CHAP, ii.] 



RESPIRATION. 



619 



is a double act consisting of an inspiration and an expiration, and 

 nervous impulses may especially affect the one or the other. One 

 mode of breathing may differ from another in the depth of the 

 individual breath, in the volume of air taken in and given" out ; 

 and nervous impulses may increase or may diminish the depth of a 

 breath, the volume of air respired. One mode of breathing again 

 differs from another in the rapidity with which one breath succeeds 

 another, that is, in the rate of rhythm ; and nervous impulses may 

 slow or may quicken the rate of rhythm. Then, again, combinations 

 of effects so numerous 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 hardly any, if any, 

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

 passing along it, may not be the instrument of influencing 

 the respiratory 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 im- 

 pulses. 



Certain afferent nerves however appear to be more closely con- 

 nected 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 



FIG. 77. EFFECT ON BESPIBATION 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. 71, 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 6. Expiration begins at 6 and ends at c. The lever gradually falls 

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



402 



