tion spreads eccentrically towards the margins of the bone, where ultimately the sutures are formed. Here the growth rendered necessary by the expansion of the cranium takes place through the agency of an intervening layer of vascular connective tissue rich in osteoblasts; but in course of time the activity of this is reduced until only a thin layer of intermediate tissue persists along the line of the suture; this may eventually become absorbed, leading to the obliteration of the suture by the osseous union of the contiguous bones. Whilst the expansion of the bone in all directions is thus provided for, its increase in thickness is determined by the activity of the underlying and overlying strata. These form the periosteum, and furnish the lamella which constitute the inner and outer compact osseous layers.

Ossification in Cartilage.-Cartilage bones are those which are preformed in cartilage, and include most of the bones of the skeleton. Their growth is often described as endochondral and ectochondral, the former term implying the deposition of membrane bone in the centre of the cartilage, while the latter signifies a deposit of membrane bone on the surface of the cartilage, the osteogenetic layer on the surface of the cartilage being named the perichondrium till once bone has been formed, when it is called the periosteum.

In a cartilage bone changes of a similar nature occur. The cartilage, which may be regarded histologically as white fibrous tissue + chondro-sulphuric acid and a certain amount of lime salts, undergoes the following changes :-First, the cartilage cells being arranged in rows, become enlarged; secondly, the matrix between the cartilage cells becomes calcified by the deposition of an additional amount of lime salts; thirdly, the rows of cells become confluent; and, fourthly, into the spaces so formed extend the blood-vessels derived from the vascular layer of the periosteum. Accompanying these vessels are osteoblasts and osteoclasts, the former building up true bone at the expense of the calcified cartilage, the latter causing an absorption of the newly formed bone, and leading to its conversion into a marrow cavity, so that in due course all the cartilage or its products disappear. At the same time that this is taking place within the cartilage, the perichondrium is undergoing conversion into the periosteum, an investing membrane, the deeper stratum of which, highly vascular, furnishes a layer of osteoblast cells which serve to develop the circumferential lamellæ of the bone. It is by the accrescence of these layers externally, and their absorption internally through the action of the osteoclast cells, that growth takes place transversely. A transverse section of the shaft of a long bone shows this very clearly. Centrally there is the marrow cavity, formed primarily by the absorption of the calcified cartilage; around this the spongy tissue produced by the partial erosion of the primary periosteal bone is disposed, whilst externally there is the dense envelope made up of the more recent periosteal growth.

Growth of Bone.-The above description, whilst explaining the growth of bone circumferentially, fails to account for its growth in length; hence the necessity in long bones for some arrangement whereby ossification may take place at one or both extremities of the body. This zone of growth is situated where the ossified body becomes continuous with the cartilaginous epiphysis. In addition, within these epiphysial cartilages calcification of the cartilage takes place centrally, just as in the diaphysis. The two parts of the bone, viz., the diaphysis and epiphysis, are thus separated by a layer of cartilage, sometimes called the cartilage of conjugation, as yet uncalcified, but extremely active in growth owing to the invasion of vessels and cells from a vascular zone which surrounds the epiphysis. The nucleus of the epiphysis becomes converted into true bone, which grows eccentrically. This arrangement provides for the growth of the shaft towards the epiphysis, and the growth of the epiphysis towards the shaft; so that as long as the active intervening layer of cartilage persists, extension of growth in a longitudinal direction is possible. As might be expected, experience proves that growth takes place more actively, and is continued for a longer time, at the end of the bone where the epiphysis is the last to unite. In consequence, surgeons sometimes term this the "growing end of the bone." Subsequently, however, at variable periods the intervening layer of cartilage becomes calcified, and true bony growth occurs within it, thus leading to complete osseous union between the shaft and epiphysis. When this has taken place all further growth in a longitudinal

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

movement.

COLUMNA VERTEBRALIS.

The vertebral column of man consists of thirty-three superposed segments or vertebræ. In the adult, certain of these vertebræ 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 :

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 canalin 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 the 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

Fovea costalis
transversalis

Transverse

process

Superior

articular process Root of the vertebral arch

FIG. 106.-FIFTH THORACIC VERTEBRA, (A) as viewed from the right side, (B) as viewed from above.

laminæ 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.

The muscular processes are three in number, viz., two processus transversi— one on 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.

THE TRUE OR MOVABLE VERTEBRÆ.

Vertebræ Cervicales.

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