CHANGES IN THE AIR BY RESPIRATION. 291 



modifications of the atmosphere in respiration, therefore, react upon 

 each other and combine to produce a common result. 



The second element in the vitiation of the respired air is that due to 

 the presence of carbonic acid. The effect of this gas, as produced by 

 respiration, cannot be ascertained from that of an atmosphere consisting 

 of carbonic acid alone. A man or an animal, introduced suddenly into 

 an atmosphere of pure carbonic acid, as sometimes happens in beer-vats 

 and old wells, dies at once by suffocation. But this result is not due to 

 the influence of carbonic acid. It is simply the consequence of the 

 absence of oxygen ; and death would take place as promptly, in the 

 warm-blooded animals, by exposure to an atmosphere of pure nitrogen 

 or any other indifferent gas. It may be said that, as a general rule, for 

 birds and small mammalians, the atmosphere becomes incapable of 

 supporting life when, in addition to its normal proportion of oxygen, it 

 contains 20 per cent, of its volume of carbonic acid ; that is, five times 

 as much as is present, in man, in the expired breath. But Regnault 

 and Reiset found that dogs and rabbits could continue to breathe with- 

 out difficulty in an atmosphere containing even 23 per cent of carbonic 

 acid, provided its proportion of oxygen were at the same time increased 

 to 30 or 40 per cent. Thus a part at least of the influence of carbonic 

 acid, when present exclusively or in large quantity in the atmosphere, 

 is evidently due to its physical action in excluding or interfering with 

 the absorption of oxygen. 



When pure carbonic acid is gradually mingled with atmospheric 

 air, its immediate effects are not so fatal as they have sometimes been 

 represented. If a pigeon be confined in a glass receiver with a wide 

 open mouth, and carbonic acid be introduced through a tube placed just 

 within the edge of the vessel, so that it will not completely displace the 

 air but gradually mingle with it, its effect is to produce a rapid and 

 laborious respiration, gradually increasing in intensity ; and in a few 

 moments the pigeon falls in a state of complete insensibility. But if 

 the glass receiver be removed and fresh air allowed access, the insen- 

 sibility rapidly passes off, and in a few moments longer the animal is 

 again breathing in a natural manner, without having suffered any per- 

 ceptible permanent injury. The effect of carbonic acid alone, thus 

 mingled with the atmosphere, is very similar to that of an anaesthetic 

 vapor, like ether or chloroform, with the addition of strong symptoms 

 of dyspnoea. 



There is evidence that in man the immediate effects of carbonic acid 

 in respiration are of a similar nature. From personal experiments upon 

 this subject we have found that the inhalation of pure carbonic acid 

 from a gasometer is at first extremely difficult, owing to the stimulant 

 effect of the gas upon the mucous membrane of the larynx, which pro- 

 duces a spasmodic stricture of the glottis. If the gas, however, be 

 allowed to remain in contact with the mucous membrane for a short 

 time, this effect passes off, the glottis may be gently opened, and the 

 carbonic acid drawn into the lungs, by a full, deep inspiration, to the 



