320 COORDINATION AND SENSATION 



Respiration. Efferent fibers connect the different muscles of respi- 

 ration with a cluster of cell-bodies in the bulb, called the respiratory 

 center. This center together with the nerves and muscles in question 

 form an automatic, or self-acting, mechanism similar in some respects 

 to that of the heart. Through the impulses passing from the respira- 

 tory center to the muscles, a rhythmic action is maintained sufficient to 

 satisfy the usual needs of the body for oxygen. The demand of the 

 body for oxygen, however, varies with its activities, and to such varia- 

 tions the respiratory center alone is unable to respond. The regulating 

 factor in the respiratory movements has been found to be the condition 

 of the blood with reference to the presence of oxygen and carbon diox- 

 ide. If the blood contains much carbon dioxide and little oxygen, it 

 acts as a strong stimulus to the respiratory center, causing it, in turn, 

 to stimulate the respiratory muscles with greater intensity and fre- 

 quency. On the other hand, if the blood contains much oxygen and 

 little carbon dioxide, it acts only as a mild stimulus. This explains 

 how physical exercise increases the breathing, since the muscles at 

 work consume more oxygen than when resting and give more carbon 

 dioxide and other wastes to the blood. 



The respiratory center is also connected by afferent nerves with the 

 mucous membrane of the air passages. Irritation of the nerve endings 

 in this membrane causes impulses to pass to the center, and this leads, 

 by reflex action, to such modifications of the respiratory acts as sneezing 

 and coughing. There is also a connection between the cerebrum and 

 the respiratory center. This is shown by the fact that one can volun- 

 tarily change the rate and force of the respiratory movements, and 

 further by the fact that emotions affect the breathing. 

 Regulation of the Body Temperature. As explained in the study 

 of the skin (page 270), the nervous system regulates the body temper- 

 ature by controlling the circulation of the blood through the skin and 

 the internal organs. This is accomplished by stimulating in a reflex 

 manner the muscles in the walls of certain arteries. To prevent the 

 body from getting too hot, muscles in the arteries going to the skin 

 relax, thereby allowing more blood to flow to the surface, where the 

 heat can be disposed of through radiation and through the evaporation 

 of the perspiration. On the other hand, if the body is in danger of 

 losing too much heat, the muscles in the walls of arteries going to the 

 skin are made to contract and those to internal organs to relax, so that 

 less blood flows to the skin and more to the internal organs. In this 



