CHANGES IN THE AIR BY RESPIRATION. 289 



brane, by which it is absorbed from the blood. At ordinary tempera- 

 tures it is transparent and invisible ; but in cold weather it becomes 

 partly condensed on leaving the lungs, and appears under the form of a 

 cloudy vapor in the breath. According to the researches of Valentin, 

 the average quantity of water exhaled from the lungs is about 500 

 grammes per day. 



The exhalation of watery vapor by the lungs is a purely physical 

 process, dependent upon the moist and permeable structure of the pul- 

 monary mucous membrane and the volatility of the watery fluid, by 

 which it necessarily becomes vaporized under the requisite conditions 

 of temperature at the ordinary pressure of the atmosphere. Any moist 

 animal membrane, after death as well as during life, loses water by 

 evaporation and thus becomes gradually desiccated. Experiments upon 

 recently killed frogs have shown that the spontaneous desiccation goes 

 on rapidly at first, and afterward more slowly, as the proportion of water 

 contained in the tissues becomes diminished. In the lungs of a warm- 

 blooded animal during life all the requisite conditions are present for 

 rapid and continuous evaporation, namely, a moderately elevated tem- 

 perature, a constant renewal of atmospheric air by the movements of 

 respiration, and a continuous supply of fresh moisture to the pulmonary 

 membrane by the blood circulating in its vessels. The quantity of 

 watery vapor exhaled by the lungs in a given time is therefore increased 

 or diminished by corresponding changes in the rapidity of respiration, 

 by greater dryness or humidity of the atmosphere, and by increase or 

 diminution of the pulmonary circulation. 



In some species of animals, as in the dog, where the integument is 

 comparatively destitute of perspiratory glands, the pulmonary trans- 

 piration becomes much more active ; and it is not uncommon to see 

 these animals, in hot weather, lying at rest with their tongues protruded, 

 and the movements of respiration doubled or trebled in frequency, for 

 the purpose of increasing the watery exhalation from the lungs. 



In the human subject the precise physiological value of the pulmonary 

 transpiration is not known. Though subject to fluctuations according 

 to variation in the physical conditions above mentioned, it is a continu- 

 ous process, and even at ordinary temperatures the expired breath 

 directed upon a polished glass or metallic surface will always produce 

 an immediate dimness by the condensation of its watery vapor. It is 

 very possible that the vapor thus exhaled, beside being complementary 

 to the perspiration by the skin, may serve as a vehicle for the discharge 

 of certain other substances from the pulmonary cavity. 



Exhalation of Organic Matter by the Breath. Beside carbonic acid 

 and water, the expired air also contains a small amount of an organic 

 ingredient, which communicates a faint but perceptible odor to the 

 breath. This substance is discharged in the vaporous form, probably 

 entangled in the watery vapor exhaled by respiration. Under ordinary 

 circumstances it is present in so small a quantity as to be hardly notice- 

 able ; but if a large number of persons be confined in a small apartment 



