HAP. ii.] RESPIRATION. 487 



While the respiratory centre is thus being affected by the 

 too venous blood, it is, until exhaustion begins to set in, more 

 irritable, more easily and largely affected by afferent impulses 

 than in its normal condition. During dyspnoea a stimulus 

 which applied to the vagus or to some other sensory nerve 

 under normal conditions would produce little or no effect, may 

 start very powerful respiratory movements. 



302. Deficient aeration produces two effects in blood : it 

 diminishes the oxygen, and increases the carbonic acid. Do 

 both of these changes affect the respiratory centre, or only one, 

 and if so, which ? When an animal is made to breathe an atmos- 

 phere containing nitrogen only, the exit of carbonic acid by 

 diffusion is not affected, and the blood, as is proved by actual 

 analysis, contains no excess of carbonic acid. Yet all the phe- 

 nomena of dyspnoea are present, and if the experiment be con- 

 tinued, convulsions ensue and the animal dies in asphyxia. In 

 this case the result can only be attributed to the deficiency of 

 oxygen. On the other hand, if an animal be made to breathe 

 an atmosphere rich in carbonic acid, but at the same time con- 

 taining abundance of oxygen, though the breathing becomes 

 markedly deeper and also somewhat more frequent, there is no 

 culmination in a convulsive asphyxia, even when the quantity 

 of carbonic acid in the blood, as shewn by direct analysis, is 

 very largely increased. On the contrary, the increase in the 

 respiratory movements may after a while pass off, the animal 

 becoming unconscious, and appearing to be suffering rather 

 from a narcotic poison than from simple dyspnoea ; the excess 

 of carbonic acid in the blood appears to affect other parts of 

 the central nervous system, and especially portions of the brain, 

 more profoundly than it does the respiratory centre. It has 

 been maintained by some that while a deficiency of oxygen 

 promotes inspiratory movements, an excess of carbonic acid 

 stimulates the expiratory movements, the nervous mechanisms 

 being so arranged that a lack of oxygen leads to an effort to 

 get more of it and a too great load of carbonic acid to an effort 

 to get rid of it ; but the facts are opposed to the existence of 

 any such teleological adaptation. It is obvious however that a 

 lack of oxygen and an excess of carbonic acid affect the respir- 

 atory centre in very different ways, and that in ordinary cases 

 of interference with the interchange in the lungs, as in defi- 

 cient aeration, it is the lack of oxygen which plays the prin- 

 cipal part in developing the abnormal respiratory movements. 

 We may infer that it too is chiefly concerned in regulating the 

 more normal respiration, but cannot as yet say what is the 

 exact share to be attributed to the carbonic acid. 



We may here point out that it is not to be supposed that 

 each breath is determined by the condition of the blood flowing 

 through the capillaries of the medulla at the moment preceding 



