44 HUMAN PHYSIOLOGY. 



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 modified 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 dimin- 

 ishes 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 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 cor- 

 responding 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 



