270 



THE ARTICULATIONS, OR JOINTS 



structure. The disks are interposed between the adjacent surfaces of the bodies 

 of the vertebrae from the axis to the sacrum, and form the chief bonds of connec- 

 tion between those bones. In young children intervertebral substance exists 

 in the coccyx. These disks vary in shape, size, and thickness in different parts 

 of the vertebral column. 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 thoracic. Their size is greatest in the lumbar region. 

 In thickness they vary not only in the different regions of the vertebral column, 

 but in different parts of the same disk; thus, they are thicker in front than behind 

 in the cervical and lumbar regions, while they are uniformly thick in the thoracic 

 region. The intervertebral disks form about one-fourth of the vertebral column, 

 exclusive of the first two vertebrae; they are not equally distributed, however, 

 between the various bones; the thoracic portion of the vertebral column having, 

 in proportion to its length, a much smaller quantity than in the cervical and lumbar 



POSTERIOR COMMON 

 LIGAMENT 



C 



INTERVERTEBRAL 

 LfBROCARTILAGE 



FIG. 222. Median section of a piece of the lumbar vertebral column, right half of section viewed from the 



left. (Spalteholz.) 



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, in early life, the epiphyseal plates develop, 

 and by their circumference are closely connected in front to the anterior, and 

 behind to the posterior common ligament; while in the thoracic region they 

 are connected laterally, by means of the interarticular ligament, to the heads of 

 those ribs which articulate with two vertebrae; they, consequently, form part of 

 the articular cavities in which the heads of these bones are received. 



Structure of the Intervertebral Substance. The outer portion of the intervertebral sub- 

 stance is composed of many layers of fibrous connective tissue. This enveloping portion is called 

 the annulus fibrosus. The central portion of the disk is composed of soft, pulpy, highly elastic 

 fibrocartilage, containing some bands of connective tissue. It is called the nucleus pulposus, is of 

 a yellowish color, and rises up considerably above the surrounding level when the disk is divided 

 horizontally. This pulpy substance, which is especially well developed in the lumbar region, 

 is the remains of the notochord, and, according to Luschka, contains a small synovial cavity in its 



