THE MUSCLES 



37 



cold-blooded animals, as the turtle, contraction has been known 

 to take place for several days after the head has been cut off. 



7. The property which, in muscle, enables these movements 

 to take place is called contractility. If we grasp a muscle while 

 in exercise (for example, the large muscle in the front of the 

 arm), we notice the alternate swelling and decrease of the 

 muscle, as we move the forearm to and fro. It was at one 

 time supposed that the muscle actually increased in volume 

 during contraction. This, however, is not the case; for the 



FIG. 16. VIEW OF KNEE-JOINT. A, Thigh-bone; B, Knee-pan; C, D, Leg-bones 



muscle, while gaining in thickness, loses in length in the same 

 proportion ; and thus the volume remains the same in action 

 and at rest. 



8. Contraction is not the permanent, or normal state of a 

 muscle. It cannot long remain contracted, but after a time it 

 wearies, and is obliged to relax. After a short rest, it can then 

 again contract. It is for this reason that the heart can beat 

 all through life, night and day, by having, as we shall here- 

 after see, a brief interval of rest between successive coiitrac- 



7. Contractility ? Give the illustration. What was supposed ? What is the case ? 



8. What further in relation to contraction ? Weariness of a muscle ? Beating of the 

 heart ? Standing and walking ? 



