

EESPIRATION 309 



phase being chiefly accentuated. If the stimulus is very strong 

 respirations are stopped in the phase of inspiration. Weak 

 stimuli, on the other hand, may cause inhibition of inspiration. 



Such experiments prove that impulses are constantly travel- 

 ling from the lungs to the centre regulating the rhythmic 

 activity of the centre. 



How do these impulses originate in the lungs ? Apparently 

 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. The 

 nature of the gas, if non-irritant, with which this inflation is 

 carried out is of no consequence. If, on the other hand, the 

 lungs be collapsed by sucking air out of them, the inspiration 

 becomes more and more powerful, and may end in a spasm 

 of the inspiratory muscles. 



This shows that with each expiration a stimulus passes up 

 the vagus which acts upon the inspiratory centre to make it 

 discharge. The vagus is thus an excito-motor nerve, making 

 the centre act in a reflex manner. With each collapse of the 

 lung the vagus is thrown into action ; as the lungs expand it 

 ceases to act, and, as a result, the inspiratory centre stops 

 acting, the muscles of inspiration cease to contract and 

 expiration occurs. 



Action of other Ingoing Nerves on the Respiratory Centre. 

 The upper part of the respiratory tract, the larynx, receives 

 its sensory fibres from the vagus. Section of the superior 

 laryngeal branch of the vagus does not alter the rhythm of 

 respiration. Stimulation of the upper end of the cut nerve 

 causes first an inhibition of inspiration, and, if stronger, pro- 

 duces forced expiratory acts. This is well illustrated by the 

 very common experience of the effect of a foreign body, such 

 as a crumb, in the larynx. The fit of coughing is a series 

 of expiratory acts produced through this nerve. 



Another set of visceral nerves having an important influence 

 on the respiratory rhythm are the splanehnies. When these 

 are stimulated inspiration is inhibited. Everyone has ex- 

 perienced the "loss of wind" as the result of a blow on the 

 abdomen. 



The glossopharyngeal, which supplies the back of the 

 tongue, when stimulated, as by the passage of food in the 



