112 rilYSlOLOGY. 



ness that one experiences in a close room where there are 

 a number of people is due to the reabsorption of these or- 

 ganic 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 in- 

 jurious, 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. 



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, ancj 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. 



5. Inspiration acts in opposition to resistances, whose elastic re- 

 action performs ordinary expiration without active effort. 



6. There are four heart beats for each respiration. 



7. The lungs are never emptied. 



8. Respiratory capacity may be increased by exercise and practice. 



9. Respiration is controlled by the nervous system ; the respiratory 

 center is in the spinal bulb. 



10. Internal respiration is an oxidation in the tissues, illustrated by 

 the rusting of moist iron. 



