588 



INFLUENCE OF VAGUS NERVES. [BOOK n. 



act consisting of an inspiration and an expiration, and nervous im- 

 pulses 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 

 <ir 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 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 impulses. 



Certain afferent mrves however appear to be more closely con- 

 nected with it than others ; and of these the niost 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 



a 



J 



FIG. 77. EFFECT ON RESPIRATION OF SECTION OF ONE VAGUS. 



The vagus was divided at the point marked .r. 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 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. 



