THE STRUCTURE OF THE MOTOR ORGANS 81 



The belly of every muscle possesses the power of shorten ing- 

 forcibly under certain conditions. In so doing it pulls upon the 

 tendons, which being composed of inextensible white fibrous tissue 

 transmit the movement to the hard parts to which they are at- 

 tached, just as a pull at one end of a rope may be made to act upon 

 distant objects to which the other end is tied. The tendons are 

 merely passive cords and are sometimes very long, as for instance 

 in the case of the muscles of the fingers, the bellies of many of 

 which lie away in the forearm. 



If the tendons at each end of a muscle were fixed to the 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 



FIG. 39. The biceps muscle and the arm-bones, to illustrate how, under ordinary 

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



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 articulation it is nearly always re- 

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



