1 68 RESPIRATION 



i, 2. The effect of the will and emotions are too apparent 

 to call for comment. I and 2 are properly included in 4. 



3. A deficiency of O or an excess of CO2 in the blood will 

 increase the rate. Increase in temperature of the blood, as 

 in fever, will produce a similar effect. 



4. The most important of these agencies is found in affer- 

 ent impressions conveyed to the center. The fibers carrying 

 these impressions are chiefly in the pneumo gastric, glosso- 

 pharyngeal, trigeminal and cutaneous nerves. Of these the 

 pneumogastric is by far the most important. 



Section of a single pneumogastric is followed by variable 

 respiratory disturbances which usually disappear in less than 

 an hour. Section of both nerves is followed, after a short 

 interval of increased respiratory activity, by slow and pow- 

 erful inspirations, by forced expiration and an appreciable 

 interval before the next inspiration. Irritation of the cen- 

 tral end of the cut nerve by a very weak current seems to 

 stimulate the inhibitory part of the center, for the rate is 

 slowed, the expirations are strenuous and the inspirations 

 weak. When the current is increased to a moderate strength 

 opposite results are obtained, the accelerator portion of the 

 center being stimulated. These facts show that the pneu- 

 mogastrics possess both inspiratory and expiratory fibers, 

 and that the former are stimulated more by a moderate cur- 

 rent and the latter more by a very weak one. The rhythm 

 of respiration, therefore, includes the regular sequence of 

 inspiratory and expiratory movements upon each other. 



Now what is it that, under normal conditions, irritates the 

 terminals of the pneumogastrics and causes them to convey 

 inspiratory and expiratory impressions? It has been held 

 that a change in the composition of the alveolar air an ac- 

 cumulation of carbon dioxide irritates the nerve terminals 

 and explains the conveyance of the inspiratory impressions, 

 while the stretching of the lung tissue originates the expira- 

 tory impressions. Others ascribe both inspiratory and ex- 

 piratory impressions to lung movements movements of in- 



