6J3 



SKELETON. 



SKELETON. 



811 



upper surface also it has two elevations (1), between which the lower 

 part of the vertebra next above is received. The lamina; (3) of the 

 arch are long and narrow, and inclose a large somewhat triangular 

 vertebral foramen (4). The spinous process is short and bifurcated. 

 The transverse processes (6) are short, horizontal, and bifurcated; 

 and each has a foramen at its base, through which the vertebral artery 

 passes. The superior articular processes have their smooth oval 

 surfaces directed backwards and inwards, and they receive between 

 them the inferior processes of the vertebra next above, whose articular 

 surfaces are turned in the opposite direction. 



But these distinctive characters of the several sets of vertebra are 

 only general : they are merged at the extremes of each set, the lowest 

 dorsal being very like the upper lumbar, and the upper dorsal like 

 the lowest cervical. Some single vertebra, moreover, have particular 

 character!'. The first of the cervical set, or Atlas, is scarcely more 

 than a flat ring of bone with two long transverse processes, two 

 superior articulating processes, with largo oval concave surfaces 

 opposed to those of the occipital bone, and two inferior, with large 

 flat horizontal sui faces, which articulate with those of the second 

 vertebra. By the former joint the chief movements of depression 

 and elevation of the head upon the nock are permitted ; by the latter, 

 those of rotation. The second cervical vertebra (named Axis, or 

 Vertebra Dentata) has a large pointed process, which rises from the 

 upper part of its body, and is inclosed in a riug formed by the anterior 

 half of the Atlas, and a transverse ligament passing from one side to 

 the other of its body. lu this ring the process of the Axis rotates 

 freely ; or rather, the Atlas, with the head supported on it, moves 

 round that process, and upon the flat superior articulating surfaces of 

 the Axis. The seventh cervical vertebra has a remarkably long 

 spinous process, to which is attached the Ligamentum Nuchos, a 

 strong elastic band for the support of the head when inclined forwards, 

 but which, as well as the npiuous processes of all the adjacent verte- 

 bne, is much more developed in animals that move horizontally and 

 graze, than in man. This vertebra is also marked by having a small 

 rib-like process in front of its transverse process; it is a rudimental 

 rib, and is analogous to the cervical ribs of serpents and many other 

 animals iu whom the chest is more elongated than in man. The first 

 and the last three dorsal vertebra! have each, at the upper borders of 

 the body, surfaces for articulation with the whole head of the corre- 

 sponding rib. The fifth lumbar has its lower surface cut obliquely 

 upwards to articulate with the top of the sacrum and form the pro- 

 montory of the pelvis. 



Viewed as a whole, the human vertebral column forms a kind of 

 pyramid with its base at the sacrum, and its truncated summit at the 

 head. It is not however regularly pyramidal ; for, as seen from the 

 front, it becomes gradually smaller from the base to the fourth dorsal 

 vertebra; then it widens to the seventh cervical, and then again 

 becomes narrower to the second. In the adult it has well-marked 

 c m-viitures. (Fig. 2.) From the head it is first curved slightly for- 

 wards to the last cervical vertebra ; then its dorsal portion forms an 

 arch with its convexity backwards and ending at the last dorsal; and 

 then again in the lumbar region it arches forwards to the base of the 

 last lumbar vertebra. These directions of the column have relations 

 to the naturally erect posture of the human body [MAN] : in corre- 

 spondence with them the bodies of the cervical and lumbar vertebra;, 

 and their intervertebral cartilages, are thicker before than behind, and 

 those of the dorsal thicker behind than before. 



The Spine serves several offices in the economy. One is that of 

 guarding the spinal marrow, which, with the roots of its nerves, is 

 inclosed in the long canal formed by the superposed rings of the 

 several vertebrae. The spini-cerebrate form of nervous system, which 

 consists of a brain and longitudinal axis, both placed on the same side 

 of the digestive canal, is intimately connected with all the rest of the 

 organisation of the animals in which it exists; and being always in- 

 closed in a skull and spinal canal, the vertebral column is taken as the 

 most obvious character of the four classes of animals which have this 

 plan of nervous system. These therefore, namely, Mammals, Birds, 

 Fish, and Reptiles, are called Vertebrata; and the other portion of 

 the nnimal kingdom, whatever be the plan of their nervous centres, 

 Inrertebrutn. 



The spine is also the main support of all the rest of the skeleton. 

 The head, the rib?, and the pelvis directly articulate with it; an 1 

 through the medium of the pelvis and sternum, it suspends both tlie 

 lower and upper extremities. It is the passive instrument of all the 

 motions of the trunk, and the centre about which each of the limbs 

 as a whole is moved. For these purposes it is adapted by combining 



firmness with flexibility and lightness. Flexibility is obtained by its 

 being composed of so many pieces separated from each other by layers 

 of elastic tissue ; and its strength is secured by these layers, which are 

 at the same time firm bonds of union, and by numerous other strong 

 ligaments passing from bone to bone. In its own movements, extent 

 is combined with security by each vertebra (except the first) having 

 but little motion on those next to it; the larger movements being the 

 result of the combination of a number of such small ones. The 

 directions of the processes and the diverse modes in which they are 

 locked one within another, determine the degrees in which, in each part 

 of the column, the several motions of flexion and extension in all direc- 

 tions, and of rotation, can be performed. The pyramidal form of the 

 whole is adapted to the accumulated weight which the lower vertebra? 

 have to bear. The curvature in the back increases the capacity of the 

 chest. The spinous and transverse processes especially serve for the 

 attachments of muscles of the head, chest, back, shoulders, and pelvis. 

 The elastic cartilages interposed between the bodies break the shock 

 of any violence upon one end of the body, and both they and the 

 iut erlamiuar ligaments tend to keep the spine straight, and so diminish 

 the muscular action necessary to hold the body erect. 



The base of the spinal column rests on the top of the Sacrum 

 (fig. 2, 5), which, though commonly described as part of the pelvis, is 

 indeed a continuation of the column, and is composed of five or six 

 rudimental vertebrae, which after about the tenth year become con- 

 solidated. The Sacrum (fig. 6, A) has a triangular outline, the base 

 being above ; and it articulates with the last lumbar vertebra, so as to 

 form an obtuse angle, the promontory, by means of an intervertebral 

 substance and the other parts common to the rest of the vertebral 

 joints. Its anterior surface, which in the erect posture looks obliquely 

 downwards, is concave, and on it are four or five transverse lines, the 

 traces of the divisjons between the bodies of the original vertebras. 

 At each end of these lines are as many holes (the anterior sacral fora- 

 mina), which give passage to the anterior branches of the sacral nerves. 

 Outside these holes the Sacrum has a smooth surface composed of the 

 coalesced transverse processes of its several vertebra. Along the 

 posterior convex surface the Sacrum presents corresponding traces of 

 its composition. Its upper border is surmounted by two regularly- 

 formed articular processes, which are connected with those of the last 

 lumbar vertebra, and leading downwards from these, in converging 

 lines on either side of the middle, is a series of slight elevations, the 

 traces of other rudimental articular processes. Along the middle Hue 

 are three or four higher ridges, the traces of spinous processes, and 

 between these and the former are on either side four or five foramina, 

 which give passage to the posterior branches of the sacral nerves. 

 These and the anterior sacral'foratniua already mentioned are analogous 

 to the intervertebral foramina; and they both lead into the sacral 

 canal, which runs through the whole length of the Sacrum, and con- 

 tains the cauda equina, or tuft of the last roots of the spinal nerves. 

 The outline of the sacral canal is triangular ; it grows smaller from 

 above downwards, and is closed iu behind by a layer analogous to 

 the arches of the regular vertebra;. It is continuous above with the 

 spinal canal, and below is, in the dry bones, open in the middle line, 

 the arch of the last sacral vertebra being deficient ; but in the recent 

 subject is closed by dura mater and dense ligament. The sides of the 

 Sacrum are thick above, and become gradually thinner below. In the 

 former situation they are marked by large rough oval surfaces, directed 

 backwards and somewhat outwards, by which the Sacrum is on either 

 side articulated immoveably with the iliac bones to form the sac'-j- 

 iliac symphyses. The lower end of the Sacrum has a plain oval 

 surface, which is fitted to the upper surface of the first bone of the 

 coccyx. 



The Coccyx is the lowest part of the whole vertebral column. Its 

 bones form the interior frame of the tail in brutes, but in man are 

 email, short, and not more than four or five in number. The upper- 

 most is by far the largest, and is surmounted by two processes called 

 Cornua, the extremities of which arc adapted to those of two similar 

 processes by which the sides of the lower end of the sacral canal are 

 bounded. The three or four lower pieces of the Coccyx have a some- 

 what oval outline, and are rather deeper than they are broad. Up to 

 a late period of life they are articulated moveably with thin layers of 

 interposed cartilage. 



The Sacrum and Coccyx form the middle posterior part of the 

 Pelvis ; its sides and front are formed by the boues called Ossa Inuo- 

 riiiiin M (fiy. 6, B). Each of these is in the young subject composed of 

 three parts, which are usually described separately, as the Ilium, or 

 Haunch-Bone (a), the Ischium (6), and the Pubes (c). These thr<e 

 meet at the acetabulum (1), the hemispherical cavity in which the 

 head of the thigh bone is lodged, and of which the Ischium forms 

 nearly three-fifths, the Ilium somewhat more than one-fifth, and the 

 Pubes rather less than one-fifth. 



The Ilium forms the upper broad and expanded part of the Pelvis. 

 Its outline is somewhat fan-shaped, and in the greater part of its 

 extent it is flat and thin. That surface which is directed forwards 

 and inwards towards the cavity of the pelvis is slightly concave, and 

 gives attachment to the strong iliac muscle by which the thigh is 

 raised towards the pelvis. Its upper border has a thick strong rim 

 (2), the cristii ilii, to which parts of the three broad muscles of the 

 abdomen are attached, and which serves for a fixed point towards 



