112 PHYSIOLOGY. 



ness that one experiences in a close room where there are 

 a number of people is due to the reabsorption of these 

 organic matters. It is not due to lack of oxygen, for the 

 oxygen may be reduced to 13 per cent without causing 

 discomfort. A person may breathe air containing i per 

 cent of carbon dioxid, with a corresponding reduction of 

 oxygen, when the carbon dioxid is generated by ordinary 

 chemical processes (as in a small room with a large kero- 

 sene lamp, or a gasoline stove); but air having I per 

 cent of carbon dioxid produced by breathing is highly 

 injurious, because it contains the organic impurities above 

 noted, and the term "crowd poison" has been employed 

 for this material. Later investigators, however, maintain 

 that there is nothing injurious in the freshly expired breath. 



Alcohol and Consumption. At one time it was widely 

 believed that alcohol was a cure for consumption. This is 

 now known to be so far from the real facts of the case 

 that it is well established that certain forms of consump- 

 tion are directly attributable to the use of alcoholic drinks. 

 Under the former mistaken view many consumptives used 

 alcoholic liquors to their own injury. But time and ex- 

 perience have taught that they only exaggerate the trouble. 



Summary. i . In the lungs the air and blood are brought very 

 close together, only the wall of the capillary and that of the air vesicle 

 intervening. 



2. Through these two layers oxygen passes from the air vesicle 

 into the blood. Carbon dioxid, water vapor, and other wastes pass 

 from the blood into the air vesicle. 



3. The mucous membrane of the air passages secretes mucus which 

 is driven toward the nostrils by the cilia. 



4. The chest is lengthened by the depression of the diaphragm, and 

 widened by the elevation of the ribs, giving greater space, which is 

 filled by external air expanding the lungs. 



