132 STRUCTURE OF CARTILAGE. 



4. Rotation is the movement of a bone upon its own axis, and is illi ?- 

 trated in the hip and shoulder, or better in the rotation of the cup of the 

 radius against the eminentia capitata of the humerus. Rotation is also 

 observed in the movements of the atlas upon the axis, in which the odon- 

 toid process serves as a pivot around which the atlas turns. 



The structures entering into the composition of a joint are Done, carti 

 lage, fibrous tissue, adipose tissue, and synovial membrane. Cartilag 

 forms a thin coating to the articular extremities of bones, sometimes pre 

 senting a smooth surface which moves on a corresponding smooth surface 

 of the articulating bone ; sometimes forming a plate smooth on both sur 

 faces and interposed between the cartilaginous ends of two bones, inter- 

 articular; and sometimes acting as the connecting medium between bones 

 without any free surface, interosseous. Fibrous tissue enters into the con- 

 struction of joints under the form of ligament, in one situation constituting 

 bands of various breadth and thickness, in another a layer which extends 

 completely around the joint, and is then called a capsular ligament. All 

 ihe ligaments of joints are composed of that variety of fibrous tissue term- 

 ed white fibrous tissue, but in some situations ligaments are found which 

 consist of yellow fibrous tissue, for example, the ligamenta subflava of the 

 arches of the vertebral column. Adipose tissue exists in variable quantity 

 in relation with joints, where it performs, among other offices, that of a 

 valve or spring, which occupies any vacant space that may be formed 

 during the movements of the joint, and effectually prevents the occurrence 

 of a vacuum in those cavities. This purpose of adipose tissue is exempli- 

 fied in the cushion of fat at the bottom of the acetabulum, and in the 

 similar cushion behind the ligamentum patella?. Synovial membrane 

 constitutes the beautiful smooth and polished lining of a joint, and con- 

 tains the fluid termed synovia, by means of which the adapted surfaces 

 are enabled to move upon each other with the perfect ease and freedom 

 which are known to exist. 



CARTILAGE. In the structure of joints, cartilage serves the double pur- 

 pose of a connecting and separating medium. In the former capacity 

 possessing great strength, and in the latter smoothness and elasticity. In 



Fig. 64.f 



A portion of articular cartilage from the head of the fibula, showing the appcxuanne 

 and arrangement of the cells near to the bone. The section is made vertically to the 

 surface, and magnified 155 diameters. The irregular line to the right is the bowdary 

 M!' the bone 



* A viest of the same section, at about midway between the bone and the free sur 

 lace 



