The Muscles 55 



A 80. Various Kinds of Muscles. There are about four 

 hundred muscles in the human body, all necessary for its 

 various movements. They vary greatly in shape and size, 

 according to their position and use. Some are from one 

 to two feet long, others only a fraction of an inch. Some 

 are long and spindle-shaped, others thin and broad, while 

 still others form rings. Thus, some of the muscles of the 

 arm and thigh are long and tapering, while the abdominal 

 muscles are thin and broad. Again, the muscular fibers 

 which surround and by their contraction close certain 

 orifices, as those of the eyelids and lips, sometimes radiate 

 like the spokes of a wheel, and sometimes form complete 

 circles or ellipses. 



The part of. a muscle which remains fixed when it 

 contracts is called its origin ; the end connected with the 

 movable part is its insertion. 



81. ' How Muscles are named. Muscles are named accord- 

 ing to their shape, position, division of origin or insertion, 

 and their function. Thus we have the recti (straight) and the 

 deltoid (A, delta), the brachial (arm), pectoral (breast), and the 

 intercosials (between the ribs), so named from their position. 

 Again, we have the biceps (two-headed), triceps (three-headed), 

 and many others with similar names, so called from the points 

 of origin and insertion. We find other groups named after 

 their special use. The muscles which 'bend the limbs are 

 called flexor s, while those which straighten them are known as 

 extensors. 



Experiment 19. To show how muscles relax and contract. Lay 

 your left forearm on a table ; grasp with the right hand the mass of 

 flesh on the front of the upper arm. Now gradually raise the fore- 

 arm, keeping the elbow on the table. Note that the muscle thickens 

 as the hand rises. This illustrates the contraction of the biceps, and 

 is popularly called "trying your muscle." Reverse the act Keep 

 the elbow in position, bring the forearm slowly to the table, and the 

 biceps appears to become softer and smaller, it relaxes. 



