822 SPECIAL PHYSIOLOGY. 



is nearer that of the body ; but the difference is less than might be sup- 

 posed. With the thermometer at from 50 to 68, the expired air has 

 a temperature of 95 to 100; whilst if the external temperature be 

 32 or freezing-point, then the expired air is not more than 86. Ac- 

 cording to Valentin, however, in ordinary breathing the temperature 

 of the expired air in the winter is only 1 less than that of the air 

 expired in summer. In tranquil respiration, the expired air becomes 

 comparatively warmer than in rapid breathing, as if, in the latter case, 

 sufficient time was not allowed for the air to gain warmth. The in- 

 creased warmth of the expired air, necessarily causes an increase of 

 volume, but this is partly neutralized by a small loss of air in respi- 

 ration, owing, as we shall see, to the absorption of more atmospheric 

 oxygen by the lungs, than is equal to the carbonic acid exhaled. The 

 actual volumes of the inspired and the expired air, are as 97.2 to 99.5; 

 but, after the equalization of their temperature, the volume of the ex- 

 pired air is so reduced that it becomes less, by the amount of oxygen 

 absorbed in excess of that of the carbonic acid given out. 



The surplus of watery vapor in the expired air, as compared with 

 that in the inspired air, depending on the hygrometric condition of the 

 air before it is breathed, it becomes difficult to estimate the daily quan- 

 tity of water actually exhaled from the lungs. This has, however, 

 been variously calculated at from 11 to 16 oz., or, as an approximate 

 average, at 15 oz., per diem; but the quantity, according to modifying 

 circumstances, ranges from 6 to 27 oz. The source of this vapor is 

 the water of the blood, and thus, like the aqueous basis of the secre- 

 tions and excretions, it must assist in regulating the degree of fluidity 

 of the blood. Some of the vapor of the breath, comes from the fauces, 

 mouth, and nose, but the greater part, from the air-cells and air-tubes. 

 It would seem that a small quantity of hydrogen is converted into 

 water in the respiratory process, and may also come to be thus expelled. 

 After taking food, or alcohol, the pulmonary exhalation is said to be 

 increased, but it is lessened during fasting. As a rule, the expired 

 air is almost completely saturated with vapor, holding as much as it 

 can dissolve, according to its temperature; but this is true only of 

 calm respiration; for in hurried breathing, neither can the air be 

 elevated to its highest temperature, nor can it be completely saturated 

 with moisture. The more calmly air is breathed, the greater the loss 

 of water by the lungs. Lastly, the drier the inspired air, the greater 

 must be the amount of pulmonary exhalation ; for, in breathing air 

 already perfectly saturated, only such further quantity of water can 

 be added to it, as its increase of temperature in the lungs will enable 

 it to dissolve. The inhalation of actual vapor stops the pulmonary 

 exhalation, So, too, when the temperature of the surrounding air is 

 100 or 102, and it is already saturated, the temperature of the blood 

 itself being about the same, no further exhalation of water from the 

 lungs is possible; nor can the skin then give off more watery vapor. 

 Under such circumstances, the kidneys, and perhaps also, though to a 

 slight extent, the mucous membrane of the intestines, excrete more 

 actively. The pulmonary exhalation contains, besides water, traces 



