690 PHYSIOLOGY OF RESPIRATION. 



only after section of both nerves. This result is not a temporary 

 one due to the stimulation of cutting, but is permanent, and there- 

 fore we may conclude that some influence has been cut off 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 distributed 

 to the lungs. It is the constant effect of these fibers on the respira- 

 tory center which maintains the normal rhythm; when they are 

 severed the center drops into a slower, unregulated rhythm. Ex- 

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

 divided vagus is stimulated artificially the respiratory 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 vagi on the respiratory movements 

 (rabbit). The right vagus was cut at x and caused a slight 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. (Dawson.) 



the thorax in the stage of passive expiration (Fig. 277), or, second, 

 the rate of the inspiratory movements may 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 perplexing, especially with strong 



