THE STRUCTURE OF THE MOTOR ORGANS 79 



movement is permitted, and so when the muscle pulls it alters 

 the relative positions of the parts to which its tendons are fixed. 

 In the great majority of cases a true joint lies between the bones on 

 which the muscle can pull, and when the latter contracts it produces 

 movement at the joint. Many muscles even pass over two joints 

 and can produce movement at either, as the biceps of the arm 

 which, fixed at one end to the scapula and at the other to the 

 radius, can move the bones at either the shoulder or elbow-joint. 

 Where a muscle passes over an articulation it is nearly always re- 

 duced to a narrow tendon ; otherwise the bulky bellies lying around 

 the joints would make them extremely clumsy and limit their 

 mobility. 



Origin and Insertion of Muscles. Almost invariably that part of 

 the skeleton to which one end of a muscle is fixed is more easily 



FIG. 43. The biceps muscle and the arm-bones, to illustrate how, under ordi- 

 nary circumstances, the elbow-joint is flexed when the muscle contracts. 



moved than the part on which it pulls by its other tendon. The 

 less movable attachment of a muscle is called its origin, the more 

 movable its insertion. Taking for example the biceps of the arm, 

 we find that when the belly of the muscle contracts and pulls on its 

 upper and lower tendons, it commonly moves only the forearm, 

 bending the elbow-joint as shown in Fig. 43. The shoulder is so 

 much more firm that it serves as a fixed point, and so that end is 

 the origin of the muscle, and the forearm attachment, P, the in- 

 sertion. It is clear, however, that this distinction in the mobility 

 of the points of fixation of the muscle is only relative, for, by chang- 

 ing the conditions, the insertion may become the stationary and 



