INNERVATION OF THE RESPIRATORY MOVEMENTS. 687 



cut oft" which normally keeps the respiratory movements at a more 

 rapid rate. Experiment has shown that this influence consists in 

 the tonic action of sensory fibers contained in the vagus and dis- 

 tributed to the lungs. It is the constant eftect of these fibers on the 

 respiratory center which maintains the normal rhythm ; when they 

 are severed the center drops into a slower, unregulated rhythm. 

 Experiment has shown, also, that when the central stump of the 

 divided vagus is stimulated artificially the respirator^y center is 

 affected, as indicated by the respiratory movements, in a variety 

 of ways, depending upon the strength of the stimulus and the 

 condition of the center. The two results which are most constantly 

 obtained and which may therefore be especially emphasized are as 

 follows: first, with weak stimuli the inspiratory movements are in- 

 hibited partially or completely, giving either smaller movements or, 

 in a condition of narcosis, complete cessation of respirations, with 



Fig. 278. — ^To show the effect of section of the vaf;i on tlie leNiiiiatory movements 

 (rabbit). The right vagus was cut at x and caused a sHght augmentation and slowing 

 of the movements. The left vagus was cut at xx and caused first a short inhibition (due 

 to mechanical stimulation) which was then followed by the typical slow and deep respi- 

 rations seen under these conditions. — (Daivson.) 



the thorax in the stage of passive expiration (P'ig. 277), or, second, 

 the rate of the inspiratory movements msiy be increased and this 

 may end finally in an inspiratory standstill, — that is, the respiratory- 

 movements cease with the chest in an inspiratory position (Fig. 279), 

 the inspiratory muscles being in a condition of tetanic contraction. 

 When both the inspiratory and expiratory muscles are considered, 

 the variety of effects that may be obtained from stimulation of the 

 afferent fibers of the vagus is jjerplexing, especially with strong 

 stimuli, and has led to much difference of opinion among investi- 



