THE MECHANISM OF JOINTS. 235 



side of it, exert more or less continuous pull upon the bones, jamming 

 the articular surfaces together. This contraction has been estimated at 

 about 20 kilos, in the case of the hip-joint, even during the resting 

 condition of the limb, but will be a very considerable power when the 

 muscles are voluntarily or reflexly contracted. 



Edward Weber drew attention to the action of atmospheric pressure in 

 keeping the articular surfaces in contact. He assumed that the joints, 

 having closed cavities, cannot be enlarged, as they would be if the articular 

 surfaces were separated, without the action of a force greater than 

 that of the pressure of the atmosphere. In the case of a large joint 

 like the hip, the atmospheric pressure would be such that a weight 

 of about 20 kilos, would be capable of overcoming it, and of 

 pulling the surfaces of the joint apart and rendering the joint-ligaments 

 tense. These ligaments would, owing to their inextensible nature, lie 

 able to resist a weight of over 400 kilos, before complete rupture could 

 take place. Weber divided all the muscles surrounding the hip-joint, 

 leaving, however, the capsule of the joint intact. The weight of the 

 limb was not sufficient to draw the articular surfaces apart. On opening 

 the joint cavity, by boring a hole through the acetabulum, the head at 

 once fell downwards. It is, however, doubtful whether the result of 

 this experiment may be applied to any other than the hip-joint. A 

 strain of 500 grms. put upon a metacarpophalangeal joint during 

 muscular relaxation causes the articular surfaces to come apart, the 

 space being rilled by the surrounding substance closing in. For the 

 living subject the articular cavity of the hip-joint may perhaps like- 

 wise be considered as being in partial communication with the outer 

 pressure. 



Ball-and-socket joints. — A spherical surface is produced by the 

 rotation of a circle round any one of its diameters. If such a surface 

 be received into a cup of the same, or of a slightly longer, radius of 

 curvature, these two surfaces may be caused to move in relationship to 

 each other. Putting on one side any " translation " of the cup-and-ball 

 in space, and considering the cup as fixed — for example, when the arm or 

 leg is raised from the shoulder or hip — we find that the centre of the 

 spherical mass will remain fixed in space, and movements of the ball 

 will take place around axes passing through the fixed point. This 

 point is therefore termed the centre of rotation, and the axes are 

 termed the axes of rotation. If, on the other hand, the ball be fixed 

 in space, the socket may move upon it, as when the thigh or arm are 

 fixed, and the trunk moves upon one of these fixed limbs. Here we 

 have a centre and axis of rotation for the cup, but these do not lie 

 within the substance of the cup. We must consider them as a point 

 and line in space, the space occupied or partly occupied by the ball 

 itself. In the body we may have simultaneous rotations of the ball and 

 of the socket, as in many movements of the arm, where not only dots 

 the humerus rotate, but the shoulder-blade and glenoid cavity change 

 their positions too. Confining our attention to the rotation of the ball, 

 and remembering that the centre of rotation is a point fixed in space, 

 any movement of the ball is equivalent to a rotation around a certain 

 axis passing through the centre of rotation. 



If the displacement of any two points in the sphere be known, neither 

 of which is a point lying in the centre of rotation, nor in a line with it and 



