24$ MOTION AND COORDINATION 



many of the cells have branches by which they are united 

 to the cells in neighboring fibers. In this way they inter- 

 lace more or less with each other, but 

 are also cemented together. They con- 

 tract quickly and with great force, but are 

 not under control of the will. Muscular 

 tissue of this variety seems excellently 



m 



adapted to the work of the heart. 



The Muscular Stimulus. The in- 

 FIG. 112. Muscle 



cells from the heart, active, or resting, condition of a muscle 

 highly magnified (after js that of relaxation. It does work 

 Schafer). through contracting. It becomes active, 



or contracts, only when it is being acted upon by some force 

 outside of itself, and it relaxes again when this force is with- 

 drawn. Any kind of force which, by acting on muscles, 

 causes them to contract, is called a muscular stimulus. 

 Electricity, chemicals of different kinds, and mechanical 

 force may be so applied to the muscles as to cause them to 

 contract. These are artificial stimuli. So far as known, 

 muscles are stimulated naturally in but one way. This 

 is through the nervous system. The nervous system sup- 

 plies a stimulus called the nervous impulse, which reaches 

 the muscles by the nerves, causing them to contract. By 

 means of nervous impulses, all of the muscles (both vol- 

 untary and involuntary) are made to contract as the needs 

 of the body for motion require. 



Energy Transformation in the Muscle. The muscle 

 serves as a kind of engine, doing work by the transforma- 

 tion of potential into kinetic energy. Evidences of this 

 are found in the changes that accompany contraction. 

 Careful study shows that during any period of contrac- 

 tion oxygen and food materials are consumed, waste prod- 

 ucts, such as carbon dioxide, are produced, and heat is 



