THE PHYSIOLOGY OF THE SKELETON. 39 



gravity, cohesion, friction, elasticity, etc. The active forces by which these 

 latter are opposed and overcome through the intermediation of the bony 

 levers are found in the muscles attached to them. For the execution of all 

 these movements, it is essential that the relation of the various portions of the 

 bony skeleton to one another shall be such as to permit of movement while 

 yet retaining close apposition. This is accomplished by the mechanical 

 conditions which have been evolved at the points of union of bones, and 

 which are technically known as articulations or joints. 



A consideration of the body movements involves an account of (i) the static 

 conditions, or those states of equilibrium in which the body is at rest e. g., 

 standing, sitting; (2) the dynamic conditions, or those states of activity 

 characterized by movement e. g., walking, running, etc. In this connection, 

 however, only those physical and physiologic peculiarities of the skeleton, 

 especially in its relation to joints, will be referred to which underlie and 

 determine both the static and dynamic states of the body. 



Structure of Joints. The structures entering into the formation of 

 joints are: 



1. Bones, the articulating surfaces of which are often more or less expanded, 

 especially in the case of long bones, and at the same time variously modi- 

 fied and adapted to one another in accordance with the character and 

 extent of the movements which there take place. 



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

 each bone. The smoothness of this form of cartilage facilitates 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 fibre-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 membrane is composed 

 largely of connective tissue, the inner surface of which is lined by endoth^lial 

 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 inextensible but pliant, ligaments 

 assist in keeping the bones in apposition, and prevent displacement while 

 yet permitting of free and easy movements. 



