104 



PHYSIOLOGY 



CHAP. 



absolutely rigid and the other movable; much more frequently 

 both the bones are movable, but in different degrees. The muscle 

 or muscles attached to the two bones exert in contraction an 

 equal traction upon the two points of insertion and tend to dis- 

 place the two bones equally, but since the resistances opposed to 

 the displacement of the two bones differ, it follows that they are 

 unequally displaced. The distinction of fixed and movable in- 

 sertions of a muscle really has only a very relative value. As a 

 rule, however, one of the muscular insertions is less displaced 

 than the other, generally that which is nearer the axis of the 

 trunk, or the root of the limb. 



Fio. 65. 'A, Flexor movements of forearm for contraction of anterior brachial, which causes 

 backward rotation of arm in scapulo-humeral articulation. (O. Fischer.) B, Extensor move- 

 ments of forearm produced by triceps, and associated with forward rotation of arm in 

 scapulo-humeral articulation. (O. Fischer.) The two diagrams represent an experiment made 

 on a mechanical model. 



To demonstrate the mobility of the points of insertion of the 

 muscle, Fischer (1895) employed a wooden model to represent 

 the humerus and ulna articulating together, flexed by contrac- 

 tion of the anterior brachial muscles, and extended by the con- 

 traction of the triceps. He found that the movement of flexion 

 is associated with the backward displacement of the humerus, and 

 the movement of extension with its forward displacement (Fig. 

 65, A, B). 



The relation between the movements of the shoulder and of 

 the elbow joints which occur in consequence of the contraction 

 of the flexor or extensor muscles of the elbow varies when the 

 mass of the limb is increased. If, for instance, a weight is held 

 in the hand, and the elbow is flexed, the movement at the shoulder 

 is increased. 



