332 RESPIRATION 



If then the first act of respiration be induced by some property of the 

 blood, it follows with a high degree of probability that the respiratory center 

 is roused to activity in the same way throughout life. This is confirmed also 

 by a large number of experimental facts. Thus it has been shown that every- 

 thing that tends to heighten the combustion in the body or to render more 

 difficult the elimination of the gaseous products of decomposition or the 

 absorption of oxygen, produces an augmented respiration. This condition of 

 things is described as dyspnoea, if it involves the cooperation of the accessory 

 muscles of respiration. 



One might conceive that the products of combustion present in the blood 

 in increased quantity stimulate the end arborizations of the afferent nerves, 

 and that the augmented respiration now under consideration is therefore 

 reflex in nature. Even if this were true, experiment has shown in the clearest 

 possible manner that, in muscular work, for example, the increased respiration 

 is not due to this cause alone; for it appears when the hinder parts of the 

 body, cut off from every possible nervous communication with the fore parts, 

 are stimulated to active contractions, but is entirely wanting if the return 

 flow of blood from the hinder parts is prevented. The blood returning from 

 the posterior active parts has therefore a direct stimulating effect upon the 

 respiratory center (Zuntz and Geppert). 



Finally it has been shown that the respiratory movements react very deli- 

 cately to any change in the carbon-dioxide content of the blood, since, the 

 respiratory frequency remaining almost unchanged, the breath volume of 

 the individual respirations increases with an increasing quantity of C0 2 in 

 the inspired air (Miescher). On the other hand, considerable changes in the 

 oxygen content of the surrounding air (12.5 to 60 volumes per cent) influence 

 the respiration relatively little. 



We conclude, therefore, that the respiratory center is excited by the direct 

 effect of the blood or the lymph, but that its action is regulated by all kinds 

 of afferent nerves, especially by the vagi and the brain pathways. 



The condition of apncea, or respiratory standstill, which is induced by ex- 

 cessive inflation of the lungs, or in man by one or more very deep inhalations, 

 has often been regarded as a very important fact in support of the conception 

 here presented. Apnoea might have its justification in the unusual opportunity 

 which the blood has, in consequence of unusually ample ventilation, of becom- 

 ing saturated with oxygen and of freeing itself of carbon dioxide, so that the 

 next respiration would be less necessary. But the matter is not so simple. It 

 has been made clear, for example, that in the rabbit apncea is much more diffi- 

 cult to obtain if the vagi are cut. These nerves must have something to do, 

 therefore, with bringing about this condition. Moreover, apnoea appears as the 

 result of inflation with hydrogen and can be induced by forcing the same air 

 into the lungs over and over (Gad). Finally, it ceases only after the other 

 organs have shown signs of asphyxiation. We may say, therefore, that apnoea 

 depends at least in part upon an inhibitory action of the vagus upon the respira- 

 tory center. But the condition of the blood is not without its importance also, 

 as the following experiment shows. Two dogs were operated upon in such a 

 way that the carotid blood of the first was led into the head of the second. A 

 condition of apncea was then induced in the second dog by artificial respiration 

 applied to the first (Fredericq). We might distinguish this form of apncea 



