RELATION OF RESPIRATION TO THE NERVOUS SYSTEM 281 



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, and 

 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 

 p"a"ssjed 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 still 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 im- 

 portant the regulation of respiration by afferent nervous impulses 

 may be, the normal discharge of the respiratory centre is intimately 

 associated with the gases of the blood. 



It is generally acknowledged 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 is 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. 



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 

 cent, and upwards in rabbits) are inhaled, a condition of narcosis 

 comes on without any previous respiratory distress. For many 

 substances act differently in large and in small doses. HaHane has 

 pointed out how exquisitely sensitive the respiratory centre is to even 

 small changes in the partial pressure of carbon dioxide in the alveo- 

 lar air, and therefore in the arterial blood and the centre itself, and 

 has demonstrated that this is the way in which the amount of the 



