374 



RESPIRATION. 



division of the nerves had become slow, is quickened again (Fig. 101), and 

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

 rhythm may for a time be restored. Upon the cessation 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 expira- 

 tion of the preceding breath is completed (Fig. 102), and this may go on 



FIG. 102. 



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 attach- 

 ing a lever to the needle. (See 271.) The lever rises with each contraction of the diaphragm, 

 so that inspiration begins at a and ends at b, expiration begins at 6 and ends at c, the interval be- 

 tween 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 expira- 

 tions are complete. 



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, stimulation produces a still further slowing, the pauses between 

 each expiration and the succeeding inspiration are prolonged (c/. Fig. 103), 

 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 dia- 

 phragm 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 extremes ; the 

 diaphragm stands still in a prolonged contraction in a position which is inter- 

 mediate between the height of inspiration and expiration. 



These results suggest the conclusion that the vagus nerve (we are dealing 

 now with the main trunk of the nerve) contains afferent fibres of two kinds 

 connected with the respiratory centre ; one kind augmenting the action of 

 the centre somewhat in the same way as the augmentor cardiac fibres aug- 

 ment the beat of the heart, and the other kind having an inhibitory effect. 

 Apparently sometimes the one and sometimes the other kind is, according to 

 circumstances, most provoked by the stimulation, much in the same way 

 as stimulation of the vagus in the frog, which, as we have seen, 144, is 

 the channel for both inhibitory and augmentor cardiac impulses, produces 

 sometimes inhibition, sometimes augmentation of the heart-beat. To affect 

 the heart of course the stimulation of the vagus must be centrifugal, 

 directed toward the periphery, whereas to affect the respiration it must be 

 centripetal, applied to the part of the nerve connected with the brain ; and 



