THE MUSCULAR SYSTEM 



71 



length from the shoulder, the muscles of the arm soon 

 become exhausted and incapable of sustaining the weight. 

 But a moment's rest restores the contractile power, and 

 the weight may again be held 

 out. 



91 . Muscle Waste. When a 

 muscle contracts certain chem- 

 ical changes take place in the 

 substance of its cells. Some 

 of the matter in the muscular 

 fiber becomes oxidized, and 

 new substances are formed 

 which are harmful to the body 

 if not removed. These are 

 called waste products, and that 

 which appears in largest quan- 

 tity is carbon dioxide. These 

 waste products are taken up 

 by the blood which flows along 

 the muscle cells and are finally 

 removed from the body by 

 means, mainly, of the lungs 

 and the kidneys. If the waste 

 matter is not removed, the 

 effect soon appears in the cen- 

 tral nervous system, to which 

 the poison is carried by the 

 blood. 



92. Voluntary Movement. 

 Let us suppose that a man 



seeing an apple within reach puts forth his hand to take 

 it (Fig. 47). In such a case the light from the apple 

 enters the organ of sight and stimulates the nerve endings 



Fig. 47. Diagram of the path of 

 a nervous impulse which re- 

 sults in the hand reaching to 

 seize an object seen with the 

 eye. 



G cerebrum. 



Cb cerebellum. 



M medulla oblongata. 



MC motor center in brain. 



OC optic center in brain. 



P pons Varolii. 



Q corpora quadrigemina. 



S spinal cord. 



T optic thalami. 



