632 EFFECTS OF DEFICIENT ARTERIALIZATIOK [BOOK n. 



by the too venous blood; but the doubts which we previously 

 urged hold good in these cases also. 



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 respira- 

 tory movements. 



372. 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 atmosphere 

 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 phenomena 

 of dyspnoea are present, and if the experiment be continued, 

 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 containing 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 respi- 

 ratory 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 respiratory 

 centre in very different ways, and that in ordinary cases of inter- 

 ference with the interchange in the lungs, as in deficient aeration, 

 it is the lack of oxygen which plays the principal 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 



