226 



THE ARTICULATIONS 



Each disc is composed of two portions — -a circumferential laminar, and a central pulpy- 

 portion; the former tightly surrounds and braces in the latter, and forms somewhat more than 

 half the disc. The fibrous ring [annulus fibrosus] or laminar portion consists of alternating 

 layers of fibrous tissue and fibro-cartilage; the component fibres of these layers are firmly con- 

 nected with two vertebrae, those of one passing obliquely down and to the right, those of the 

 next down and to the left, making an X -shaped arrangement of the alternate layers. A few 

 of the superficial lamellae project beyond the edges of the bodies, theif fibres being connected 

 with the edges of the anterior and lateral surfaces; and some do not completely surround the 

 rest, but terminate at the intervertebral foramina, so that on horizontal section the circum- 

 ferential portion is seen to be thinner posteriorly. The more central lamella; are incomplete, 

 less firm, and not so distinct as the rest; and as they near the pulp they gradually assume its 

 characters, becoming more fibro-cartilaginous and less fibrous, and have cartilage cells in their 

 structure. 



The pulpy nucleus [nucleus pulposus] or central portion is situated somewhat behind the 

 centre of the disc, forming a ball of very elastic and tightly compressed material, which bulges 

 freely when the confining pressure of the laminar portion is removed by either horizontal or 

 vertical section. Thus, it has a constant tendency to spring out of its confinement in the 

 direction of least resistance, and constitutes a pivot round which the bodies of the vertebrae 

 can twist, tilt, or incline. It is yellowish in colour, and is composed of fine white and elastic 



Fig. 



265. — The Anterior Longitudinal Ligament, the Radiate, the Interarticular,. 

 AND the Anterior Costo-transverse Ligaments. 



The interarticular ligament 



The anterior costo- 

 transverse ligaments 



' j , I — The radiate ligament 



fibres amidst which are ordinary connective-tissue cells, and peculiar cells of various sizes which 

 contain one or more nuclei. Together with the most central laminie, it is separated from im- 

 mediate contact with the bone by a thin plate of articular cartilage. The central pulp of the 

 intervertebral sul)stance is the persistent part of the notochord. 



The intervertebral substances vary in shape with the bodies of the vertebrae they unite, 

 /and are widest and thickest in the lumbar region. In the cervical and lumbar regions they are 

 [thicker in front than Ijehind, and cnusc the convexity forward of the cervical, and increase that 

 of the lumbar; the curve in the thoracic region, almost entirely due to the shape of the bodies, 

 is, however, somewhat increased by the discs. Without the discs the column loses a quarter 

 of its length, and assumes a curve with the concavity forward, most marked a little below the . 



I mid-thoracic region. Such is the curve of old age, which is due to the shrinking and drying m 



iup of the intervertebral sul^stances. The disc between the epistropheus and third cervical is'^'-f. ^^ 

 the thinnest of all (fig. 2U0); that Jjetwoon tlie fifth lumbar and sacrum is the thickest, and is < 



much thicker in front than behind (fig. 271). 'i'ho intervertebral discs are in relation, in front 

 . with the anterior longitudinal ligament; l)oliind, with the posterior longitudinal ligament; 

 Uaterally, with the short lateral; and in the thoracic region, with the interarticular and radiate 

 uigaiiients. - — -V 



a''^ In the cervical region lateral diarthrodial joints are placed one on each side of the inter-\ 

 /vertebral discs. They are of small extent and are confined to the intervals between the promi- \ 

 I nent lateral lips of the upper surface of the body below and the bevelled lateral edges of the lower \ 

 I surface of the body above. Situated in front of the issuing spinal nerves and between those i 

 \ parts of the })0(lies formed from t he noiu-al arches, they are homologous with the joints between 

 ^ the atlas and epistropheus, and l»ctween the atlas and occipital bone. ,_.,_^' 



