THE NERVOUS MECHANISM OF RESPIRATION. 



373 



Certain afferent nerves, however, appear to be more closely connected 

 with it than others ; and of these the most conspicuous and important are 

 the two vagus nerves, which we have already mentioned in this connection. 

 Their importance is well illustrated by the following experiments: If one 

 vagus be divided in an ordinary way, without any special precautions, the 

 respiration is either not materially changed, or if affected becomes slower 

 (Fig. 99). If both be divided (Fig. 100) it becomes very slow, the pauses 

 between expiration and inspiration being markedly prolonged. The cha- 

 racter of the respiratory movement, too, is markedly changed ; each respira- 

 tion is fuller and deeper, so much so, indeed, that, according to some observ- 

 ers, what is lost in rate is gained in extent, the amount of carbonic acid pro- 



FIG. 100. 



f\ 



Effect on Respiration of Section of Both Vagus Nerves. The curve was obtained in the same 

 way as Fig. 99. The second vagus nerve was divided at x. 



duced and oxygen consumed in a given period remaining after division of 

 the nerves about the same as when these were intact ; but it is undesirable 

 to insist too much on the exactness of this compensation. 



FIG. 101. 



Quickening of Respiration by Gentle Stimulation of the Central End of the Vagus Trunk. The 

 curve was obtained in the same way as Figs. 99, 100. Stimulation of the vagus began at x and 

 ended at y. 



When after division of both vagus nerves in the neck, the medulla being 

 intact, the central stump, that connected with the central nervous system, of 

 one of them is stimulated with a gentle interrupted current, the effects are 

 not always the same ; one of two results may follow, and that whichever of 

 the two nerves be used. In a certain number of cases, and these may, per- 

 haps, be regarded as the more typical ones, the respiration, which from the 



