350 THE RESPIRATION 



are placed 'on the vagus nerve and connected with a galvanometer, a 

 current of action occurs toward the end of each inspiration in quiet 

 breathing; and when the respirations are forced, a current of action 

 appears during both inspiration and expiration. Another reason for 

 believing that the vagi have some important function to perform in con- 

 nection with the control of respiration is the fact, observed by F. H. Scott, 16 

 that in an intact animal, when atmospheres containing increasing percent- 

 ages of carbon dioxide are respired, the respirations become both deeper 

 and quicker, whereas in one whose vagi have been cut the carbon diox- 

 ide causes only a deepening of the respirations. From this result it 

 would appear that the vagi exert an influence on the rate of the respira- 

 tions but not on their depth, this effect, as we shall see later, being de- 

 pendent primarily on changes in the composition of the blood supplying 

 the respiratory center. It is probable that both controlling agencies act 

 together, the one serving to maintain the center in a proper state of 

 excitability, and being active to a greater or less extent all the time; 

 while the other acts only occasionally on the " tuned up" center. There 

 is, of course, no doubt that it is through the nerves that the occasional 

 alterations of respiration occur. They appear also to have a certain 

 influence on the rhythm, for Stewart, Pike and Guthrie 17 observed that, 

 after resuscitation from acute brain anemia, the respirations when they 

 returned were of the same rhythm as that of the artificial respirations 

 employed during the resuscitation. 



Afferent Impulses from Other Parts of the Body 



To the first group belong afferent nerves from practically every part of 

 the body. That impressions from the skin affect the respiratory center 

 is well known by the increased breathing caused by applications of cold 

 water. The influence of these afferent impulses is often very marked, 

 and is frequently taken advantage of in stimulating a newborn infant to 

 take the first breath. Stimulation of the terminations of the fifth nerve 

 in the mucous membrane of the nose, as by inhaling a pungent odor, 

 immediately inhibits respiration. To these occasionally acting afferent 

 impulses may be added the impulses that are conveyed to the respiratory 

 center from the higher nerve centers of the cerebrum. These impulses 

 are largely voluntary in nature, and enable us to hold our breath at will. 

 Some of the cerebral impulses are however also involuntary, their exist- 

 ence being seen by observing the respirations of an animal before and 

 after sectioning the pons or peduncles. The respirations for a time at least 

 become distinctly affected, but they later return with perfect regularity. 

 They may become very irregular, however, if the vagi as well as the pons 

 are cut. Other experimental evidence of the existence of cerebral respir- 



