588 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



The air in the air cells will, therefore, be richer in carbon dioxide and 

 poorer in oxygen than fresh air taken in in inspiration ; as a consequence^ 

 we have the phenomena of diffusion at once taking place, the oxygen of 

 the inspired air diffusing down into the deepest portions of the lungs, 

 while the carbon dioxide diffuses up from the air-cells into the atmos- 

 phere through the larger bronchi and trachea. As the air penetrates, 

 therefore, by diffusion deeper and deeper in the bronchial tubes it loses 

 oxygen and receives more and more carbon dioxide, but the rate of 

 diffusion is not interfered with : for in the deeper portions of the lungs 

 the descending atmosphere is meeting with an increasing tension of 

 carbon dioxide and a decreasing oxygen tension, so that, although the 

 atmosphere now is poorer in oxygen than the external air, it is yet richer 

 than the air in the air-cells, and diffusion, therefore, continues. This 

 difference in tensions remains constant, for we have continually fresh air 

 taken in in inspiration, and just as continually the removal of the oxygen 

 from the air in the lower portions of the lungs and the addition to it of 

 carbon dioxide. 



The changes which the air undergoes in respiration may be recog- 

 nized by a comparison of the composition and the physical character- 

 istics of the expelled air as contrasted with the inspired air. 



The temperature of the expelled air is, as a rule, higher than that of 

 the inspired air, from the fact that the temperature of the body is almost 

 invariably higher than that of the surrounding medium. The expired 

 air is, as a rule, saturated with watery vapor, the water coming directly 

 from the blood and the mucous membrane, the quantity being estimated 

 in man as about one and one-half pounds. The watery vapor of the 

 expired air becomes readily perceptible through its condensation in cold 

 weather. 



Expired air contains, further, numerous organic substances derived 

 from the blood, resulting partly from the decomposition of the materials 

 entering into the composition of the tissues, and partly from matters 

 taken into the blood from without by absorption. Thus, various drugs y 

 such as turpentine, are excreted by the pulmonary mucous membrane, 

 readily revealing this in the odor of the expired air. Ammonia, also, is 

 contained in small amount in the air leaving the lungs, the amount given 

 off in ordinary respiration in twenty-four hours being calculated at 0.014 

 per cent. The presence of organic matter in the expired air may be 

 readily proved by breathing into a vessel, stopping it tightly, and allowing 

 it to stand a short time ; the odor of putrefaction will develop itself, and it 

 is, without doubt, to these organic substances in the inspired air that the 

 odor of the breath is due. It has been estimated that in man about three 

 and one-half grains of organic matter are so removed in the twenty-four 

 hours in expired air. Many of these substances which are organic in 



