THE STRUCTURE OF THE MOTOR ORGANS. 115 



same bone the muscle would clearly be able to produce no 

 movement, unless by bending or breaking the bone; the 

 probable result in such a case would be the tearing of the 

 muscle by its own efforts. In the Body, however, the two 

 ends of a muscle are always attached to different parts, 

 usually two bones, between which more or less 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 ar- 

 ticulation it is nearly always reduced 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 



FIG. 52. The biceps muscle and the arm-bones, to illustrate how, under ordinary 

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



fixed is more easily 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. 52. The shoulder is so much 

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



