THE MUSCLES AND HOW TO USE THEM 47 



Under the microscope these muscles are found to con- 

 sist, not of fibers, but of long, spindle-shaped cells, bound 

 together in such a way as to form bands or ribbons. They 

 are often called unstriated or unstriped 

 muscles. 



70. How Muscles contract. Muscles 

 have a peculiar power of their own. 

 This is the power to contract throughout 

 their length or to become shorter and 

 thicker. 



Contraction is not, however, the nat- 

 ural state of a muscle. After a longer 

 or shorter time it is tired and begins 

 to relax. Even the heart, the hardest 

 working muscle of the body, rests be- 

 tween its beats. 



In order to contract, a muscle must FlG - 37- A Spindle 

 be stimulated. This stimulus, or that 

 which gives it the power to contract, is 

 a nervous impulse which is sent along the 

 nerve fibers from the central nervous system. This stim- 

 ulus acts upon the muscles and causes them to shorten in 

 length and swell in girth. 



71. Why Muscles contract. The purpose of the contrac- 

 tion of muscles is plain. By contracting, muscles become 

 shorter and thicker and the two ends are brought closer 

 together. Now, if one end of a muscle is attached to a 

 fixed point and the other end is fastened to some object 

 which is free to move, the contraction of the muscle will 

 bring the movable body nearer to the fixed point. 



Thus, the muscles cause the motion of the parts to 

 which they are attached. This motion in turn gives rise to 



