292 HUMAN PHYSIOLOGY 



again interrupted by another more or less long and strong 

 discharge. 



Separation of the respiratory centre from the vagi and 

 upper brain tracts brings about a loss of its rhythmic 

 action, but does not stop its activity. The centre owes the 

 rhythmic nature of its action to afferent impulses. These 

 afferent impulses reach it normally through the vagi, but 

 when these are cut the upper brain takes upon itself the 

 function of maintaining the rhythm. 



To investigate further this influence of the vagus it is 

 necessary to study the effect of stimulating the nerve. 



Strong stimulation of the pulmonary branches of one 



d' 



FlO. 137. Tracings of the Respirations Downstrokes, Inspiration; Upstrokes, 

 Expiration. At a one Vagus Nerve was cut ; at 6 the second was divided ; 

 at c the Upper Brain Tracts also were cut off ; d and d' show the effect of 

 Stimulating the Glossopharyngeal Nerve. 



vagus (vagus below the origin of the superior laryngeal) 

 causes the respiration to become more and more rapid, the 

 inspiratory phase being chiefly accentuated. If the stimulus 

 is very strong respirations are stopped in the phase of in- 

 spiration. Weak stimuli, on the other hand, may cause 

 inhibition of inspiration. 



Such experiments prove that impulses are constantly 

 travelling from the lungs to the centre whereby the rhythmic 

 activity of the centre is maintained. 



How do these impulses originate in the lungs ? Appa- 

 rently from their alternate expansion and contraction. 



If the lungs be forcibly inflated e.g. with a bellows the 

 inspiration becomes feebler and feebler and finally stops. 



