590 



INFLUENCE OF VAGUS NERVES. [BOOK n. 



of two results may follow and that whichever of the two nerves be 

 used. In a certain number of cases, and these may perhaps be 

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

 division of the nerves had become slow, is quickened again (Fig. 79); 

 and with care, by a proper application of the stimulus, the normal 

 respiratory rhythm may for a time be restored. Upon the cessa- 

 tion of the stimulus, the slower rhythm returns. If the current 

 be increased in strength, the rhythm may in some cases be so 

 accelerated that inspiration begins before the expiration of the 



FIG. 80. STIMULATION OF VAGUS LEADING TO INSPIRATORY INCREASE. 



This curve, unlike the preceding, was obtained by inserting a needle through the 

 body wall so as to rest on the diaphragm and attaching a lever to the needle ; see 

 328. The lever rises with each contraction of the diaphragm so that inspiration 

 begins at a and ends at 6, expiration begins at 6 and ends at c, the interval between 

 c and a corresponding to the pause. 



Stimulation of the vagus begins at x. It will be seen that upon stimulation 

 the inspiratory rises of the lever begin long before the preceding expirations are 

 complete. 



preceding breath is completed, Fig. 80 ; and this may go on until 

 at last the diaphragm is brought into a condition of prolonged 

 tetanus, and a standstill of respiration in an extreme inspiratory 

 phase is the result. On the other hand in a certain number of 

 cases the result is of an opposite character. Even though the 

 respiration be already slowed by division of the nerves, stimula- 

 tion produces a still further slowing, the pauses between each 

 expiration and the succeeding inspiration are prolonged (cf. Fig. 

 81), and in a certain number of cases, actual standstill is brought 

 about, but a standstill of a kind the opposite of the one just 

 described, since the diaphragm which in that case was in 

 prolonged tetanus is, in this case, completely relaxed, and remains 

 for some time in the condition in which it is at the close of an 

 ordinary breath. In a certain number of cases, and these are not 

 uncommon, the result is intermediate between the two above 



