200 UNCONSCIOUS NERVOUS OPERATIONS 



279. Reflex Action of Respiratory Center. Though the 

 action of the breathing center in the medulla oblongata is 

 shown by experiment to originate efferent nervous impulses 

 independent of irritation conveyed to it by sensory fibers, 

 it is also largely under reflex control. For instance, when 

 the mucous membrane of the air passages is irritated by 

 foreign substances, a sudden sneeze or cough results. A 

 sudden dash of cold water upon the skin causes a quick, 

 long inspiration. So it appears that in normal respiration 

 the movements are not reflex, while reflex movements are 

 also possible. 



280. The Normal Excitation of the Respiratory Center is due, 

 more than to any other cause, to the relative amounts of 

 oxygen and carbon dioxide in the blood which reaches it. 

 If the blood contains too little oxygen and too much carbon 

 dioxide, the center is stimulated, and the resulting respira- 

 tory movements tend to correct the evil. 



Afferent nervous influences brought to the center along 

 the .vagus nerves also excite its action. The main trunk 

 of this tenth nerve contains fibers which excite inspiration, 

 and other fibers which inhibit, or check, inspiratory move- 

 ments. It is supposed that the expansion of the air cells 

 in the lungs, where many of the vagus fibers end, produces 

 impulses along those fibers which result in inhibitory or 

 expiratory impulses from the center in the medulla ; while, 

 on the other hand, collapse of the air cells excites along 

 other fibers contrary impulses which result in inspiration. 

 Thus the action of the lungs becomes that of a self-regulat- 

 ing pump. Other afferent impulses along branches of the 

 vagus, or from other nerve fibers connected with it, may 

 also affect respiration. 



