THE CONNECTIVE TISSUES AND THE SKELETAL SYSTEM. 209 



cavity are the most highly differentiated, the cell bodies being large and ap- 

 parently swollen, and there is gradually less differentiation as the distance from 

 the surface increases, until finally they merge with the ordinary type of con- 

 nective tissue cells of the joint capsule (Clarke). The more mobile joints of 

 the body are all representatives of this type. 



Joint cavity 



Synovial membrane 



FIG. 193. From longitudinal section of finger of child at birth, showing joint cavity and synovial 

 membrane between adjacent ends of the first metacarpal and proximal phalanx. Other 

 description same as in Fig. 192. Photograph. 



Anomalies. 

 THE AXIAL SKELETON. 



THE VERTEBRAE. The number of cervical vertebras in man is remarkably 

 constant. Cases where the number is but six are extremely rare. The 

 thoracic vertebrae may vary in number in different individuals from eleven to 

 thirteen, twelve being the usual number. The lumbar vertebrae may vary 

 from four to six, five being the usual number. The sacral vertebrae, fused in the 

 adult to form the sacrum, are usually five in number, sometimes four, sometimes 



