48 OUR BODIES AND HOW WE LIVE 



locomotion or other movements of the body. In other 

 words, muscles by their contraction are able to do work. 



The part of a muscle which remains fixed when it con- 

 tracts is called its origin. 



The end which is connected with the movable part is its 

 insertion. 



Experiment 15. To show how muscles relax and contract. This 

 experiment illustrates the contraction of the biceps, and is popularly 

 called " trying your muscle." 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 forearm, keeping the elbow on the 

 table. Note that the muscle thickens as the hand rises. 



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. 



Experiment 16. Repeat the same experiment with other muscles. 

 With the right hand grasp firmly the extended left forearm. Extend 

 and bend the fingers vigorously. Note the effect on the muscles and 

 tendons of the forearm. 



72. How the Muscles work in Harmony. A single muscle 

 rarely or never contracts alone, but always in harmony 

 with a number of other muscles. Even the simplest move- 

 ment we can make requires the combined action of several 

 muscles to carry it out. If the movement is at all difficult, 

 such as playing on the piano or riding on the bicycle, the 

 different muscles must contract in a certain order and with 

 a certain strength and rapidity. 



This harmonious working of the muscles is spoken of as 

 muscular coordination. Unless the muscles work in perfect 

 harmony, the complicated movements will be performed 

 in a clumsy and imperfect way. 



73. The Function of Tendons. If we bend the leg or 

 arm and grasp the inside bend of the joint with the hand, 



