THE PHYSIOLOGY OF THE SKELETON. 



59 



2. Hyaline cartilage, which is closely applied to the articulating end 



of each bone. The smoothness of this form of cartilage facili- 

 tates the movements of the opposing surfaces, while its elasticity 

 diminishes the force of shocks and jars imparted to the bones 

 during various muscular acts. In a number of joints, plates 

 or discs of white fibro-cartilage are inserted between the surfaces 

 of the bones. 



3. A synovial membrane, which is attached to the edge of the hyaline 



cartilage, entirely inclosing the cavity of the joint. This mem- 

 brane is composed largely of connective tissue, the inner surface 

 of which is lined by endothelial cells, which secrete a clear, 

 colorless, viscid fluid the synovia. This fluid not only fills up 

 the joint-cavity, but, flowing over the articulating surfaces, 

 diminishes or prevents friction. 



4. Ligaments, tough, inelastic_bands, composed of white fibrous 



tissue, which pass from bone to bone in various directions on 

 the different aspects of the joint. As white fibrous tissue is in- 

 extensible but pliant, ligaments assist in keeping the bones in 

 apposition, and prevent displacement while yet permitting of 

 free and easy movements. 

 Classification of Joints. All joints may be divided, according 



to the extent and kind of movements permitted by them, into (i) 



diarthroses; (2) amphiarthroses ; (3) synarthroses. 



i. Diarthroses. In this division of the joints are included all those 

 which permit of free movement. In the majority of instances 

 the articulating surfaces are mutually adapted to each other. 

 If the articulating surface of one bone is convex, the opposing 

 but corresponding surface is concave. Each surface, therefore, 

 represents a section of a sphere or a cylinder, which latter arises 

 by rotation of a line around an axis in space. According to the 

 number of axes around which the movements take place all 

 diarthrodial joints may be divided into: 



i. Uniaxial Joints. In this group the convex articulating surface is 

 a segment of a cylinder or cone, to which the opposing surface 

 more or less completely corresponds. In such a joint the single 

 axis of rotation, though nearly, is not exactly at right angles 

 to the long axis of the bone, and hence the movements flexion 

 and extension which take place are not confined to one plane. 

 Joints of this character e. g., the elbow, knee, ankle, the pha- 

 langeal joints of the fingers and toes are, therefore, termed 

 ginglymi, or hinge- joints. Owing to the obliquity of their 

 articulating surfaces, the elbow and ankle are cochleoid or screw- 

 ginglymi. Inasmuch as the axes of these joints on the opposite 

 sides of the body are not coincident, the right elbow and left 

 ankle are right-handed screws; the left elbow and right ankle, 



