478 



INFLUENCE OF VAGUS NERVES. [Book n. 



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 expiration of the preceding breath is com- 

 pleted, Fig. 94 ; and this may go on until at last the diaphragm 

 is brought into a condition of prolonged tetanus, and a stand- 

 still 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 



Fig. 94. 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 § 251. The lever rises with each contraction of the diaphragm so that inspira- 

 tion begins at a and ends at b, expiration begins at b 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. 



slowed by division of the nerves, stimulation produces a still 

 further slowing, the pauses between each expiration and the 

 succeeding inspiration are prolonged (cf. Fig. 95), 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 extremes ; the 

 diaphragm stands still in a prolonged contraction in a position 

 which is intermediate 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 



