320 



THE ARTICULATIONS 



canal, and extends along the posterior surface of the bodies of the vertebrae from 

 the body of the axis above, where it is continuous with the occipito-axial ligament, 

 to the sacrum below. It is broader above than below, and thicker in the dorsal 

 than in the cervical or lumbar regions. In the situation of the intervertebral 

 substance and contiguous margins of the vertebrae, where the ligament is more 

 intimately adherent, it is broad, and presents a series of dentations with inter- 

 vening concave margins ; but it is narrow and thick over the centre of the bodies, 

 from which it is separated by the vence basis vertebrae. This ligament is composed 

 of smooth, shining, longitudinal fibres, denser and more compact than those of the 

 anterior ligament, and composed of a superficial layer 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 



ANTERIOR COMMON 



POSTERIOR 

 COMMON 

 LIGAMENT. 



FIG. 227. Vertical section of two vertebree and their ligaments, from the lumbar region. 



the spinal cord by some loose connective tissue which is very liable to serous 

 infiltration. 



The Intervertebral Substance (Figs. 227, 236) is a lenticular disk of composite 

 structure 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 disk : thus, they are much thicker in front than behind in the cervical 

 and lumbar regions, while they are uniformly thick in the dorsal region. 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 a thin layer of hyaline cartilage which 

 covers the upper and under surfaces of the bodies of the vertebrae, and in which the 

 epiphysial plate develops, and by their circumference are closely connected in. 



