OF THE SPINE. 



189 



occupying the interval between three or four vertebrae, 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 infiltration. 



The Intervertebral Substance (fig. 116) is a lenticular disk of fibro-cartilage, 

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

 axis to the sacrum, forming the chief bond of connection between these 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, circular in the dorsal. 

 Their size is greatest in the lumbar region. In thinness, 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 contribute, in a great measure, to the curvatures 



Fig. 116. Vertical Section of two Vertebrae and their Ligaments, from the Lumbar Region. 



ANTERIOR 

 COMMON 



IICT 



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 vertebras. The intervertebral disks 

 form about one-fourth of the spinal column, exclusive of the first two vertebrae ; 

 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 vertebrae ; and by their 

 circumference are closely connected in front to the anterior, and behind to the 

 posterior, common ligament ; whilst, in the dorsal region, they are connected 

 laterally to the heads of those ribs which articulate with two vertebrae, by means 

 of the interarticular ligament. 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 



