RELATION OF RESPIRATION TO THE NERVOUS SYSTEM 275 



terminal ' spindles/ or by the chemical action on them of certain 

 waste products produced in contraction. It is quite likely that this 

 is one way in which the adjustment is achieved. But this is not 

 the only, and perhaps not the most important, way. For an in- 

 crease in the respiratory movements is caused by tetanizing the 

 muscles of a limb whose nerves have been completely severed, ancj, 

 which is indeed connected with the rest of the body by no other 

 structures than its bloodvessels. This can only be due to two things : 

 a direct action on the respiratory centre by the blood that has 

 passed through, and been altered in, the contracting muscles, or an 

 action exerted by the blood indirectly on the centre through the 

 excitation of afferent respiratory nerves whose connection with it 

 is stilt intact for example, the other muscular nerves or the pul- 

 monary branches of the vagus. That the action is direct is shown 

 by the fact that after section of the vagi, the sympathetic, and the 

 spinal cord below the origin of the phrenics, an increase in the 

 respiratory movements is still produced by tetanizing a limb. 



The Chemical Regulation of the Respiration. However important 

 the regulation of respiration by afferent nervous impulses may be, 

 the normal discharge of the respiratory centre is intimately associ- 

 ated with the gases of the blood. 



It is generally aclmowledged that the centre may be excited both 

 by blood that is rich in carbon^ dioxide and by blood that is poor in 

 oxygen. Stimulation by deficiency of oxygen has to some minds 

 presented a metaphysical difficulty namely, that it k not easy to 

 see how the absence of a thing could cause stimulation. The diffi- 

 culty does not exist, but none the less there is some evidence that 

 when oxygen is lacking the respiratory centre can be excited by 

 substances like lactic acid, which are easily oxidizable and rapidly 

 disappear from properly oxygenated blood. On the other hand, it 

 it stated that, when the oxidative processes of the medullary centres 

 are decreased by the administration of carbon monoxide or sodium 

 cyanide, the latent period which precedes the excitation of the 

 respiratory (and other) centres is so short that the stimulation 

 cannot be attributed to the accumulation of acid products, and 

 that the mere oxygen want is of itself a stimulus for these centres 

 (Rosenthal, Gasser and Loevenhart). 



Be that as it may, it has been the subject of long-continued dis- 

 cussion whether excess of carbon dioxide or deficiency of oxygen is 

 the more potent stimulus for the respiratory centre. The best evi- 

 dence points to the conclusion that comparatively small alterations 

 in the amount of carbon dioxide in the inspired air cause a relatively 

 great increase in the respiration, while in the case of the oxygen the 

 departure from the normal proportion must be much more decided 

 to bring about any notable effect. Nor is it at all out of harmony 

 with this that, when very large quantities of carbon dioxide (30 per 



