292 RESPIRATION. 



amount of from 800 to 1200 cubic centimetres. At first it produces in 

 the chest only a sensation of warmth and moderate stimulus. But at 

 the end of two or three seconds there comes on very suddenly a sense 

 of extreme dyspnoea, with rapid and laborious respiration, accompanied 

 immediately by dimness of vision, slight confusion of mind, and partial 

 insensibility, all of which are soon terminated, as respiration returns to 

 its normal condition, leaving only a feeling of quietude and tendency to 

 sleep. 



Notwithstanding, however, the intense feeling of dyspnoea produced 

 by such an inhalation of pure carbonic acid, the external signs of actual 

 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, and the access of rapid and 

 laborious respiration, come on as before. The dyspnoea, suffusion of 

 the face, and lividity are all less marked than after breathing pure 

 carbonic acid, but the subsequent condition of quiescence and partial 

 anaesthesia is more decided and of longer continuance. 



A mixture of one volume of carbonic acid with three volumes of 

 atmospheric air may be inspired without difficulty, producing a rather 

 agreeable sensation by contact with the lungs. After about 3000 cubic 

 centimetres have been inhaled in successive inspirations, a sense of 

 dyspnoea comes on, which however is not particularly increased by con- 

 tinuing the inspiration of the mixture to 6000 cubic centimetres. The 

 nervous symptoms produced are moderate in degree, but similar to the 

 preceding. 



On the other hand, pure nitrogen has no taste or odor, nor does it 

 have any stimulating effect on the mucous membrane. It may be inspired 

 from the gasometer to the amount of 6000 cubic centimetres, without 

 producing any sense of dyspnoea, or any perceptible effect on the nervous 

 system. 



These results indicate that the presence of carbonic acid in the lungs 

 acts as a stimulus to respiration by causing a sense of the want of air ; 

 and that furthermore its principal toxic effect, when in abnormal quan- 

 tity, is that of producing more or less insensibility or anaesthesia. The 

 sense of drowsiness and inattention experienced by an audience in an 

 imperfectly ventilated lecture-room or theatre is probabry due to this 

 cause, especially as the burning gas-lights are at the same time contribu- 

 ting to the formation of carbonic acid. The temporary nature of these 

 sensations, and their immediate relief on coming into the open air, are 

 matters of common observation. 



The third element in the vitiation of air by the breath is the accumu- 

 lation of its organic vapor. This is the least understood, but probably 

 the most deleterious ingredient of the atmosphere produced by respira- 



