RESPIRATION 179 



tion alternate in the usual way, although the blood is growing 

 steadily more venous. The failure of inspiration to bring about 

 aeration of the blood does not lead to a prolongation of the 

 inspiratory effort. Inspiration is cut off and expiration estab- 

 lished in regular sequence. In performing " artificial respira- 

 tion " (cf. p. 184) for the purpose of saving life, in cases in which 

 respiration has ceased owing to the lungs being filled with 

 water, or for other reasons, the chest is enlarged by raising the 

 arms above the head, and diminished by pressing the elbows 

 against the sides. Enlargement promotes a tendency to ex- 

 piration, compression a tendency to a natural inspiratory effort. 

 Evidently there is a connection between the movements of the 

 chest and the stimulation of the respiratory centre. If respira- 

 tion is being carried on artificially, by forcing air from a bellows 

 into the trachea, the nostrils dilate as the chest is distended, 

 and contract as it is emptied, so long as the vagus nerve is 

 intact, just as they do in normal respiration. This shows that, 

 when the chest is emptied, a message is sent through to the 

 nucleus of origin of the nerve which supplies the dilator muscles 

 of the nostril. When the lungs are full, a message calls upon 

 the nostrils to contract. The only factor which is common to 

 pressing in and pulling out the ribs, and filling and exhausting 

 the lungs with a bellows, is the alteration in the form of the 

 lungs which is produced by the two methods. It is impossible 

 to resist the conclusion that the stretching of the tissue of the 

 lungs stimulates the nerve-endings of the vagus. The impulses 

 thus induced automatically stop inspiration, and lead to an 

 expiratory effort. 



There are many indications that the nervous mechanism of 

 respiration is a double one, certain stimuli inducing expiration, 

 with inhibition of inspiration, others inhibiting expiration and 

 inducing inspiration. There are, however, many difficulties in 

 the way of formulating a satisfactory theory of the relation of 

 these antagonistic actions. We may frequently observe indi- 

 cations of such an antagonism between the two phases of the 

 respiratory mechanism. Cold water dashed on the back of the 

 head (when the head is being shampooed) induces a long in- 

 spiration with inhibition of expiration. A blow in the pit of 

 the stomach " knocks all the wind out of a man." Expiration 

 is prolonged until the lungs are unusually empty, and yet the 



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