at either end of the bone with the epiphyseal cartilage, but less closely with the body of the bone. From which it is separated by a layer of soft tissue, containing a number of granular core puzzles, or osteoblasts, by which ossification proceeds on the exterior of the young bone. Later in life the periosteum is thinner and less vascular, and the osteoblasts are converted into an epithelioid layer on the deep surface of the periosteum. The periosteum serves as a nidus for the ramification of the vessels previous to their distribution in the bone, hence the liability of bone to exfoliation or necrosis when denuded of this membrane by injury or disease. Fine nerves and lymphatics, which generally accompany the arteries, may also be demonstrated in the periosteum. Marrow The marrow not only fills up the cylindrical cavities in the bodies of the long bones, but also occupies the spaces of the cancerous tissue and extends into the larger bony canals, halversian canals, which contain the blood vessels. It differs in composition in different bones. In the bodies of the long bones the marrow is of a yellow colour and contains, in 100 parts, 96 of fat, 1 of areola tissue and vessels, and 3 of fluid with extractive matter. It consists of a basis of connective tissue supporting numerous blood vessels and cells, most of which are fat cells but some are marrow cells, such as occur in the red marrow to be immediately described. In the flat and short bones the articular ends of the long bones in the bodies of the vertebrae in the cranial deploy and in the sternum and ribs of the marrow is of a red colour and contains in 100 parts 75 of water and 25 of solid matter consisting of cell globulin, nucleoprotein, extractives, salts and only a small proportion of fat. The red marrow consists of a small quantity of connective tissue, blood vessels and numerous cells, some few of which are fat cells but the great majority are roundish nucleated cells, the true marrow cells of colica. These marrow cells proper, or myelocytes, resemble in appearance lymphoid corpuscles and like them are amyboid. They generally have a higher line protoplasm, though some show granules either oxyfil or basophil in reaction. A number of eosinophil cells are also present. Among the marrow cells may be seen smaller cells which possess a slightly pinkish hue. These are the erythroblasts or norma blasts for which the red corpuscles of the adult are derived and which may be regarded as descendants of the nucleated coloured corpuscles of the embryo. Giant cells, myeloplaxis, osteoclasts, large multi-nucleated protoplasmic masses are also to be found in both sorts of adult marrow, but more particularly in red marrow. They were believed by colica to be concerned in the absorption of bone matrix and hence the name which he gave to them, osteoclasts. They excavate in the bone small shallow pits or cavities which are named house ships foveoli and in these they are found lying. Vessels and nerves of bone The blood vessels of bone are very numerous. Those of the compact tissue are derived from a close and dense network of vessels ramifying in the periosteum. From this membrane vessels pass into the minute orifices in the compact tissue and run through the canals which traverse its substance. The cancerous tissue is supplied in a similar way but by less numerous and larger vessels which, perforating the outer compact tissue, are distributed to the cavities of the spongy portion of the bone. In the long bones numerous apertures may be seen at the ends near the articular surfaces. Some of these give passage to the arteries of the largest set of vessels referred to but the most numerous and largest apertures offer some of the veins of the cancerous tissue which emerge apart from the arteries. The marrow in the body of a long bone is supplied by one large artery or sometimes more which enters the bone at the nutrient foramen situated in most cases near the centre of the body and perforates obliquely the compact structure. The medullary or nutrient artery, usually accompanied by one or two veins, sends branches upward and downward which ramify in the medullary membrane and give twigs to the adjoining canals. The ramifications of this vessel unestimals with the arteries of the cancerous and compact tissues. In most of the flat and in many of the short spongy bones one or more large apertures are observed which transmit to the central parts of the bone vessels corresponding to the nutrient arteries and veins. The veins emerge from the long bones in three places, colica. One, one or two large veins accompany the artery. Two, numerous large and small veins emerge at the articular extremities. Three, many small veins pass out of the compact substance. In the flat cranial bones the veins are large, very numerous and run in tortuous canals in the diploic tissue, the sides of the canals being formed by thin lamellae of bone, perforated here and there for the passage of branches from the adjacent cancelli. The same condition is also found in all cancerous tissue, the veins being enclosed and supported by osseous material and having exceedingly thin coats. When a bone is divided the vessels remain patchless and do not contract in the canals in which they are contained. Lymphatic vessels, in addition to those found in the periosteum, have been traced by cookshank into the substance of bone and Klein describes them as running in the haversian canals. Nerves are distributed freely to the periosteum and accompany the nutrient arteries into the interior of the bone. They are said by colica to be most numerous in the articular extremities of the long bones in the vertebrae and in the larger flat bones. Reignute anatomy. A transverse section of dense bone may be cut with a saw and ground down until it is sufficiently thin. If this be examined with a rather low power the bone will be seen to be mapped out into a number of circular districts, each consisting of a central hole surrounded by a number of concentric rings. These districts are termed haversian systems. The central hole is an haversian canal and rings are layers of bony tissue arranged concentrically around the central canal and termed lamellae. Moreover, on closer examination it will be found that between these lamellae and therefore also arranged concentrically around the central canal are a number of little dark spots, the lacunae. And that these lacunae are connected with each other and with the central haversian canal by a number of fine dark lines which radiate like the spokes of a wheel and are called canalicule. Filling in the irregular intervals which are left between these circular systems are other lamellae with their lacunae and canalicule running in various directions but more or less curved. They are termed interstitial lamellae. Again, other lamellae, found on the surface of the bone are arranged parallel to its circumference. They are termed circumferential or by some authors primary or fundamental lamellae to distinguish them from those laid down around the axis of the haversian canals which are then termed secondary or special lamellae. The haversian canals, seen in a transverse section of bone as round holes at or about the centre of each haversian system may be demonstrated to be true canals if a longitudinal section be made. It will then be seen that the canals run parallel with the longitudinal axis of the bone for a short distance and then branch and communicate. They vary considerably in size some being as much as 0.12 mm in diameter. The average size is however about 0.05 mm. Near the medullary cavity the canals are larger than those near the surface of the bone. Each canal contains one or two blood vessels with a small quantity of delicate connective tissue and some nerve filaments. In the larger ones there are also lymphatic vessels and cells with branching processes which communicate through the canalicule with the branched processes of certain bone cells in the substance of the bone. Those canals near the surface of the bone open upon it by minute orifices and those near the medullary cavity open in the same way into this space so that the whole of the bone is permeated by a system of blood vessels running through the bony canals in the centres of the haversian systems. The lamellae are thin plates of bony tissue encircling the central canal and may be compared, for the sake of illustration, to a number of sheets of paper pasted one over another around a central hollow cylinder. After macerating a piece of bone in dilute mineral acid these lamellae may be stripped off in a longitudinal direction as thin films. If one of these be examined with a high power of the microscope it will be found to be composed of a finely reticular structure made up of very slender transparent fibres decosating obliquely and coalescing at the points of intersection. These fibres are composed of fine fibres identical with those of white connective tissue. The intercellular matrix between the fibres is impregnated by calcareous deposit which the acid dissolves. In many places the various lamellae may be seen to be held together by tapering fibres which run obliquely through them pinning or bolting them together. They were first described by Sharpie and were named by him perforating fibres. The lacunae are situated between the lamellae and consist of a number of oblong spaces. In an ordinary microscopic section viewed by transmitted light they appear as fusiform opaque spots. Each lacunae is occupied during life by a branched cell termed a bone cell or bone corpuscle the processes from which extend into the canalicule. The canalicule are exceedingly minute channels crossing the lamellae and connecting the lacunae with neighbouring lacunae and also with the haversian canal. From the haversian canal a number of canalicule are given off which radiate from it and open into the first set of lacunae between the first and second lamellae. From these lacunae a second section of canalicule is given off. These run outward to the next series of lacunae and so on until the periphery of the haversian system is reached. Here the canalicule given off from the last series of lacunae do not communicate with the lacunae of neighbouring haversian systems but after passing outward for a short distance form loops and return to their own lacunae. Thus every part of an haversian system is supplied with nutrient fluids derived from the vessel in the haversian canal and distributed to the canalicule and lacunae. The bone cells are contained in the lacunae which however they do not completely fill. They are flattened and nucleated branched cells homologous with those of connected tissue. The branches, especially in young bones pass into the canalicule from the lacunae. In thin plates of bone as in the walls of the spaces of cancerous tissue the haversian canals are absent and the canalicule open into the spaces of the cancerous tissue, medullary spaces which thus have the same function as the haversian canals. End of section 10 Section 11 of Grey's Anatomy Part 1 This is a LibriVox recording. All LibriVox recordings are in the public domain. For more information or to volunteer please visit LibriVox.org Recording by Morgan Scorpion Anatomy of the Human Body Part 1 by Henry Grey Bones Part 2 Chemical Composition Bone consists of an animal and an earthy part intimately combined together. The animal part may be obtained by immersing a bone for a considerable time in dilute mineral acid after which process the bone comes out exactly the same shape as before but perfectly flexible so that a long bone, one of the ribs for example can easily be tied in a knot. If now a transverse section is made the same general arrangement of the haversian canals lemeli, lacunae and canaliculi is seen. The earthy part may be separately obtained by calcination by which the animal matter is completely burnt out. The bone will still retain its original form but it will be white and brittle will have lost about one third of its original weight and will crumble down with the slightest force. The earthy matter is composed chiefly of calcium phosphate about 58% of the weight of the bone Calcium carbonate about 7% Calcium fluoride and magnesium phosphate from 1 to 2% each and sodium chloride less than 1% They confer on bone its hardness and rigidity while the animal matter, ocean, determines its tenacity. Osification Some bones are preceded by membrane such as those forming the roof and sides of the skull others, such as the bones of the limbs are preceded by rods of cartilage hence two kinds of ossification are described the intramembranous and the intracartillaginous Intramembranous ossification In the case of bones which are developed in membrane no cartillaginous mould precedes the appearance of the bony tissue the membrane which occupies the place of the future bone is of the nature of connective tissue and ultimately forms the periosteum it is composed of fibres and granular cells in a matrix The peripheral portion is more fibrous while in the interior the cells or osteoblast predominate the whole tissue is richly supplied with blood vessels At the outset of the process of bone formation a little network of spicules is noticed radiating from the point or centre of ossification These rays consist at their growing points of a network of fine clear fibres and granular corpuscles with an intervening ground substance The fibres are termed osteogenic fibres and are made up of fine fibres differing little from those of white fibres tissue The membrane soon assumes a dark and granular appearance from the deposition of calcareous granules in the fibres and in the intervening matrix and in the calcified material some of the granular corpuscles are enclosed By the fusion of the calcareous granules the tissue again assumes a more transparent appearance but the fibres are no longer so distinctly seen The involved osteoblasts from the corpuscles of the future bone the spaces in which they are enclosed constituting the lacunae The involved osteoblasts form the corpuscles of the future bone the spaces in which they are enclosed constituting the lacunae The genetic fibres grow out to the periphery they continue to calcify and give rise to fresh bone spicules Thus the network of bone is formed the meshes of which contain the blood vessels and a delicate connective tissue crowded with osteoblasts The bony trabeculi thickened by the addition of fresh layers of bone formed by the osteoblasts on the surface and the meshes are correspondingly encroached upon Subsequently, successive layers of bony tissue are deposited under the periosteum and around the larger vascular channels which become the herversium canals so that the bone increases much in thickness Enter cartilaginous ossification Just before ossification begins the mass is entirely cartilaginous and in a long bone, which may be taken as an example the process commences in the centre and proceeds towards the extremities which for some time remain cartilaginous Subsequently, a similar process commences in one or more places in those extremities and gradually extends through them The extremities do not, however, become joined to the body of the bone by bony tissue until growth has seized Between the body and either extremity a layer of cartilaginous tissue termed the epiphyseal cartage persists for a definite period The first step in the ossification of the cartilage is that the cartilage cells at the point where ossification is commencing and which is termed a centre of ossification enlarge and arrange themselves in rows The matrix in which they are embedded increases in quantity so that the cells become further separated from each other A deposit of calcareous material now takes place in this matrix between the rows of cells so that they become separated from each other by longitudinal columns of calcified matrix presenting a granular and opaque appearance Here and there the matrix between two cells of the same row become calcified and transverse bars of calcified substance would stretch from one calcareous column to another Thus there are longitudinal groups of the cartilage cells enclosed in oblong cavities the walls of which are formed of calcified matrix which cuts off all nutrition from the cells The cells in consequence atrophy leaving spaces called the primary areoli At the same time that this process is going on in the centre of the solid bar of cartilage certain changes are taking place on its surface This is covered by a very vascular membrane the pericondrium entirely similar to the embryonic connective tissue already described as constituting the basis of membrane bone On the inner surface of this that is to say on the surface in contact with the cartilage are gathered the formative cells the osteoblasts By the agency of these cells a thin layer of bony tissue formed between the pericondrium and the cartilage by the intramembranous mode of ossification just described There are then in this first stage of ossification two processes going on simultaneously in the centre of the cartilage the formation of a number of oblong spaces formed of calcified matrix and containing the withered cartilage cells and on the surface of the cartilage the formation of a layer of true membrane bone The second stage consists in the prolongation of the passage of processes of the deeper or osteogenetic layer of the pericondrium which has now become periosteum The processes consist of blood vessels and cells osteoblasts or bone formers and osteoclasts or bone destroyers The latter are similar to the giant cells myeloplaxis found in marrow and they excavate passages through the new formed bony layer by absorption and pass through it into the calcified matrix Wherever these processes come in contact with the calcified walls of the primary areola they absorb them and thus cause a fusion of the original cavities and the formation of larger spaces which are termed the secondary areola or medullary spaces These secondary spaces become filled with embryonic marrow consisting of osteoblasts and vessels derived in the manner described above from the osteogenetic layer of the periosteum Thus far there has been traced the formation of enlarged spaces secondary areolae the perforated walls of which are still formed by calcified cartilage matrix containing an embryonic marrow derived from the processes sent in from the osteogenetic layer of the periosteum and consisting of blood vessels and osteoblasts The walls of these secondary areolae are at this time of only inconsiderable thickness but they become thickened by the deposition of layers of true bone on their surface The process takes place in the following manner Some of the osteoblasts of the embryonic marrow after undergoing rapid division arrange themselves as an epithelioid layer on the surface of the wall of the space This layer of osteoblasts forms a bony stratum and thus the wall of the space becomes gradually covered with a layer of true osteosubstance in which some of the bone forming cells are included as bone corpuscles The next stage in the process consists in the removal of these primary bone spicules of the osteoclasts One of these giant cells may be found lying in a house ship's foveola at the free end of each spicule The removal of the primary spicules go on paripassu with the formation of permanent bone by the periosteum and in this way the medullary cavity of the body of the bone is formed This series of changes has been gradually proceeding towards the end of the body of the bone so that in the ossifying bone all the changes described above are in different parts from the true bone at the centre of the body to the higher line cartilages at the extremities While the ossification of the cartilaginous body is extending towards the articular ends the cartilage immediately in advance of the osteos tissue continues to grow until the length of the adult bone is reached During the period of growth the articular end, or epiphysis remains for some time entirely cartilaginous Then a bony centre appears and initiates in it the process of intracartilaginous ossification but this process never extends to any great distance The epiphysis remains separated from the body by a narrow cartilaginous layer for a definite time This layer ultimately ossifies The distinction between body and epiphysis is obliterated and the bone assumes its completed form and shape The same remarks also apply to such processes of bone as are separately ossified e.g. the trochanters of the femur The bones therefore continue to grow until the body has acquired its full stature They increase in length by ossification continuing to extend behind the epiphyseal cartilage which goes on growing in advance of the ossifying process They increase in circumference by deposition of new bone from the deeper layer of the periosteum on their external surface and at the same time an absorption takes place from within by which the medullary cavities are increased The permanent bone formed by the periosteum when first laid down is cancelous in structure Later the osteoblasts contained in its spaces become arranged in the concentric layers characteristic of the Haversian systems and are included as bone corpuscles The number of ossific centres varies in different bones In most of the short bones ossification commences at a single point near the centre and proceeds towards the surface In the long bones there is a central point of ossification for the body or diaphysis and one or more for each extremity the epiphysis That for the body is the first to appear The times of union of the epiphysis with the body vary inversely with the dates at which their ossifications begin with the exception of the fibula and regulate the direction of the nutrient arteries of the bones Thus the nutrient arteries of the bones of the arm and forearm are directed towards the elbow since the epiphysis at this joint become united to the bodies with those at the opposite extremities In the lower limb on the other hand the nutrient arteries are directed away from the knee that is upwards in the femur downward in the tibia and fibula and in them it is observed that the upper epiphysis of the femur and the lower epiphysis of the tibia and fibula unite first with the bodies Where there is only one epiphysis the nutrient artery is directed towards the other end of the bone as towards the acromial end of the clavicle towards the distal ends of the metacarpal bone towards the distal ends of the metacarpal bone of the thumb and the metatarsal bone of the great toe and towards the proximal ends of the other metacarpal and metatarsal bones Parsons groups epiphysis under three headings 1. Pressure epiphysis appearing at the articular ends of the bones and transmitting the weight of the body from bone to bone 2. Traction epiphysis associated with the insertion of muscles and originally sesamoid structures though not necessarily sesamoid bones and 3. Atavistic epiphysis representing parts of the skeleton which at one time form separate bones but which have lost their function and only appear as separate ossifications in early life End of Section 11 LibriVox.org Anatomy of the Human Body Part 1 by Henry Gray The Vertebral Column Column the vertebralis spinal column The vertebral column is a flexuous and flexible column formed of a series of bones called vertebrae The vertebrae are 33 in number and are grouped under the names cervical, thoracic, lumbar sacral and coxigial according to the regions they occupy There are 7 in the cervical region 12 in the thoracic 5 in the lumbar 5 in the sacral and 4 in the coxigial This number is sometimes increased by an additional vertebrae in one region or it may be diminished in one region The deficiency often being supplied by an additional vertebrae in another The number of cervical vertebrae is however very rarely increased or diminished The vertebrae in the upper 3 regions of the column remain distinct throughout life and are known as true or movable vertebrae Those of the sacral and coxigial regions on the other hand are termed false or fixed vertebrae because they are united with one another in the adult to form 2 bones 5 forming the upper bone or sacrum and 4 the terminal bone or coccyx With the exception of the first and second cervical the true or movable vertebrae present certain common characteristics which are best studied by examining one from the middle of the thoracic region General characteristics of a vertebra A typical vertebra consists of 2 essential parts namely an anterior segment the body and a posterior part the vertebral or neural arch These enclose a foramen the vertebral foramen The vertebral arch consists of a pair of pedicles and a pair of laminate processes namely 4 articular 2 transverse and 1 spinous When the vertebrae are articulated with each other the bodies form a strong pillar for the support of the head and trunk and the vertebral foramina constitute a canal for the protection of the medulla spinalis or spinal cord While between every pair of vertebrae are 2 apertures the intervertebral foramina 1 on each side for the transmission of the spinal nerves and vessels Body Corpus vertebrae The body is the largest part of a vertebra and is more or less cylindrical in shape Its upper and lower surfaces are flattened and rough and give attachment to the intervertebral fibrocartilages and each presents a rim around its circumference In front the body is convex from side to side and concave from above downward Behind it is flat from above downward and slightly concave from side to side Its anterior surface presents a few small apertures for the passage of nutrient vessels On the posterior surface is a single large irregular aperture or occasionally more than one for the exit of the basa vertebral veins from the body of the vertebra Pedicles Radices archi vertebrae The pedicles are two short thick processes which project backward 1 on either side of the body at the junction of its posterior and lateral surfaces The concavities above and below the pedicles are named the vertebral notches and when the vertebrae are articulated the notches of each contiguous pair of bones form the intervertebral foramina already referred to Lamin A The lamin A are two broad plates directed backward and medial word from the pedicles They fuse in the middle line posteriorly and complete the posterior boundary of the vertebral foramen Their upper borders and the lower parts of their anterior surfaces are rough for the attachment of the ligmenta flavor Processes Spinus process Processes spinosis The spinus process is directed backward and downward from the junction of the lamin A and serves for the attachment of muscles and ligaments Articular processes Two superior and two inferior spring from the junctions of the pedicles and lamin A The superior project upward and their articular surfaces are directed more or less backward The inferior project downward and their surfaces look more or less forward The articular surfaces are coated with hyaline cartilage Transverse processes Processes transversi The transverse processes two in number project one on either side from the point where the lamin A joins the pedicle between the superior and inferior articular processes They serve for the attachment of muscles and ligaments Structure of a vertebra The body is composed of cancelous tissue covered by a thin coating of compact bone The latter is perforated by numerous orifices some of large size for the passage of vessels The interior of the bone is traversed through the channels for the reception of veins which converge toward a single large irregular aperture or several small apertures at the posterior part of the body The thin bony lamin A of the cancelous tissue are more pronounced in lines perpendicular to the upper and lower surfaces and are developed in response to greater pressure in this direction The arch and processes projecting from it of section 12 Section 13 of Grey's Anatomy Part 1 This is a LibriVox recording All LibriVox recordings are in the public domain For more information or to volunteer, please visit LibriVox.org Anatomy of the Human Body Part 1 by Henry Gray The cervical vertebrae Vertebrae Cervicales Cervical vertebrae are the smallest and true vertebrae and can be readily distinguished from those of the thoracic or lumbar regions by the presence of a foramen in each transverse process The first, second, and seventh present exceptional features and must be separately described The following characteristics are common to the remaining four The body is small and broader from side to side than from before backward The anterior and posterior surfaces are flattened and of equal depth The former is placed on a lower level than the latter and its inferior border is prolonged downward so as to overlap the upper and four part of the vertebra below The upper surface is concave transversely and presents a projecting lip on either side The lower surface is concave from before backward, convex from side to side, and presents laterally shallow concavities which receive the corresponding projecting lips of the subjacent vertebra The pedicles are directed lateral word and backward and are attached to the body midway between its upper and lower borders so that the superior vertebral notch is as deep as the inferior but it is at the same time narrower The laminate are narrow and thinner above than below The vertebral foramen is large and of a triangular form The spinous process is short and bipid, the two divisions being often of unequal size The superior and inferior articular processes on either side are fused to form an articular pillar which projects lateral word from the junction of the pedicle and lamina The articular facets are flat and of an oval form The superior look backward, upward and slightly medial word The inferior forward, downward and slightly lateral word The transverse processes are each pierced by the foramen transversarium which in the upper six vertebrae gives passage to the vertebral artery and vein and a plexus of sympathetic nerves Each process consists of an anterior and a posterior part The anterior portion is the homologue of the rib in the thoracic region and is therefore named the costal process or costal element It arises from the side of the body and is directed lateral word in front of the foramen and ends in a tubercle The posterior part The true transverse process springs from the vertebral arch behind the foramen and is directed forward and lateral word It ends in a flattened vertical tubercle The posterior tubercle These two parts are joined outside the foramen by a bar of bone which exhibits a deep sulcus on its upper surface for the passage of the corresponding spinal nerve Footnote The costal element of a cervical vertebra not only includes the portion which springs from the side of the body but the anterior and posterior tubercles and the bar of bone which connects them and a footnote First cervical vertebra The first cervical vertebra is named the atlas because it supports the globe of the head Its chief peculiarity is that it has no body and this is due to the fact that the body of the atlas has fused with that of the next vertebra Its other peculiarities are that it has no spinous process is ring-like and consists of an anterior and a posterior arch and two lateral masses The anterior arch firms about one-fifth of the ring Its anterior surface is convex and presents at its center the anterior tubercle for the attachment of the longest coli muscles Posteriorly it is concave and marked by a smooth oval or circular facet fovea dentis for articulation with the odontoid process dens of the axis The upper and lower borders respectively give attachment to the anterior atlanto-oxypital membrane and the anterior atlanto-axial ligament The former connects it with the occipital bone above and the latter with the axis below The posterior arch forms about two-fifths of the circumference of the ring It ends behind in the posterior tubercle which is the rudiment of a spinous process and gives origin to the recticapitis posterioris minoris The diminutive size of this process prevents any interference with the movements between the atlas and the skull The posterior part of the arch presents above and behind a rounded edge for the attachment of the posterior atlanto-oxypital membrane while immediately behind each superior articular process is a groove sulcus arteriae vertebralis converted into a foramen by a delicate bony spiculum which arches backward from the posterior end of the superior articular process This groove represents the superior vertebral notch and serves for the transmission of the vertebral artery which after ascending through the foramen in the transverse process winds around the lateral mass in a direction backward and medialward it also transmits the sub-oxypital first spinal nerve The undersurface of the posterior arch behind the articular facets are two shallow grooves the inferior vertebral notches The lower border gives attachment to the posterior atlanto-axial ligament which connects it with the axis The lateral masses are the most bulky and solid parts of the atlas in order to support the weight of the head Each carries two articular facets a superior and an inferior The superior facets are of large size, oval, concave and approach each other in front but diverge behind They are directed upward, medialward and a little backward each forming a cup for the corresponding condyle of the occipital bone and are admirably adapted to the knotting movements of the head Not infrequently, they are partially subdivided by indentations which encroach upon their margins The inferior articular facets are circular in form, flattened or slightly convex and directed downward and medialward articulating with the axis and permitting the rotary movements of the head Just below the medial margin of each superior facet is a small tubercle For the attachment of the transverse atlanto-ligament which stretches across the ring of the atlas and divides the vertebral foramen into two unequal parts The anterior or smaller receiving the odontoid process of the axis The posterior transmitting the spinalis and its membranes This part of the vertebral canal is of considerable size much greater than is required for the accommodation of the medulla spinalis and hence lateral displacement of the atlas may occur without compression of this structure The transverse processes are large They project lateralward and downward from the lateral masses and serve for the attachment of muscles which assist in rotating the head They are long and their anterior tubercles are fused into one mass The foramen transversarium is directed from below upward and backward Second cervical vertebra The second cervical vertebra is named the epistrophias or axis because it forms the pivot upon which the first vertebra carrying the head rotates The most distinctive characteristic of this bone is the strong odontoid process which rises perpendicularly from the upper surface of the body is deeper in front than behind and prolonged downward anteriorly so as to overlap the upper and fore part of the third vertebra It presents in front a median longitudinal ridge separating two lateral depressions for the attachment of the longest coli muscles Its under surface is concave from before backward and convex from side to side The dense or odontoid process exhibits a slight constriction or neck where it joins the body On its anterior surface is an oval or nearly circular facet for articulation with that on the anterior arch of the atlas On the back of the neck and frequently extending onto its lateral surfaces is a shallow groove for the transverse atlanta ligament which retains the process and position The apex is pointed and gives attachment to the apical odontoid ligament Below the apex the process is somewhat enlarged and presents on either side a rough impression for the attachment of the allar ligament These ligaments connect the process to the occipital bone The internal structure of the odontoid process is more compact than that of the body The pedicles are broad and strong especially in front where they coalesce with the sides of the body and the root of the odontoid process They are covered above by the superior articular surfaces The laminate are thick and strong and the vertebral foramen large but smaller than that of the atlas The transverse processes are very small and each ends in a single tubercle Each is perforated by the foramen transversarium which is directed obliquely upward and lateral The superior articular surfaces are round, slightly convex directed upward and lateral and are supported on the body pedicles and transverse processes The inferior articular surfaces have the same direction as those of the other cervical vertebrae The superior vertebral notches are very shallow and lie behind the articular processes The inferior lie in front of the articular processes as in the other cervical vertebrae The spinous process is large very strong, deeply channeled on its under surface and presents a bipid tuberculated extremity The seventh cervical vertebra The most distinct characteristic of this vertebra is the existence of a long and prominent spinous process hence the name vertebra prominence This process is thick nearly horizontal in direction not bifurcated but terminating in a tubercle into which the lower end of the lingumentum nuce is attached The transverse processes are of considerable size Their posterior roots are large and prominent while the anterior are small and faintly marked The upper surface of each has usually a shallow sulcus for the eighth spinal nerve This extremity seldom presents more than a trace of bifurcation The foramen transversarium may be as large as that in the other cervical vertebrae but is generally smaller on one or both sides Occasionally it is double, sometimes it is absent On the left side it occasionally gives passage to the vertebral artery More frequently the vertebral vein traverses on both sides but the usual arrangement is for both artery and vein to pass in front of the transverse process and not through the foramen Sometimes the anterior root of the transverse process attains a large size and exists as a separate bone which is known as a cervical rib End of section 13 Section 14 of Grey's Anatomy Part 1 This is a LibriVox recording All LibriVox recordings are in the public domain For more information or to volunteer please visit LibriVox.org Anatomy of the Human Body Part 1 by Henry Gray The thoracic vertebrae The thoracalis The thoracic vertebrae are intermediate in size between those of the cervical and lumbar regions They increase in size from above downward the upper vertebrae being much smaller than those in the lower part of the region They are distinguished by the presence of facets on the sides of the bodies for articulation with the heads of the ribs and facets on the transverse processes of all except the 11th and 12th for articulation with the tubercles of the ribs The bodies in the middle of the thoracic region are heart-shaped and as broad in the anterior-posterior as in the transverse direction At the ends of the thoracic region they resemble respectively those of the cervical and lumbar vertebrae They are slightly thicker behind than in front flat above and below convex from side to side in front and slightly constricted laterally and in front They present on either side two costal demi-facets one above near the root of the pedicle the other below in front of the inferior vertebral notch These are covered with cartilage in the fresh state and when vertebrae are articulated with one another form with the intervening intervertebral fibrocartilages oval surfaces for the reception of the heads of the ribs The pedicles are directed backward and slightly upward and the inferior vertebral notches are of a large size deeper than in any other region of the vertebral column The laminate are broad thick and implicated that is to say they overlap those of sub-adjacent vertebrae like tiles on a roof The vertebral foramen is small and of a circular form The spinous process is long triangular on coronal section directed obliquely downward and ends in a tuberculated extremity These processes overlap from the fifth to the eighth but are less oblique in direction above and below The superior articular processes are thin plates of bone projecting upward from the junctions of the pedicles and laminate Their articular facets are practically flat and are directed backward and a little lateral word and upward The inferior articular processes are fused to a considerable extent with the laminate and project but slightly beyond their lower borders Their facets are directed forward and a little medial word and downward The transverse processes arise from the arch behind the superior articular processes and pedicles They are thick, strong, and of considerable length directed obliquely backward and lateral word and each ends in a clubbed extremity on the front of which is a small concave surface for articulation with the tubercle of a rib The first, ninth, tenth, eleventh, and twelfth thoracic vertebrae present certain peculiarities and must be specially considered The first thoracic vertebrae has on either side of the body an entire articular facet for the head of the first rib and a demi-facet for the upper half of the head of the second rib The body is like that of a cervical vertebra being broad transversely, its upper surface is concave and lipped on either side The superior articular surfaces are directed upward and backward The spinous process is thick, long, and almost horizontal The transverse processes are long and the upper vertebral notches are deeper than those of the other thoracic vertebrae The ninth thoracic vertebrae may have no demi-facets below In some subjects, however, it has two demi-facets on either side When this occurs, the tenth has only demi-facets at the upper part The tenth thoracic vertebrae has, except in the cases just mentioned an entire articular facet on either side which is placed partly on the lateral surface of the pedicle In the eleventh thoracic vertebrae, the body approaches in its form and size to that of the lumbar vertebrae The articular facets for the heads of the ribs are of large size and placed chiefly on the pedicles which are thicker and stronger in this and the next vertebrae than in any other part of the thoracic region The spinous process is short and nearly horizontal in direction The transverse processes are very short, tuberculated at their extremities and have no articular facets The twelfth thoracic vertebrae has the same general characteristics as the eleventh but may be distinguished from it by its inferior articular surfaces being convex and directed lateralward, like those of the lumbar vertebrae By the general form of the body, laminae and spinous process in which it resembles the lumbar vertebrae and by each transverse process being subdivided into three elevations the superior, inferior and lateral tubercles the superior and inferior correspond to the mammillary and accessory processes of the lumbar vertebrae Traces of similar elevations are found on the transverse processes of the tenth and eleventh thoracic vertebrae The lumbar vertebrae Vertebrae lumbalis The lumbar vertebrae are the largest segments of the movable part of the vertebral column and can be distinguished by the absence of a foramen in the transverse process and by the absence of facets on the sides of the body The body is large, wider from side to side than from before backward and a little thicker in front than behind It is flattened or slightly concave above and below, concave behind and deeply constricted in front and at the sides The pedicles are very strong, directed backward from the upper part of the body Consequently, the inferior vertebral notches are of considerable depth The laminate are broad, short and strong The vertebral foramen is triangular, larger than in the thoracic but smaller than in the cervical region The spinous process is thick, broad and somewhat quadrilateral It projects backward and ends in a rough uneven border, thickest below where it is occasionally notched The superior and inferior articular processes are well defined projecting respectively upward and downward from the junctions of pedicles and laminate The facets on the superior processes are concave and look backward and medialward Those on the inferior are convex and are directed forward and lateralward The former are wider apart than the latter Since in the articulated column, the inferior articular processes are embraced by the superior processes of the sub-adjacent vertebra The transverse processes are long, slender and horizontal in the upper three lumbar vertebrae They incline a little upward in the lower two In the upper three vertebrae, they arise from the junctions of the pedicles and laminate but in the lower two, they are set farther forward and spring from the pedicles and posterior parts of the bodies They are situated in front of the articular processes instead of behind them, as in the thoracic vertebrae and are homologous with the ribs Of the three tubercles, noticed in connection with the transverse processes of the lower thoracic vertebrae the superior one is connected in the lumbar region with the back part of the superior articular process and is named the mammillary process The inferior is situated at the back part of the base of the transverse process and is called the accessory process The fifth lumbar vertebrae is characterized by its body being much deeper in front than behind which accords with the prominence of the sacrovertible articulation by the smaller size of its spinous process by the wide interval between the inferior articular processes and by the thickness of its transverse processes which spring from the body as well as from the pedicles End of section 14 Section 15 of Grey's Anatomy Part 1 This is a LibriVox recording All LibriVox recordings are in the public domain For more information or to volunteer please visit LibriVox.org Anatomy of the Human Body Part 1 by Henry Gray The sacral and coxidual vertebrae The sacral and coxidual vertebrae consist at an early period of life of nine separate segments which are united in the adult so as to form two bones five entering into the formation of the sacrum four into that of the coccyx Sometimes the coccyx consists of five bones occasionally the number is reduced to three The sacrum, os sacrum The sacrum is a large triangular bone situated in the lower part of the vertebral column and at the upper and back part of the pelvic cavity where it is inserted like a wedge between the two hip bones Its upper part or base articulates with the last lumbar vertebra its apex with the coccyx It is curved upon itself and placed very obliquely Its base projecting forward and forming the prominent sacrovertebral angle when articulated with the last lumbar vertebra Its central part is projected backward so as to give increased capacity to the pelvic cavity Pelvic surface Faces, Pelvina The pelvic surface is concave from above downward and slightly so from side to side Its middle part is crossed by four transverse ridges the positions of which correspond with the original separation between the five segments of the bone The portions of bone intervening between the ridges are the bodies of the sacral vertebrae The body of the first segment is of large size and in form resembles that of a lumbar vertebra The succeeding ones diminished from above downward are flattened from before backward and curved so as to accommodate themselves to the form of the sacrum being concave in front, convex behind At the ends of the ridges are seen the anterior sacral foramina Four in number on either side somewhat rounded in form diminishing in size from above downward and directed lateralward and forward They give exit to the anterior divisions of the sacral nerves and entrance to the lateral sacral arteries Lateral to these foramina are the lateral parts of the sacrum Each consisting of five separate segments at an early period of life in the adult these are blended with the bodies and with each other Each lateral part is traversed by four broad shallow grooves which lodge the anterior divisions of the sacral nerves and are separated by prominent ridges of bone which give origin to the piriformis muscle If a sagittal section be made to the center of the sacrum the bodies are seen to be united at their circumferences by bone, wide intervals being left centrally which in the fresh state are filled by the intervertebral fibrocartilages In some bones this union is more complete between the lower than the upper segments dorsal surface faces dorsalis The dorsal surface is convex and narrower than the pelvic In the middle line it displays a crest the middle sacral crest surmounted by three or four tubercles the rudimentary spinous processes of the upper three or four sacral vertebrae On either side of the middle sacral crest is a shallow groove the sacral groove which gives origin to the multi-phytus the floor of the groove being formed by the united laminate of the corresponding vertebrae The laminate of the fifth sacral vertebra and sometimes those of the fourth fail to meet behind and thus a hiatus or deficiency occurs in the posterior wall of the sacral canal On the lateral aspect of the sacral groove is a linear series of tubercles produced by the fusion of the articular processes which together form the indistinct sacroarticular crests The articular processes of the first sacrovertebra are large and oval in shape Their facets are concave from side to side look backward and medialward and articulate with the facets on the inferior processes of the fifth lumbar vertebra The tubercles which represent the inferior articular processes of the fifth sacrovertebra are prolonged downward as rounded processes which are named the sacral cornea and are connected to the cornea of the coccyx Lateral to the articular processes are the four posterior sacral foramina They are smaller in size and less regular in form than the anterior and transmit the posterior divisions of the sacral nerves On the lateral side of the posterior sacral foramina is a series of tubercles which represent the transverse processes of the sacral vertebrae and form the lateral crests of the sacrum The transverse tubercles of the first sacral vertebra are large and very distinct They, together with the transverse tubercles of the second vertebra, give attachment to the horizontal parts of the posterior sacroiliac ligaments Those of the third vertebra give attachment to the oblique fasciculi of the posterior sacroiliac ligaments and those of the fourth and fifth to the sacro tuberous ligaments Lateral surface The lateral surface is broad above but narrowed into a thin edge below The upper half presents in front an ear-shaped surface the auricular surface covered with cartilage in the fresh state for articulation with the ilium Behind is a rough surface the sacral tuberosity on which are three deep and uneven impressions for the attachment of the posterior sacroiliac ligament The lower half is thin and ends in a projection called the inferior lateral angle Medial to this angle is a notch which is converted into a foramen by the transverse process of the first piece of the coccyx and transmits the anterior division of the fifth sacral nerve The thin lower half of the lateral surface gives attachment to the sacro tuberous and sacrospinus ligaments to some fibers of the gluteus maximus behind and to the coccyxius in front Base Basis-os sacri The base of the sacrum, which is broad and expanded is directed upward and forward and the middle is a large oval articular surface The upper surface of the body of the first sacral vertebra which is connected with the under surface of the body of the last lumbar vertebra behind this is the large triangular orifice of the sacral canal which is completed by the laminae and spinous process of the first sacral vertebra The superior articular processes project from it on either side They are oval, concave directed backward and medial word like the superior articular processes of a lumbar vertebra They are attached to the body of the first sacral vertebra and to the alley by short thick pedicles On the upper surface of each pedicle is a vertebral notch which forms the lower part of the foramen between the last lumbar and first sacral vertebrae On either side of the body is a large triangular surface which supports the psoas major and lumbo sacral trunk and in the articulated pelvis is continuous with the iliac fossa This is called the alley It is slightly concave from side to side convex from before backward and gives attachment to a few of the fibers of the iliacus The posterior fourth of the alley represents the transverse process and its anterior three fourths the costal process of the first sacral segment Apex The apex is directed downward and presents an oval facet for articulation with the coccyx Vertebral canal Canalis sacralis sacral canal The vertebral canal runs throughout the greater part of the bone Above it is triangular in form Below its posterior wall is incomplete from the non-development of the laminae and spinous processes It lodges the sacral nerves and its walls are perforated by the anterior and posterior sacrophoramina through which these nerves pass out Structure The sacrum consists of cancerous tissue enveloped by a thin layer of compact bone articulations The sacrum articulates with four bones The last lumbar vertebra above The coccyx below and the hip bone on either side Differences in the sacrum of the male and female In the female the sacrum is shorter and wider than in the male The lower half forms a greater angle with the upper The upper half is nearly straight The lower half presenting the greatest amount of curvature This increases the size of the pelvic cavity and renders the sacrovertebral angle more prominent In the male the curvature is more evenly distributed over the whole length of the bone and is altogether greater than in the female Variations The sacrum in some cases consists of six pieces, occasionally the number is reduced to four The bodies of the first and second vertebrae may fail to unite Sometimes the uppermost transverse tubercles may not join to the rest of the ala on one or both sides Or the sacral canal may be open through a considerable part of its length in consequence of the imperfect development of the laminate and spinous processes The sacrum also varies considerably with respect to its degree of curvature The coccyx Os coccygius The coccyx is usually formed of four rudimentary vertebrae The number may however be increased to three or diminished to three In each of the first three segments may be traced a rudimentary body in articular and transverse processes The last piece, sometimes the third is a mere nodule of bone All the segments are destitute of pedicles, laminate and spinous processes The first is the largest It resembles the lowest sacral vertebra and often exists as a separate piece The last three diminish in size from above downward and are usually matched with one another Surfaces The anterior surface is slightly concave and marked with three transverse grooves which indicate the junctions of the different segments It gives attachment to the anterior sacrococcygeal ligament and the levatoris ani and supports part of the rectum The posterior surface is convex marked by transverse grooves similar to those on the anterior surface and presents on either side the rudimentary articular processes of the coccygeal vertebrae Of these, the superior pair are large and are called the coccygeal canua They project upward and articulate with the cornea of the sacrum and on either side complete the foramen for the transmission of the posterior division of the fifth sacral nerve Borders The lateral borders are thin and exhibit a series of small eminences which represent the transverse processes of the coccygeal vertebrae Of these, the first is the largest It is flattened from before backward and often ascends to join the lower part of the thin lateral edge of the sacrum Thus completing the foramen for the transmission of the anterior division of the fifth sacral nerve The others diminish in size from above downward and are often wanting The borders of the coccyx are narrow and give attachment to either side to the sacro tuberis and sacrospinus ligaments to the coccygeus in front of the ligaments and to the gluteus maximus behind them Base The base presents an oval surface for articulation with the sacrum Apex The apex is rounded and has attached to it the tendon of the sphincter ani externus It may be bipid and is sometimes deflected to one or the other side Osification of the vertebral column Each cartilaginous vertebra is ossified from three primary centers two for the vertebral arch and one for the body Footnote A vertebra is occasionally found in which the body consists of two lateral portions a condition which proves that the body is sometimes ossified from two primary centers one on either side of the middle line and a footnote Osification of the vertebral arches begins in the upper cervical vertebra about the seventh or eighth week of fetal life and gradually extends down the column The ossific granules first appear in the situations where the transverse processes afterward project and spread backward to the spinous process forward into the pedicles and lateral word into the transverse and articular processes Osification of the bodies begins about the eighth week in the lower thoracic region and subsequently extends upward and downward along the column The center for the body does not give rise to the whole of the body of the adult vertebra the posterior lateral portions of which are ossified by extensions from the vertebral arch centers The body of the vertebra during the first few years of life shows therefore two synchondroses Neurocentral synchondroses transversing it along the planes of junction of the three centers In the thoracic region the facets for the heads of the ribs are central synchondroses and are ossified from the centers for the vertebral arch At birth the vertebra consists of three pieces the body and the halves of the vertebral arch During the first year the halves of the arch unite behind union taking place first in the lumbar region and then extending upward through the thoracic and cervical regions About the third year the bodies of the upper cervical vertebra are joined to the arches on either side In the lower lumbar vertebra the union is not completed until the sixth year Before puberty no other changes occur excepting a gradual increase of these primary centers the upper and under surfaces of the bodies and the ends of the transverse and spinous processes being cartilaginous About the sixteenth year five secondary centers appear one for the tip of each transverse process one for the extremity of the spinous process one for the upper and one for the lower surface of the body these fuse with the rest of the bone about the age of 25 Exceptions to this mode of development occur in the first, second and seventh cervical vertebrae and in the lumbar vertebrae Atlas The Atlas is usually ossified from three centers of these one appears in each lateral mass about the seventh week of fetal life and extends backward At birth these portions of bone are separated from one another behind by a narrow interval filled with cartilage between the third and fourth years they unite either directly or through the medium of a separate center developing in the cartilage At birth the anterior arch consists of cartilage in this a separate center appears about the end of the first year after birth and joins the lateral masses from the sixth to the eighth year the lines of union extending across the anterior portions of the superior particular facets Occasionally there is no separate center the anterior arch being formed by the forward extension and ultimate junction of the two lateral masses sometimes this arch is ossified from two centers one on either side of the middle line Epistrophias or axis The axis is ossified from five primary and two secondary centers The body and vertebral arch are ossified in the same manner there are corresponding parts in the other vertebrae namely one center for the body and two for the vertebral arch The centers for the arch appear about the seventh or eighth week of fetal life that for the body about the fourth or fifth month The dense erodontoid process consists originally of a continuation upward of the cartilaginous mass in which the lower part of the body is formed About the sixth month of fetal life two centers make their appearance in the base of this process They are placed laterally and joined before birth to form a conical bilobid mass deeply cleft above The interval between the sides of the cleft and the summit of the process is formed by a wedge-shaped piece of cartilage The base of the process is separated from the body by a cartilaginous disc which gradually becomes ossified at its circumference but remains cartilaginous in its center until advanced age In this cartilage, rudiments the upper epiphyseal lamella of the atlas and the upper epiphyseal lamella of the axis may sometimes be found The apex of the odontoid process has a separate center which appears in the second and joins about the twelfth year This is the upper epiphyseal lamella of the atlas In addition to these there is a secondary center for a thin epiphyseal plate on the under surface of the body of the bone The seventh cervical vertebra The posterior or costal part of the transverse process of this vertebra is sometimes ossified from a separate center which appears about the sixth month of fetal life and joins the body and posterior part of the transverse process between the fifth and sixth years Occasionally the costal part persists as a separate piece and becoming lengthened, lateral, and forward constitutes what is known as a cervical rib Separate osophic centers have also been found of the fourth, fifth, and sixth cervical vertebrae Lumbar vertebrae The lumbar vertebrae have each two additional centers for the mammillary processes The transverse process of the first lumbar is sometimes developed as a separate piece which may remain permanently ununited with the rest of the bone thus forming a lumbar rib a peculiarity, however, rarely met with Sacrum Each sacral vertebra is ossified from a primary center and two epiphyseal plates one for its upper and another for its under surface while each vertebral arch is ossified from two centers The anterior portions of the lateral parts have six additional centers two for each of the first three vertebrae These represent the costal elements and make their appearance above and lateral to the anterior sacral foramina On each lateral surface epiphyseal plates are developed one for the auricular surface and another for the remaining part of the thin lateral edge of the bone Footnote The ends of the spinous processes of the three sacrovertebrae are sometimes developed from three separate epiphyseas and faucet states that a number of epiphyseal nodules may be seen in the sacrum at the age of 18 years These are distributed as follows one for each of the mammillary processes of the first sacrovertebra twelve, six on either side in connection with the costal elements two each for the first and second and one each for the third and fourth and eight for the transverse processes four on either side one each for the first, third, fourth, and fifth He is further of opinion that the lower part of each lateral surface of the sacrum is formed by the extension and union of the third and fourth costal and fourth and fifth transverse epiphyseas and a footnote Periods of ossification About the eighth or ninth week of fetal life ossification of the central part of the body of the first sacrovertebra commences and is rapidly followed by deposit of ossific matter in the second and third ossification does not commence in the bodies of the lower two segments until between the fifth and eighth months of fetal life Between the sixth and eighth months ossification of the vertebral arches takes place and about the same time the costal centers for the lateral parts make their appearance The junctions of the vertebral arches with the bodies take place in the lower vertebra as early as the second year but are not affected in the uppermost until the fifth or sixth year About the sixteenth year the epiphyseal plates for the upper and under surfaces of the bodies are formed and between the eighteenth and twentieth years those for the lateral surfaces make their appearance The vertebrae are during early life separated from each other by intervertebral fibrocartilages but about the eighteenth year the two lowest segments become united by bone and the process of bony union gradually extends upward with the result that between the twenty-fifth and thirtieth years of life all the segments are united On examining a sagittal section of the sacrum the situations of the intervertebral fibrocartilages are indicated by a series of oval cavities The coccyx is ossified from four centers one for each segment the ossific nuclei make their appearance in the following order in the first segment between the first and fourth years in the second between the fifth and tenth years in the third between the tenth and fifteenth years in the fourth between the fourteenth and twentieth years as age advances the segments unite with one another the union between the first and second segments being frequently delayed until after the age of twenty-five or thirty at a late period of life especially in females the coccyx often fuses with the sacrum and of section fifteen section sixteen of Grey's Anatomy part one this is a LibriVox recording all LibriVox recordings are in the public domain for more information or to volunteer please visit LibriVox.org Anatomy of the human body part one by Henry Gray the vertebral column as a whole the vertebral column is situated in the median line as the posterior part of the trunk its average length in the male is about seventy-one centimeters of this length the cervical part measures twelve point five centimeters the thoracic about twenty-eight centimeters the lumbar eighteen centimeters and the sacrum and coccyx twelve point five centimeters the female column is about sixty-one centimeters in length curves viewed laterally the vertebral column represents several curves which correspond to the different regions of the column and are called cervical, thoracic, lumbar and pelvic the cervical curve convex forward begins at the apex of the odontoid process and ends at the middle of the second thoracic vertebra it is the least marked of all the curves the thoracic curve concave forward begins at the middle of the second and ends at the middle of the 12th thoracic vertebra its most prominent point behind corresponds to the spinous process of the seventh thoracic vertebra the lumbar curve is more marked in the female than in the male it begins at the middle of the last thoracic vertebra and ends at the sacrovertebral angle it is convex anteriorly the convexity of the lower three vertebra being much greater than that of the upper two the pelvic curve begins at the sacrovertebral articulation and ends at the point of the coccyx its concavity is directed downward and forward the thoracic and pelvic curves are termed primary curves because they alone are present during fetal life the cervical and lumbar curves are compensatory or secondary and are developed after birth the former when the child is able to hold up its head at three or four months and to sit upright at nine months the latter at 12 or 18 months when the child begins to walk the vertebral column also has a slight lateral curvature the convexity of which is directed toward the right side this may be produced by muscular action most persons using the right arm in preference to the left especially in making long continued efforts when the body is curved to the right side in support of this explanation it has been found that in one or two individuals who were left-handed the convexity was to the left side by others this curvature is regarded as being produced by the aortic arch and upper part of the descending thoracic aorta a view which is supported by the fact that in cases where the viscera are transposed and the aorta is on the right side the convexity of the curve is directed to the left side surfaces anterior surface when viewed from in front the width of the bodies of the vertebrae from the second cervical to the first thoracic there is then a slight diminution in the next three vertebrae below this there is again a gradual and progressive increase in width as low as the sacrovertebral angle from this point there is rapid diminution to the apex of the coccyx posterior surface the posterior surface of the vertebral column presents in the median line the spinous processes in the cervical region with the exception of the second and seventh vertebrae these are short and horizontal with byfit extremities in the upper part of the thoracic region they are directed obliquely downward in the middle they are almost vertical and in the lower part they are nearly horizontal in the lumbar region they are nearly horizontal the spinous processes are separated by considerable intervals in the lumbar region by narrower intervals in the neck and are closely approximated in the middle of the thoracic region occasionally one of these processes deviates a little from the median line a fact to be remembered in practice as irregularities of this sort are attendant also on fractures or displacements of the vertebral column on either side of the spinous processes is the vertebral groove formed by the laminate in the cervical and lumbar regions where it is shallow and by the laminate in transverse processes in the thoracic region where it is deep and broad these grooves lodge the deep muscles of the back lateral to the vertebral grooves are the articular processes and still more laterally the transverse processes in the thoracic region the transverse processes stand backward on a plane considerably behind that of the same processes in the cervical and lumbar regions in the cervical region the transverse processes are placed in front of the articular processes lateral to the pedicles and between the intervertebral foramina in the thoracic region they are posterior to the pedicles intervertebral foramina and articular processes in the lumbar region they are in front of the articular processes but behind the intervertebral foramina lateral surfaces the lateral surfaces are separated from the posterior surface by the articular processes in the cervical and lumbar regions and by the transverse processes in the thoracic region the sides of the bodies of the vertebrae marked in the thoracic region by the facets for articulation with the heads of the ribs more posteriorly are the intervertebral foramina formed by the juxtaposition of the vertebral notches oval in shape smallest in the cervical and upper part of the thoracic regions and gradually increasing in size to the last lumbar they transmit the spinal nerves and are situated between the transverse processes and in front of them in the thoracic and lumbar regions vertebral canal the vertebral canal follows the different curves of the column it is large and triangular in those parts of the column which enjoy the greatest freedom of movement namely the cervical and lumbar regions and is small and rounded in the thoracic region where motion is more limited abnormalities occasionally the coalescence generally a cleft is left in the arches of the vertebrae through which a protrusion of the spinal membranes dura mater and arachnoid and generally the medulla spinalis itself takes place constituting the malformation known as spina bifida this condition is most common in the lumbosacral region but it may occur in the thoracic or cervical region or the arches throughout the whole length of the canal may remain incomplete end of section 16 section 17 of gray's anatomy part 1 this is LibriVox recording all LibriVox recordings are in the public domain for more information or to volunteer please visit LibriVox.org recording by Jennifer Stearns anatomy of the human body part 1 by Henry Gray the thorax the skeleton of the thorax or chest is an osteocartilaginous cage containing and protecting the principal organs of respiration and circulation it is conical in shape being narrow above and broad below flattened from before backward and longer behind than in front it is somewhat renaform on transverse section on account of the projection of the vertebral bodies into cavity boundaries the posterior surface is formed of 12 thoracic vertebrae and the posterior parts of the ribs it is convex from above downward and presents on either side of the middle line a deep groove in consequence of the lateral and backward direction which the ribs take from the vertebral extremities to their angles the anterior surface formed by the sternum and costal cartilages is flattened and slightly convex and inclined from above downward and forward the lateral surfaces are convex they are formed by the ribs separated from each other by the intercostal spaces 11 in number which are occupied by the intercostal muscles and membranes the upper opening of the thorax is renaform in shape being broader from side to side than from before backward it is formed by the first thoracic vertebra behind the upper margin of the sternum in front and the first rib on either side it slopes downward and forward so that the interior part of the opening is on a lower level than the posterior its anterior posterior diameter is about 5 cm and its transverse diameter about 10 cm the lower opening is formed by the 12th thoracic vertebra behind by the 11th and 12th ribs at the sides and in front by the cartilages of the 10th, 9th, 8th and 7th ribs which ascend on either side and form an angle, the subcostal angle into the apex of which the cyphoid process projects the lower opening is wider transversely than from before backward and slopes obliquely downward and backward it is closed by the diaphragm which forms the floor of the thorax the thorax of the female differs from that of the male as follows 1. its capacity is less 2. the sternum is shorter 3. the upper margin of the sternum is on a level with the lower part of the body of the third thoracic vertebra whereas in the male it is on a level with the lower part of the body of the second 4. the upper ribs are more movable and so allow a greater enlargement of the upper part of the thorax the sternum breastbone the sternum is an elongated flattened bone forming the middle portion of the thorax its upper end supports the clavicles and its margins articulate with the cartilages of the first 7 pairs of ribs it consists of 3 parts named from above, downward the manubrium the body or gladiolus and the cyphoid process in early life the body consists of 4 segments or sternabray in its natural position the inclination of the bone is a bleak from above downward and forward it is slightly convex in front and concave behind broad above becoming narrowed at the point where the manubrium joins the body after which it again widens a little below the middle of the body and then narrows to its lower extremity its average length in the adult is about 17 cm and is rather greater in the male than in the female manubrium manubrium sterni the manubrium is of a somewhat quadrangular form broad and thick above narrow below at its juncture with the body surfaces its anterior surface convex from side to side concave from above downward is smooth and affords attachment on either side to the sternal origins of the pectoralis major and sternocleinomastoidus sometimes the ridges limiting these muscles are very distinct its posterior surface concave and smooth affords attachment on either side to the sternohydiotis and sternothyriotis borders the superior border is the thickest and presents at its center the jugular or pre-sternal notch on either side of the notch is an oval articular surface directed upward backward and lateral for articulation with the sternal end of the clavicle the inferior border, oval and rough is covered in a fresh state with a thin layer of cartilage for articulation with the body the lateral borders are each marked above by a depression for the first costal cartilage and below by a small facet which with a similar facet on the upper angle of the body forms a notch for the reception of the coastal cartilage of the second rib between the depression for the first and the semi-facet for the second is a narrow curved edge which slopes from above downward and medialward body the body considerably longer, narrower and thinner than the manubrium attains its greatest breadth close to the lower end surfaces its anterior surface is nearly flat directed upward and forward and marked by three transverse ridges which cross the bone opposite the third and fifth articular depressions footnote Patterson the human sternum 1904 who examined 524 specimens points out that these ridges are altogether absent in 23.7% that in 69% a ridge exists opposite the third costal attachment and 39% opposite the fourth 4% only opposite the fifth and footnote it affords attachment on either side of the sternum origin of the pectoralis major at the junction of the third and fourth pieces of the body is occasionally seen in orifice the sternum foremen are varying size and form the posterior surface slightly concave is also marked by three transverse lines less distinct however than those in front from its lower part on either side transversus thoracus takes origin the superior border is oval and articulates with the manubrium junction of the two forming the sternum angle angulus ludovici footnote named after the French surgeon Antoine Louis 1723-1792 the Latin name angulus ludovici is not infrequently mistranslated into English as the angle of Ludwig and footnote the inferior border is narrow with a xyphoid process each lateral border at its superior angle has a small facet with which a similar facet on the manubrium forms a cavity for the cartilage of the second rib below this are four angular depressions which receive the cartilages of the third, fourth, fifth and sixth ribs while the inferior angle has a small facet which with a corresponding one on the xyphoid process forms a notch for the cartilage where the two particular depressions are separated by a series of curved interarticular intervals which diminish in length from above downward and correspond to the intercostal spaces most of the cartilages belong to the true ribs as will be seen from the foregoing description articulate with the sternum at the lines of junction of its permitted component segments this is well seen in many of the lower animals where the parts of the bone remain longer than in man xyphoid process processes xyphodias enziform or xyphoid appendix the xyphoid process is the smallest of the three pieces it is thin and elongated cartilaginous in structure in youth but more or less ossified at its upper part in the adult surfaces its anterior surface affords attachment on either side to the anterior costo xyphoid ligament and a small part of the rectus abdominis its posterior surface to the posterior costo xyphoid ligament and to some of the fibers of the diaphragm and transversus thoracis its lateral borders to the aponeurosis of the abdominal muscles above it articulates the lower end of the body and on the front of each superior angle presents a facet for part of the cartilage of the seventh rib by its pointed extremity it gives attachment to the linea alba the xyphoid process varies much in form it may be broad and thin pointed, bipid perforated, curved or deflected considerably to one or other side structure the sternum is composed of highly vascular cancerous tissue covered by a thin layer of compact bone which is thickest in the manubrium and has a variety of facets for the clavicles ossification the sternum originally consists of two cartilaginous bars situated one on either side of the median plane and connected with the cartilages of the upper nine ribs of its own side these two bars fuse with each other along the middle line to form the cartilaginous sternum which is ossified from six centers one for the manubrium the ossific centers appear in the intervals between the articular depressions for the cost of cartilages in the following order in the manubrium and first piece of the body during the sixth month in the second and third pieces of the body during the seventh month of fetal life in its fourth piece during the first year after birth and in the xyphoid process between the fifth and eighteenth years the centers make their appearance and proceed gradually downward footnote out of 141 sternum between the time of birth and the age of 16 years Patterson found the fourth and lowest center for the body present only in 38 cases that is 26.9% end footnote to these may be added the occasional existence of two small episternal centers which make their appearance on either side of the jugular notch they are probably vestiges of the episternal bone of the monotromata and lizards occasionally some of the segments are formed from more than one center the number and position of which vary thus the first piece may have two, three, or even six centers when two are present they are generally situated one above the other the upper being the larger the second piece has seldom more than one the third, fourth, and fifth pieces are often formed from two centers placed laterally the irregular union of which explains the rare occurrence of the sternal foreman or of the vertical fissure which occasionally intersects as part of the bone constituting the alformation known as fissura sterni these conditions are further explained by the manner in which the cartilaginous sternum is formed more rarely still the upper end of the sternum may be divided by a fissure union of the various centers of the body begins about purity and proceeds from below upward by the age of 25 they are all united the xyphoid process may become joined to the body before the age of 30 this occurs more frequently after 40 on the other hand it sometimes remains ununited in old age in advanced life the manubrium is occasionally joined to the body by bone when this takes place however the body tissue is generally only superficial the central portion of the interfering cartilage remains unalcified articulations the sternum articulates on either side with the clavicle and upper 7 costal cartilages end of section 17 recording by Jennifer Stearns Concord, New Hampshire