476 



INFLUENCE OF VAGUS NERVES. [BOOK n. 



rate of rhythm. Then, again, combinations of effects so numer- 

 ous and varied as almost to baffle description may result from 

 the influence of various nervous impulses. Emotions may affect 

 a single breath or a long series of breaths, may quicken the 

 rhythm while making each breath more shallow or may at the 

 same time make each breath deeper, or may slow the rhythm in 

 either the one or the other manner, and may bear chiefly on 

 inspiration or on expiration. Moreover there is not an afferent 

 nerve in the body which, by means of afferent impulses passing 

 along it, may not be the instrument of influencing the respira- 

 tory centre. Of all the automatic centres in the body the 

 respiratory centre is the one whose independence is most 

 obscured by the repeated effects of afferent nervous impulses. 



Certain afferent nerves however appear to be more closely 

 connected with it than others ; and of these the most conspicu- 

 ous 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, 



/SA/S 



/v 



\J 



/V* 



A, 



\J 



\ 



FIG. 91. EFFECT ON RESPIRATION OF SECTION OF ONE VAGUS. 



The vagus was divided at the point marked x. The curve was obtained by 

 means of a tambour connected with a receiver into which the animal (rabbit) 

 breathed as shewn in Fig. 85, the lever falling in inspiration as air is sucked out 

 of the tambour, and rising in expiration as the air returns. Inspiration begins 

 at a and ends at b. Expiration begins at b and ends at c. The lever gradually 

 falls between c and a owing to the escape of air from the apparatus. 



the respiration is either not materially changed, or if affected 

 becomes slower (Fig. 91). If both be divided (Fig. 92) it 

 becomes very slow, the pauses between expiration and inspira- 

 tion being markedly prolonged. The character of the respira- 

 tory movement too is markedly changed; each respiration is 

 fuller and deeper, so much so indeed that, according to some 

 observers, what is lost in rate is gained in extent, the amount 



