RESPIRATION. 247 



contains 20 per cent, 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 without diffi- 

 culty in an atmosphere containing even 23 per cent, of carbonic acid,' 

 provided its oxygen were increased to 30 or 40 per cent. Thus a part, 

 at least, of the influence of carbonic acid, when in large quantity, is due to 

 its action in excluding or interfering with the absorption of oxygen. 



Pure carbonic acid, mixed with atmospheric air of normal constitu- 

 tion, is not so fatal in its effect as sometimes 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 gradually mingle with the air, it produces rapid 

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

 few moments the animal falls in a state of insensibility. But if the 

 receiver be removed, allowing the free access of fresh air, the insen- 

 sibility soon passes off, and in a few moments the animal is again 

 breathing in a natural manner, without having suffered any permanent 

 injury. The action of carbonic acid, administered in this way, is 

 similar to that of an anaesthetic vapor, like ether or chloroform, with 

 the addition of strong symptoms of dyspnoea. 



In man the immediate effects of carbonic acid in the inspired air are 

 of a similar nature. The inhalation of pure carbonic acid from a gas- 

 ometer is at first extremely difficult, as its stimulating effect on the 

 mucous membrane produces spasmodic stricture of the glottis. If 

 the gas, however, be allowed to remain for a short time in contact 

 with the mucous membrane this effect passes off, the glottis may be 

 gently opened, and the carbonic acid drawn into the lungs, by a deep 

 inspiration, to the amount of from 800 to 1200 cubic centimetres. At 

 first it produces only a sensation of warmth and moderate stimulus in 

 the chest. But at the end of two or three seconds there comes on very 

 suddenly a sense of extreme dyspnoea, with rapid and laborious respi- 

 ration, followed by dimness of vision, slight confusion of mind, and 

 partial insensibility, all of which symptoms soon disappear, as respira- 

 tion returns to its normal condition, leaving a feeling of quietude and 

 tendency to sleep. 



Notwithstanding, however, the intense feeling of dyspnoea produced 

 by such an inhalation, the external signs of suffocation are very slight, 

 and bear no proportion to the severity of the sensations. They are 

 confined to a little suffusion of the face, with partial lividity of the 

 lips ; and the pulse is but little if at all affected. 



A mixture of carbonic acid and atmospheric air in equal volumes pro- 

 duces a perceptible feeling of warmth and pungency at the glottis, but 

 may still be readily drawn into the lungs. After two or three deep 

 inspirations, the strong sense of want of air, with rapid and laborious 

 respiration, comes on as before. The dyspnoea, suffusion of face, and 

 lividity are less marked than after breathing the pure gas, but the 

 subsequent condition of quiescence and partial anaesthesia, is more 

 decided and of longer continuance. 



