42 THE HUMAN BODY. 



PROPERTIES OP MUSCLE. 



SECTION II. 1, Muscle has a peculiar power of short- 

 ening itself. There is a kind of elastic tissue in the body, 

 of which a few ligaments are made, which, like rubber, 

 will contract after it has been stretched ; but muscle is 

 the only tissue that contracts without being stretched 

 first. When a muscle grows shorter, it grows thicker at 

 the same time, just as the body of a worm will shorten 

 and thicken. If, for example, I place my left hand on 

 the biceps of my right arm, and then bend my elbow, I 

 shall feel the biceps swelling, and growing hard : at the 



Fig. 24. 



1. BICEPS MUSCLE. The dotted lines indicate the changed shape of the biceps when 

 the fore-arm is drawn up. 



same time it has shortened, and thus drawn the bone 

 to which it is attached up toward the shoulder. We do 

 not understand how it is that muscle contracts when we 

 will that it shall. We can only say that it does so. 

 2, Involuntary muscle has the same power of shorten- 



