REGULATION OF RESPIRATION 119 



he found that, on stimulating with a strong current, 

 expiratory standstill was induced, but with a weak current 

 inspiratory standstill occurred. He therefore concluded 

 that the pulmonary branches of the vagus conveyed two 

 sets of fibres, differing both in their sensitivity to stimuH 

 and in their central effects. 



Is the activity of the vagus thus experimentally pro- 

 duced a measure of what occurs normally ? The only way 

 in which this question can be answered is by testing the 

 vagus for current of action. This is done by placing two 

 non-polarisable electrodes on the trunk of the nerve and 

 connecting them with the string galvanometer. The 

 current of action waxes with inspiration and wanes with 

 expiration. 



We have already seen that respiration is practically 

 unaffected by separation of the respiratory centre from 

 the higher parts of the brain. But when this operation 

 is combined with section of the va^ respiration ceases. 

 Evidently the centre is affected by nervous impulses from 

 two sources, normally from the vagus and vicariously from 

 the higher centres. 



We may therefore summarise the mechanism of the 

 regulation of respiration as follows :■ — The respiratory centre 

 is played upon by afferent impulses of two kinds, the 

 chemical impulse of the hydrogen ion concentration of the 

 blood and nervous impulses arising in the pulmonary nerve 

 endings. Of these the latter have the effect of hmiting the 

 respiratory excursion, while the former is responsible for 

 the adaptation of lung ventilation to the needs of the body. 

 There is evidence, too, that impulses from the higher centres 

 increase the susceptibility of the respiratory centre to 

 the H. ion concentration of the blood. Whether the 

 alternation of movement is due to rhythmic variation in 

 the sensitiveness of one centre or to the alternating activity 

 of two centres, one inspiratory the other expiratory, we 

 do not know. 



