THE HUMAN STRUCTURE 



103 



FIG, 100. 



Knee Joint. 



K 



75. Details ol the Types of Joints, (1) Joints with one axis 

 or only one degree of freedom, are the 

 most numerous. This type is the 

 spherical joint, having a spherical head 

 penetrating into a hemispherical cavity; 

 but it is very rarely found in such a 

 regular shape, for, as a rule, the surface 

 has a groove S, forming part of the 

 arc of a circle (fig. 1 10) . The movement 

 which results takes place in one plane, and is either a flexion 

 or an extension. The mobile bone turns by means of a hinge, 

 whence the name hinge joints. There are examples of this in 



>- s ' 



FIG. 110. 



