ARTICULATIONS OF THE VERTEBRAL COLUMN 



289 



Pedicle (cut) 



Intervertebral 

 fibrocartilage 





The Intervertebral Fibrocartilages (fibrocartilagines intervertebrales; intervertebral 

 disks') (Figs. 301, 313). The inter-vertebral fibrocartilages are interposed between 

 the adjacent surfaces of the bodies of the vertebrae, from the axis to the sacrum, 

 and form the chief bonds of connection between the vertebras. They vary in shape, 

 size, and thickness, in different parts of the vertebral column. In shape and size 

 they correspond with the surfaces of the bodies between which they are placed, 

 except in the cervical region, where they are slightly smaller from side to side than 

 the corresponding bodies. In thickness they vary not only in the different -regions 

 of the column, but in different parts of the same fibrocartilage; they are thicker 

 in front than behind in the cervical and lumbar 

 regions, and thus contribute to the anterior con- 

 vexities of these parts of the column; while they 

 are of nearly uniform thickness in the thoracic 

 region, the anterior concavity of this part of 

 the column being almost entirely owing to the 

 shape of the vertebral bodies. The interverte- 

 bral fibrocartilages constitute about one-fourth 

 of the length of the vertebral column, exclusive 

 of the first two vertebrae; but this amount is 

 not equally distributed between the various 

 bones, the cervical and lumbar portions having, 

 in proportion to their length, a much greater 

 amount than the thoracic region, with the result 

 that these parts possess greater pliancy and 

 freedom of movement. The intervertebral 

 fibrocartilages are adherent, by their surfaces, 

 to thin layers of hyaline cartilage which cover 

 the upper and under surfaces of the bodies of 

 the vertebra?; in the lower cervical vertebrae, 

 however, small joints lined by synovial membrane are occasionally present between 

 the upper surfaces of the bodies and the margins of the fibrocartilages on either 

 side. By their circumferences the intervertebral fibrocartilages are closely con- 

 nected in front to the anterior, and behind to the posterior, longitudinal liga- 

 ments. In the thoracic region they are joined laterally, by means of the inter- 

 articular ligaments, to the heads of those ribs which articulate with two vertebrae. 



Fia. 302. Posterior longitudinal ligament, in 

 the thoracic region. 



Structure of the Intervertebral Fibrocartilages. Each is composed, at its circumference, of 

 laminae of fibrous tissue and fibrocartilage, forming the annulus fibrosus; and, at its center, of 

 a soft, pulpy, highly elastic substance, of a yellowish color, which projects considerably above 

 the surrounding level when the disk is divided horizontally. This pulpy substance (nucleus 

 pulposus), especially well-developed in the lumbar region, is the remains of the notochord. The 

 laminae are arranged concentrically; the outermost consist of ordinary fibrous tissue, the others 

 of white fibrocartilage. The laminae are not quite vertical in their direction, those near the cir- 

 cumference being curved outward and closely approximated; while those nearest the center 

 curve in the opposite direction, and are somewhat more widely separated. The fibers of which 

 each lamina is composed are directed, for the most part, obliquely from above downward, the 

 fibers of adjacent laminae passing in opposite directions and varying in every layer; so that the 

 fibers of one layer are directed across those of another, like the limbs of the letter X. This laminar 

 arrangement belongs to about the outer half of each fibrocartilage. The pulpy substance presents 

 no such arrangement, and consists of a fine fibrous matrix, containing angular cells united to 

 form a reticular structure. 



The intervertebral fibrocartilages are important shock absorbers. Under pressure the highly 

 elastic nucleus pulposus becomes flatter and broader and pushes the more resistant fibrous laminae 

 outward in all directions. 



2. Articulations of Vertebral Arches. The joints between the articular pro- 

 cesses of the vertebrae belong to the arthrodial variety and are enveloped by 

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