RESPIRATION. 245 



Discharge of Water in Eespiration. The water exhaled with the 

 breath is given off by the pulmonary mucous membrane, by which it 

 is absorbed from the blood. At ordinary temperatures it is a transpa- 

 rent, invisible vapor ; but in cold weather it becomes partly condensed 

 on leaving the lungs, and appears as a cloudy precipitate in the breath. 

 According to Valentin, the average quantity exhaled from the lungs is 

 about 500 grammes per day. 



The exhalation of water by the lungs is a physical process, 

 dependent on the moist and permeable structure of the pulmonary 

 membrane and the vaporization of watery fluid at the ordinary press- 

 ure of the atmosphere. Any moist animal membrane, after death as 

 well as during life, loses water by evaporation and becomes gradually 

 desiccated. Experiments on recently killed frogs show that sponta- 

 neous desiccation goes on at first rapidly, and afterward more slowly, 

 as the proportion of water in the tissues is diminished. In the lungs 

 of a warm-blooded animal during life all the requisite conditions for 

 rapid evaporation are present, namely, a moderately elevated tempera- 

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

 respiration, and a continuous supply of moisture 'by the circulating 

 blood. The watery vapor exhaled is therefore increased or diminished 

 according to the rapidity of respiration, dryness or humidity of the 

 atmosphere, and the activity of the pulmonary circulation. 



In some animals, as in the dog, where the integument is compara- 

 tively deficient in perspiratory glands, the pulmonary transpiration 

 becomes more active ; and it is not uncommon for these animals, in 

 hot weather, to lie at rest with their tongues protruded, and breathing 

 from one hundred to two hundred times per minute, for the purpose 

 of increasing the watery exhalation from the lungs. 



In man the precise physiological value of the pulmonary transpira- 

 tion is not known. Though varying according to the physical condi- 

 tions above mentioned, it is a continuous process, and even at ordinary 

 temperatures the expired breath received on a polished glass or metallic 

 surface will produce an immediate dimness by the condensation of 

 moisture. It is possible that the vapor thus exhaled, beside being 

 complementary to the cutaneous perspiration, may also serve as a 

 vehicle for the discharge of other substances. 



Exhalation of Organic Matter by the Breath. Beside carbonic acid 

 and water, the expired air contains an organic ingredient, which com- 

 municates a faint but perceptible odor to the breath. This substance 

 is discharged as an ingredient in the watery vapor of respiration. 

 Under ordinary circumstances it is in so small quantity as to be hardly 

 noticeable ; but if a large number of persons remain for some hours in 

 an apartment with insufficient ventilation, it accumulates in the atmos- 

 phere to such an extent that its odor becomes offensive. According to 

 Carpenter, the watery fluid condensed from the expired air, if kept in 

 a closed vessel at ordinary temperatures, exhales, after a time, a putres- 

 cent odor which could only come from decomposing organic substances. 



