192 



THE A R TICVLA TfOXS 



lateral surfaces; and some do not ronipletely surround the rest, but terminate at 

 the intervertebral foramina, so that on horizontal section the circumferential portion 

 is seen to be thinner posteriorly. The more central lamelUt' 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 central portion is situated somewhat behind the centre of the disc, form- 

 ing 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 com- 

 posed of a fine fibrous matrix containing cartilage cells and fluid in its meshes. 

 Together with the most central laminae, it is separated from innnediate contact 



Fig. 201. — Posterior Common Ligament of the Spine. (Thoracic regiou.) 

 (Pedicles cut through, and posterior arches of vertebrse removed.) 



Lateral expanded portion 



Median longitudinal band 



with thf bone by a thin plate of articular cartilage. The central pulp of the inter- 

 vertebral substance is the persistent part of the notochord. 



The interverte})ral substances vary in shape with the bodies of the vertebrae thev 

 unite, and are widest and thickest in the lum1)ar region. In the cervical and 

 luml)ar regions they are thicker in front than behind, and caa.^c the convexity for- 

 wards of tlie cervical, and increase that of the lumbar; the curve in the thoracic 

 region, almost entirely due to the shape of the bodies, is, however, somcAvhat 

 increased Ijy the discs. Without the discs the column loses a quarter of its length, 

 and assumes a curve with the concavity forwards, most marked a little below the 

 mid-thoracic region. 8ucli is the curve of old age. which is due to the shrinking 

 and drying up of tlie intervertebral substances. The disc between the axis and 

 third cervical is the thinnest of all (fig. 195); that between the fifth lumbar and 

 sacrum is the thickest, and is much thicker in front than beliind (fig. 20G). The 

 intervertebral discs are in relation, in front with tlie anterior common ligament; 



