Muscles and Motion. 25 



developed muscles, not only because they give beauty of 

 form, but because extra strength and endurance may be 

 needed in case of accident, to save one's own life or that 

 of others. In a case of fire the ability to climb, to go up 

 or down a rope " hand over hand," may be all-important. 

 Any one's life may depend on his ability to run far and 

 swiftly, to swim, to jump, or to lift a heavy weight 



The Number of Muscles. There are over five hundred 

 muscles in the human body. These vary in size from less 

 than an inch in length, in the ear and in the larynx, to a 

 foot and a half long in the thigh. 



The Arrangement of Muscles. The muscles of the two 

 sides of the body are paired, and naturally are about equal 

 in size and strength. The muscles of the limbs are further 

 paired into flexors, which bend, and the extensors, which 

 straighten the limbs. The muscles are also arranged more 

 or less in layers. There is generally an outer layer and a 

 more deep-seated layer. 



Forms of Muscles. Muscles are of various shapes. 

 The prevailing form in the limbs is spindle-shaped, or 

 fusiform. Some muscles are flat, some have their fibers 

 arranged like the barbs of a feather, and are hence called 

 penniform. Some muscles have a tendon in the middle 

 which runs through a loop, as in the case of the muscle 

 which depresses the lower jaw. Muscles which close open- 

 ings are circular, and are called sphincter muscles. 



Peculiar Muscles. The diaphragm is a sheet of muscle 

 that forms a partition between the chest and the abdomen. 

 It is arched, and has a clear tendinous center. The ab- 

 dominal muscles form a wall to hold the organs of the 

 abdominal cavity. These muscles also aid in breathing, 



