OF THE VERTEBRAL COLUMN. 267 



gins of the vertebras, where the ligament is more intimately adherent, it is 

 broad, and presents a series of dentations with intervening concave margins ; 

 but it is narrow and thick over the centre of the bodies, from which it is sepa- 

 rated by the venae basis vertebrse. This ligament is composed of smooth, shining, 

 longitudinal fibres, denser and more compact than those of the anterior liga- 

 ment, and composed of a superficial layer occupying the interval between 

 .three or four vertebrse, and of a deeper layer which extends between one 

 vertebra and the next adjacent to it. It is separated from the dura mater of 

 the spinal cord by some loose filamentous tissue, very liable to serous in- 

 filtration. 



The Intervertebral Substance (Fig. 183) is a lenticular disk of fibro-cartilage, 

 interposed between the adjacent surfaces of the bodies of the vertebrae, from 

 the axis to the sacrum, and forming the chief bond of connection between 

 those bones. These disks vary in shape, size, and thickness, in different parts 

 of the spine. In shape they accurately correspond with the surfaces of the 

 bodies between which they are placed, being oval in the cervical and lumbar 

 regions, and circular in the dorsal. Their size is greatest in the lumbar region. 

 In thickness they vary not only in the different regions of the spine, but in 

 different parts of the same region : thus, they are uniformly thick in the lumbar 

 region ; thickest, in front, in the cervical and lumbar regions which are convex 

 forwards ; and behind, to a slight extent, in the dorsal region. They thus con- 

 tribute, in a great measure, to the curvatures of the spine in the neck and 

 loins ; whilst the concavity of the dorsal region is chiefly due to the shape of 

 the bodies of the vertebrae. The intervertebral disks form about one-fourth of 

 the spinal column, exclusive of the first two vertebra ; they are not equally 

 distributed, however, between the various bones ; the dorsal portion of the 

 spine having, in proportion to its length, a much smaller quantity than in the 

 cervical and lumbar regions, which necessarily gives to the latter parts greater 

 pliancy and freedom of movement. The intervertebral disks are adherent, 

 by their surfaces, to the adjacent parts of the bodies of the vertebra? ; and 

 by their circumference are closely connected in front to the anterior, a'nd behind 

 to the posterior common ligament ; whilst, in the dorsal region, they are con- 

 nected laterally, by means of the interarticular ligament, to the heads of those 

 ribs which articulate with two vertebras ; they, consequently, form part of the 

 articular cavities in which the heads of these bones are received. 



The intervertebral substance is composed, at its circumference, of laminae of 

 fibrous tissue and fibro-cartilage ; and, at its centre, of a soft, elastic, pulpy 

 matter. The laminae are arranged concentrically one within the other, with 

 their edges turned towards the corresponding surfaces of the vertebrae, and 

 consist of alternate plates of fibrous tissue and fibro-cartilage. These plates 

 are not quite vertical in their direction, those near the circumference being 

 curved outwards and closely approximated; whilst those nearest the centre curve 

 in the opposite direction, and are somewhat more widely separated. The fibres 

 of which each plate is composed, are directed, for the most part, obliquely from 

 above downwards ; the fibres of an adjacent plate have an exactly opposite 

 arrangement, varying in their direction in every layer; whilst in some few they 

 are horizontal. This laminar arrangement belongs to about the outer half of 

 each disk, the central part being occupied by a soft, pulpy, highly elastic sub- 

 stance, of a yellowish color, which rises up considerably above the surrounding 

 level, when the disk is divided horizontally. This substance presents no con- 

 centric arrangement, and consists of white fibrous tissue, with cells of variable 

 shape and size interspersed. The pulpy matter, which is especially well de- 

 veloped in the lumbar region, is separated from immediate contact with the 

 vertebrae by the interposition of thin plates of cartilage. 



