RESPIRATION 533 



vagus, below the origin of the superior laryngeal, generally 

 causes the respirations to become more rapid, the inspiratory 

 phase being chiefly accentuated. Weak stimuli, on the other 

 hand, may cause inhibition of the respirations. 



Such experiments prove that impulses are constantly travel- 

 ling from the lungs to the centre to regulate its rhythmic 

 activity. 



Positive and Negative Ventilation, i.e. passively inflating 

 and deflating the lungs, shows that two sets of fibres come into 

 play in normal respiration. 



If the lungs be forcibly inflated, the inspirations become 

 feebler and finally stop. The nature of the gas, if non-irritant. 



rvrvi, 



d d' 



Fm. 216.— Tracings of the Respiratiou — Downstroke is inspiration ; Upstroke 

 is expiration. At a one vagus nerve was cut ; at h 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. 



with which this inflation is carried out, is of no consequence. 

 If, on the other hand, air is sucked out of the lungs, inspira- 

 tions become more dominant, and may end in a spasm of the 

 inspiratory muscles. 



Inspiration is thus checked by one set of fibres and expira- 

 tion by another, and tlie vagus thus regulates the action of the 

 respiratory nervous mecJtanism, much as the petidulmn regu- 

 lates the action of a clock. Under some conditions, e.g. after 

 gassing, the activity of this reflex may be increased so that 

 inspiration and expiration are checked too soon, and the breath- 

 ing may thus be made shallow and quick. 



(2) Other Ingoing Nerves. — (a) Section of the superior laryn- 



