THE PARTS OF A MUSCLE. 69 



working part ; nerves from the brain or spinal cord enter 

 it, and when its nerve is excited, whether involuntarily or 

 by what we call an act of " will," the belly contracts; it 

 forcibly changes its shape so as to become shorter and 

 thicker. In so doing it drags on the tendons, which are 

 passive inextensible cords and transmit the pull to the 

 parts to which they are attached. 



The tendons are often quite long, as for example those 

 of many of the muscles moving the fingers (Fig. 28), 

 whose bellies are in the forearm. The belly of the common 

 extensor muscle of the fingers (14, Fig. 28) is seen, for 

 example, to be in the upper half of the forearm, and to 

 end above the wrist in a single tendon which divides up 

 into strips which run along the back of each finger ; the 

 muscles which straighten the thumb, 17, 18 and 19, are 

 also seen to have long slender tendons. 



Where a muscle passes over a joint it is usually reduced 

 to a narrow tendon ; the bulky bellies, if they lay there, 

 would make the joints clumsy and limit their mobility. 

 Some muscles pass over two joints and can produce move- 

 ment at either; the biceps of the arm, fixed above to the 

 scapula and below to the radius, can produce movement at 

 either the elbow or the shoulder joint. 



The shortening of a muscle when it contracts is shown 

 by the movement which it causes ; the thickening may be 



What enters the belly of a muscle? How is the nerve of a muscle 

 excited? What happens when the nerve is excited? What results 

 from the contraction of the belly of a muscle? 



Are tendons ever long? Describe the common extensor muscle of 

 the fingers and its tendons. Describe the position and length of ten- 

 dons of the muscles which extend the thumb. 



What happens to a muscle when it passes over a joint? Why? 

 Name a muscle which crosses two joints. At what joints can the 

 biceps muscle of the arm produce movement? 



How is the shortening of a contracting muscle shown? 



