388 RESPIRATION. 



need much longer immersion in water before death by asphyxia occurs than 

 do adults. Thus while in a full-grown dog recovery from drowning is 

 unusual after one and a half minutes, a newborn puppy has been known to 

 bear an immersion of as much as fifty minutes. The cause of the difference 

 lies in the fact that in the quite young or rather just-born animal the respi- 

 ratory changes of the tissues are much less active. These consume less 

 oxygen, and the general store of oxygen in the blood has a less rapid de- 

 mand made upon it. The respiratory activity of the tissues may also be 

 lessened by a deficiency in the circulation ; hence bodies in a state of syn- 

 cope at the time when the deprivation of oxygen begins can endure the loss 

 for a much longer period than can bodies in which the circulation is in full 

 swing. There being the same store of oxygen in the blood in each case, the 

 quicker circulation must of necessity bring about the speedier exhaustion of 

 the store. So also anaesthetics may diminish the effects and delay the final 

 results : large doses of anaesthetics may prevent the exaggerated and con- 

 vulsive movements. In many cases of drowning, death is hastened by the 

 entrance of water into the lungs. 



By training, the respiratory centre may be accustomed to bear a scanty 

 supply of oxygen for a much longer time than usual before dyspnoea sets in, 

 as is seen in the case of divers. 



The phenomena of slow asphyxia, where the supply of air is gradually 

 diminished, are fundamentally the same as those resulting from a sudden 

 and total deprivation. The same stages are seen, but their development takes 

 place more slowly. 



320. Deficiency of air results not only in a diminution of the oxygen 

 but also in an increase of the carbonic acid of the blood. We have seen, 

 however ( 315), that the phenomena of asphyxia are in the main due to the 

 former, and that the accumulation of carbonic acid in the blood has subsid- 

 iary effects only. 



If the percentage of oxygen in the inspired air be increased instead of 

 diminished, the total pressure of the atmosphere remaining the same, the 

 partial pressure of the oxygen alone being changed, no marked results 

 follow. We have already seen ( 297) that the percentage of oxygen in the 

 ordinary atmosphere leaves a wide margin of safety, and that ( 317) the 

 phenomena of apncea are in the main at least to be explained as the result 

 not of an increase in the oxygen of the blood but of nervous impulses ascend- 

 ing the vagus nerves. We have no satisfactory evidence that, provided the 

 respiratory mechanism is in good working order, an increase of oxygen in 

 the inspired air even to a whole atmosphere seriously modifies the respiratory 

 act ; and it may be doubted whether any effect is produced even when the 

 mechanism is impaired. 



321. The composition of the atmosphere, the pressure remaining the 

 same, may be modified by the introduction of foreign gases. To some of 

 these the respiratory mechanism is indifferent ; for instance, hydrogen may 

 be substituted for nitrogen without any change in the respiration, provided, 

 of course, that the oxygen is not diminished. Other gases may produce 

 poisonous effects, either by interfering with some of the respiratory processes 

 or in other ways. Thus carbon monoxide, by combining with the haemo- 

 globin of the red corpuscles, and so preventing the corpuscles from acting 

 as oxygen-carriers, produces asphyxia through deficiency of oxygen. Sul- 

 phuretted hydrogen interferes with the oxygenation of the blood by acting 

 as a reducing agent. Some gases while allowing the ordinary respiratory 

 changes of the blood to go on as usual produce toxic effects by acting on one 

 or other of the tissues. Thus, as we have seen, an excess of carbonic acid 

 in the blood seems to have a special effect on the central nervous system and 



