THE MUSCULAR SYSTEM 



249 



liberated. Furthermore, the blood supply to the muscle is 

 such that the materials for providing energy may be car- 

 ried rapidly to it and the products of oxidation as rapidly 

 removed. Blood vessels penetrate the muscles in all di- 

 rections and the capillaries lie very 

 near the individual cells (Fig. 113). 

 Provision is made also, through the 

 nervous system, for increasing the 

 blood supply when the muscle is at 

 work. From these facts, as well as 

 from the great force with which the 

 muscle contracts, one must conclude 

 that the muscle is a transformer of 

 energy that within its protoplasm, 

 chemical changes take place whereby 

 the potential energy of oxygen and 

 food is converted into the kinetic 

 energy of motion. 



Plan of Using Muscular Force. 

 Two difficulties have to be overcome in the using of 

 muscular force in the body. The first of these is due to 

 the fact that the muscles exert their force only when 

 tJiey contract. They can pull but not push. Hence, in 

 order to bring about the opposing movements J of the 

 body, each muscle must work against some force that 

 produces a result directly opposite to that which the 

 muscle produces. Some of the muscles (those of breath- 

 ing) work against the elasticity of certain parts of the 

 body; others (those that hold the body in an upright 

 position), to some extent against gravity ; . and others 



1 Every movement in the body has its opposing movement. This is necessary 

 both on account of the work to be accomplished and for preserving the natural 

 form of the body. 



FIG. 113. Capillaries 

 of muscles. 



