THE 1NTERVERTEBRAL DISCS. 4.i:> 



in each lamina, and run obliquely between two vertebrae ; but the 

 direction is reversed in alternate layers (fig. 180). This arrange- 

 ment is best seen in the thicker discs between the lumbar 

 vertebra? ; and it may be demonstrated by dissecting layer after 

 layer. 



The central pulpy portion of the disc (fig. 182, 6) is very soft, inner part 

 and, being tightly confined by the surrounding laminae, it projects substance 

 when two vertebrae and the interposed mass are sawn through. 

 Placed nearer the back than the front of the disc, it is more marked situation 

 in the loins and neck than in the dorsal region. It has a yellowish largest, 

 colour, and is deficient in the stratiform arrangement so conspicuous 

 in the circumferential part. 



The surfaces of the vertebrae in contact with the disc have a Cartilage 

 cartilaginous covering, which may be seen by cutting the inter- bone" 08 

 vertebral substance from the bone. 

 Over the centre of the osseous 

 surface it forms a continuous layer, 

 but it is wanting towards the 

 circumference. 



The discs are thicker between 



the lumbar and cervical, than 3I' 'T '"""""^l Thickness 



between the upper and middle 

 dorsal vertebrae ; and in the loins 

 and neck, where the spinal 

 column is convex forwards, they 

 are deepest at the anterior 

 edge, being wedge-shaped. The 

 thickest piece of all, and the FIG. 182. HORIZONTAL SECTION OF 

 most wedge-shaped, is between Ax ISTERVERTEBRAL Disc, 



the fifth lumbar vertebra and the a. Laminar external part, 

 sacrum. The total thickness of & Pulpy central substance, 

 the discs amounts to about a 

 fourth of the length of the moveable part of the spinal column. 



Use. The intervertebral discs form the chief bond of union They bind 

 between the several bones of the column ; and mainly by reason of together y 

 their strength, displacement of the vertebrae is a rare occurrence. 



In the movements of the spine the vertebrae revolve around the but permit 

 central pulpy portion of the disc; and the extent of the move- movemert ; 

 nient between two segments of the column is limited by the 

 circumferential laminar portion of the discs. 



Through their wedge-shaped form the discs are chiefly instru- render 

 mental in giving rise to the convexity of the spine in the loins and 

 neck ; and by their elasticity they moderate the effect of jars or 

 shocks transmitted through the column. 



LIGAMENTS OF THE NEURAL ARCHES AND PROCESSES. The articular several 

 processes of the vertebrae are connected by synovial joints with 

 surrounding capsules ; the neural arches are united by yellow processes, 

 ligaments ; the spinous processes have one band along their tips and 

 others in the intervals between them ; and some of the transverse 

 processes are joined by fibrous bands. 



