100 PHYSIOLOGY CHAP. 



comparatively slight force is consequently able to produce move- 

 ments of the bones. 



In articulations of the second class (arthroses and diarthroses), 

 which more especially concern us, the bones have articular heads 

 which are approximately cylindrical or spherical in shape. The 

 former constitute the hinge joints which move in a single axis ; 

 one of the articular surfaces is concave, the other convex. Both 

 are shaped like a section of a cylinder, or more exactly like a cone, 

 an ovoid, or an ellipse. The articulations of the elbow, knee, 

 and ankle belong to this class. In the second class of articula- 

 tions with round heads, the bones can rotate round a single axis, 

 as in the humero-radial and the atlanto-epistropheal joints, or 

 round many axes, as in the ball-and-socket joints, represented by 

 the scapulo-humeral and the hip-joints. 



From these articulations with one or many axes, we must 

 distinguish the articulations with two axes at right angles to 

 one another, represented by the saddle joints and the con- 

 dyloid joints. The articular saddle surfaces are convex in one 

 direction and concave in the plane vertical to it. Such is 

 the joint between the metacarpal bone of the thumb and the 

 trapezium bone of the carpus, which permits not only of flexion 

 and extension, but also of adduction and abduction in two almost 

 perpendicular axes. The joint between the radius and the bones 

 of the carpus, which permits the flexion and extension of the 

 hand, and its abduction and adduction in two axes vertical to 

 each other, is also a condyloid bi-axial articulation. 



In most articulations the surfaces of the bones are not in 

 complete apposition. There is only a small area of contact 

 between the head of the femur and the hollow of the acetabulum, 

 because, as Konig showed, their surfaces are not geometrically 

 complementary. The gap between the articular surfaces where 

 there is no direct contact is filled either by the synovia or by 

 introflexion of the capsular membrane due to external pressure. 

 These capsular introflexions always have excrescences known as 

 synovial villosities, which are rich in vessels and lined with 

 epithelium, to which the formation of synovia is mainly due. 

 There are consequently no true articular cavities. 



However small the area of contact of the articular heads, 

 it was formerly supposed that it was invariably present, but 

 Konig found an exception in the scapulo-humeral articulation. 

 On dissecting frozen subjects he discovered that there was 

 always a layer of congealed synovia between the two articular 

 surfaces. 



An important but difficult question is, what forces intervene 

 to resist displacements of the articular surfaces ? E. Weber 

 attributed this to atmospheric pressure only. He saw that if all 

 the muscles surrounding the hip-joint in a suspended corpse 



