204 HUMAN PHYSIOLOGY 



the tension of the surrounding muscles, the bones of the 

 joints are held together by atmospheric pressure. 



If, in a dead body, all the connections between the femur 

 and the pelvis are cut, and also the capsular membrane of 

 the hip-joint, the femur still remains in its socket because 

 atmospheric pressure presses the surfaces of the joint against 

 each other. The force with which the bones are held 

 together by atmospheric pressure is, in the hip-joint, about 

 22 kg, which is more than the weight of the limb. 



Limitations in tJie movements of bones connected by joints. 

 The movements of the bones are naturally limited, often 

 especially so by processes of the bone (e.g. the olecranon 

 process, which prevents the complete rotation of the elbow 

 forward) and by ligaments (e.g. the posterior crucial liga- 

 ment of the knee, which prevents the complete backward 

 bend of the knee). 



B. Action of the muscles upon the bones. By the con- 

 traction of a muscle its points of insertion are brought nearer 

 together. Hence a muscle can only act when its points of 

 insertion can approach each other. 



But the line in i^Jiieh tJie in serf ion points approach each 

 other does not always coincide with the longitudinal direc- 

 tion of the muscle fibres, because the insertion points are 

 not free to approach each other in a straight line, but the 

 nature of their movement is determined by the nature of the 

 articulation. 



If, for example, the two insertion points are attached to two 

 bones articulated by ball-and-socket joint, and if we imagine one 

 bone as immovable, then the insertion point on the other bone 

 can only assume points all of which lie in a spherical plane. If 

 the bones are articulated by a hinge-joint, the insertion joint on 

 the imaginary movable bone can move only in a circle. 



By the contraction the insertion points approach each 

 other because the muscle fibres are stretched straight 

 between the insertion points. 



If the muscle fibres are not stretched straight between the inser- 

 tion points but move over a pulley-like arrangement, the points 



