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direction ceases. In cases where the epiphysis enters into the formation of a joint, the cartilage over the articular area persists and undergoes neither calcification nor ossification.
Vascular, Lymph, and Nervous Supply of Bone. From what has been said it will be gathered that the vascular supply of the bone is derived from the vessels of the periosteum. These consist of fine arteries which enter the surface of the diaphysis and epiphysis; but in addition there is a larger trunk which enters the diaphysis and reaches the medullary cavity. This is called the nutrient artery of the bone. The direction taken by this vessel varies in different bones. In the upper limb the artery runs distally in the case of the humerus and proximally in the radius and ulna; in the lower limb the nutrient vessel of the femur is directed towards the proximal extremity of the shaft, whilst in the tibia and fibula it follows a distal course. The direction of the nutrient artery in the bone is a mechanical result of the unequal growth of the two extremities of the bone. During the greater part of intra-uterine life the principal nutrient arteries of the long bones are directed towards the distal extremity of the limb. In the process of development the point of entrance of the artery is turned away from the epiphysis which furnishes the greatest amount of bone, and thus, together with the nutrient canal, acquires an obliquity directed towards the extremity of the bone which develops last (Piollet, J. de l'Anat. et de la Phys., 1905, p. 57).
It may assist the memory to point out that when all the joints are flexed, as in the position occupied by the foetus in utero, the direction taken by the vessels is the same, and corresponds to a line passing from the head towards the tail-end of the embryo. Consequently, in the upper limb the vessels run towards the elbow, whilst in the lower limb they pass from the knee.
The veins which permeate the spongy texture of the bone are large and thinwalled. They do not accompany the arteries, and, as a rule, in long bones they escape through large openings near the articular surfaces. In flat bones they occupy channels within the diploë, and drain into an adjacent sinus, or form communications with the superficial veins of the scalp.
The lymph vessels are mainly periosteal, but enter the bone along with the blood-vessels and become perivascular.
The nerves which accompany the arteries are probably destined for the supply of the coats of these vessels. Whether they end in the bony tissue or not is unknown.
The attention of anatomists has long been directed to the elucidation of the laws which regulate bone-growth. Our present knowledge of the subject may be briefly summarised in the following generalisations:
1. In bones with a shaft and two epiphyses, the epiphysis towards which the nutrient artery is directed is the first to unite with the shaft.
2. In bones with a shaft and two epiphyses, as a rule the epiphysis which commences to ossify latest unites soonest with the shaft. (The fibula is a notable exception to this rule.)
3. In bones with a shaft and one epiphysis the nutrient artery is directed towards the end of the bone which has no epiphysis. (This arrangement holds good in the case of the clavicle, the metacarpus, metatarsus, and phalanges.)
4. When an epiphysis is ossified from more than one centre, coalescence takes place between the separate ossific nuclei before the epiphysis unites with the shaft. Highly suggestive, too, are the following propositions-That ossification first commences in the epiphysis which ultimately acquires the largest relative proportion to the rest of the bone, and that the ossification of the epiphysis is also correlated with its functional importance. In cases of long bones with only one epiphysis, the epiphysis is placed at the end of the bone where there is most
The vertebral column of man consists of thirty-three superposed segments or vertebræ. In the adult, certain of these vertebrae have become fused together in the process of growth to form bones, the segmental arrangement of which
is somewhat obscured, though even in their fully developed condition sufficient evidence remains to demonstrate their compound nature. The vertebræ so blended are termed the fixed or false vertebræ, whilst those between which osseous union has not taken place are described as the movable or true vertebræ. This fusion of the vertebral segments is met with at each extremity of the vertebral column, more particularly the lower, where the column is modified to adapt it for union with the girdle of the lower limb, and where also man's degenerated caudal appendage is situated. But a partial union of the vertebral segments also takes place at the upper end of the column, between the highest two vertebræ, in association with the mechanism necessary to provide for the movements of the head on the column.
For descriptive purposes the vertebral column is subdivided according to the regions through which it passes. Thus the vertebræ are described as cervical (vertebræ cervicales), thoracic (vertebræ thoracales), lumbar (vertebræ lumbales), sacral (vertebræ sacrales), and coccygeal (vertebræ caudales), according as they lie in the regions of the neck, thorax, loins, pelvis, and tail. The number of vertebræ met with in each region is fairly constant, though, as will be hereafter pointed out, variations may occur in the number of the members of the different series. The vertebræ in man are thus apportioned-7 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 4 or 5 coccygeal; the former three groups comprise the true or movable vertebræ, the latter two the false or fixed vertebræ. The vertebral formula may be thus expressed :
Movable or True Vertebræ.
Fixed or False Vertebræ.
The vertebræ, though displaying great diversity of characters in the regions above enumerated, yet preserve certain features in common. All possess a solid part, corpus vertebræ or body (centrum); all have articular processes by which they articulate with their fellows; most have muscular processes developed in connexion with them; whilst the majority display a vertebral foramen formed by the union of a bony arcus vertebræ (vertebral arch) with the body. These common characters may best be studied by selecting for description an intermediate member of the series. For this purpose one of the middle or lower thoracic vertebræ may be chosen.
A typical vertebra may be described as consisting of a body composed of a mass of spongy bone, more or less cylindrical in form. The size and shape of the body is liable to considerable variation according to the vertebra examined. The superior and inferior surfaces of the body are very slightly concave dorsoventrally and from side to side, due to the thickening of the bone around its margins. In the recent condition these surfaces afford attachment for the intervertebral fibro-cartilages, which are placed like pads between the bodies of the movable members of the series. The circumference of the body, formed as it is of more compact bone than the interior, is usually slightly concave from above downwards, though the dorsal surface becomes flat, where the body forms the anterior boundary of the vertebral foramen, at which point it is usually slightly concave from side to side. The vertical surfaces of the body are pierced here and there by foramina for the passage of nutrient vessels, more particularly on the dorsal surface, where a depression of considerable size receives the openings of the canals through which some of the veins which drain the body of the bone escape. Connected with the body posteriorly there is a bony vertebral arch, which, by its union with the body, encloses a foramen of variable size, called the vertebral foramen. When the vertebræ are placed on the top of each other these foramina form, with the uniting ligaments, a continuous canal-vertebral canal— in which the spinal medulla, with its coverings, is lodged. The vertebral arch, which is formed by the union of the roots of the vertebral arches (pedicles) and laminæ, besides enclosing the vertebral foramen, also supports the spinous and transverse processes, which may be regarded as a series of levers to which muscles are
attached, whilst others are articular and assist in uniting the different vertebræ together by means of a series of movable joints. The roots of the vertebral arches (O.T. pedicles) are the bars of bone which pass from the dorsum of the body of the vertebræ, one on each side, to the points where the articular processes are united to the arch. Each root is compressed from side to side, and has rounded superior and inferior borders. Since the vertical breadth of the roots is not as great as the height of the body to which they are attached, it follows that when the vertebræ are placed one above the other a series of intervals is left between the roots of the vertebral arches of the different vertebræ. These spaces, enclosed anteriorly by the bodies of the vertebræ and their intervertebral fibrocartilages and posteriorly by the coaptation of the articular processes, form a series of holes communicating with the vertebral canal; they are called the intervertebral foramina, and allow the transmission of spinal nerves and vessels. As each intervertebral foramen is bounded above and below by one of the roots of talis transthe vertebral arch, the grooved surfaces in correspondence with the upper and lower borders of the roots are called the incisurae vertebrales superior et inferior (upper and lower intervertebral notches). Posteriorly, the two roots of each vertebral arch are united by two somewhat flattened plates of bone-the lamina-which converge towards the median plane, and become fused with the root of the projecting spinous process. The vertical lengths or heights of the lamine and their sloping arrangement are such, that, when the vertebræ are articulated together, they leave little space between them, thus enclosing fairly completely the vertebral canal, of which they form the posterior wall. The edges and inner surfaces of the laminæ are rough for the attachment of the ligaments which bind them together.
process Root of the vertebral arch Fovea costalis inferior Body
The muscular processes are three in number, viz., two processus transversion either side-and one central or median, the processus spinosus. The transverse processes project laterally on either side from the arch at the point where the root of the vertebral arch joins the lamina. The spinous process extends backwards in the median plane from the point of fusion of the laminæ. The spinous processes display much variety of length and form.
The articular processes (zygapophyses), four in number, are arranged in pairsone superior, the other inferior; the former are placed on the upper part of the arch where the roots of the arch (pedicles) and laminæ join, the latter on the lower part of the arch in correspondence with the superior. Whilst differing much in the direction of their articular surfaces, the upper have generally a backward tendency, whilst the lower incline forwards.
articular process Root of the vertebral arch
FIG. 106.-FIFTH THORACIC VERTEBRA, (A) as viewed from the right side, (B) as viewed from above.
THE TRUE OR MOVABLE VERTEBRÆ.
The cervical vertebræ, seven in number, can be readily distinguished from all the other vertebræ by the fact that their transverse processes are pierced by a foramen. The highest two, and the lowest, require special description; the remaining four conform to a common type.
Their bodies, the smallest of all the true vertebræ, are oblong in shape, the transverse width being much longer than the antero-posterior diameter. The superior surface, which slopes from behind forwards and downwards, is concave from side to side, owing to the marked projection of its lateral margins. Its anterior lip is rounded off, whilst its posterior edge is sharply defined. The inferior surface, which is more or less saddle-shaped, is directed downwards and backwards. It is convex from side to side, and concave from before backwards, with a slight rounding off of the projecting anterior lip. The vertical diameter of the body is small in proportion to its width. The anterior surface is flat in the middle line, but furrowed laterally. The posterior surface, which is rough and pierced by many small foramina, is flat from side to side and above downwards; it forms part of
FIG. 107.-FOURTH CERVICAL VERTEBRA, (A) from above, and (B) from the right side.
the anterior wall of the vertebral foramen. The lateral aspect of each body, particularly in its upper part, is fused with a root of the arch and with the costal part of a transverse process, and forms the medial wall of a foramen transversarium.
The roots of the vertebral arches, which spring from the posterior half of the lateral aspects of the body, about equidistant from their superior and inferior margins, are directed horizontally backwards and laterally. The superior and inferior notches are nearly equal in depth. The lamina are long, and about as high as the bodies of the bone. The vertebral canal is larger than in the thoracic and lumbar regions; its shape is triangular, or more nearly semilunar.
The transverse processes, so called, are pierced by the foramen transversarium (vertebrarterial or transverse foramen). They consist of two parts-the part behind the foramen, which springs from the vertebral arch and is the true transverse process, and the part in front, which is homologous with a rib in the thoracic portion of the column. These two processes are united laterally by a bridge of bone, which thus converts the interval between them into a foramen, and they terminate, beyond the bridge, in two tubercles, known as the anterior and posterior tubercles.
The general direction of the transverse processes is laterally, slightly forwards, and a little downwards, the anterior tubercles lying medial to the posterior. The two tubercles are separated above by a groove directed laterally, downwards, and forwards; along this the spinal nerve trunk passes. The foramen transversarium is often subdivided by a spicule of bone. In the recent condition and in the cases of the upper six vertebræ it is traversed by the vertebral artery and vein.
The spinous processes, which are directed backwards, are short, compressed vertically, and bifid. The articular processes are supported on cylindrical masses of
bone fused with the arch where the roots of the vertebral arches and the lamina join. These cylinders are sliced away obliquely above and below, so that the superior articular facets, more or less circular in form, are directed upwards and backwards, whilst the corresponding inferior surfaces are turned downwards and forwards.
The Atlas or First Cervical Vertebra.-This bone may be readily recognised by the absence of the body and spinous process. It consists of two lateral masses, which support the articular and transverse processes. The lateral masses are themselves united by two curved bars of bone, the anterior and posterior arches, of which the former is the stouter and shorter. Each lateral mass is irregularly six-sided, and so placed that it lies closer to its fellow of the opposite side in front than behind. Its upper surface is excavated to form an elongated oval facet called the superior articular fovea, which is concave from before backwards, and inclined. obliquely medially; not infrequently this articular surface displays indications of division into two parts. The superior articular foveæ are for the reception of the condyles of the occipital bone.
The inferior articular foveæ or facets are placed on the inferior surfaces of the lateral masses. Of circular form, they display a slight side-to-side concavity, though flat in the antero-posterior direction. Their disposition is such
that their surfaces incline
1. Posterior arch.
3. Tubercle for transverse
4. Anterior arch.
tuberculum anterius (an- 5. Anterior tubercle. terior tubercle). In corre
spondence with this, on the posterior surface of this arch is a circular facet. (fovea dentis) for articulation with the dens of the epistropheus.
The medial surface of the lateral mass is rough and irregular, displaying a tubercle for the attachment of the transverse ligament of the atlas, which passes across the space included between the two lateral masses and the anterior arch, thus holding the dens of the epistropheus in position. Behind each tubercle there is usually a deep pit, opening into the bottom of which are the canals for the nutrient vessels.
FIG. 108. THE ATLAS FROM ABOVE.
6. Surface for articulation with dens. 7. Superior articular surface.
8. Foramen for vertebral artery. 9. Groove for vertebral artery. 10. Posterior tubercle.
Laterally to the lateral mass, and principally from its upper half, the transverse process arises by two roots which include between them the foramen transversarium. The transverse process is long, obliquely compressed, and down-turned; the anterior and posterior tubercles have fused to form one mass.
The posterior arch arises in part from the posterior surface of the lateral mass, and in part from the posterior root of the transverse process. Compressed from above downwards anteriorly, where it bounds a groove which curves around the posterior aspect of the superior articular process, which groove is also continuous laterally with the foramen transversarium, the posterior arch becomes thicker medially, at which point it displays posteriorly a rough irregular projection-the tuberculum posterius (posterior tubercle), the feeble representative of the spinous process. A prominent little tubercle, arising from the posterior extremity of the superior articular process, overhangs the groove above mentioned, and not in