208 ADVANCED PHYSIOLOGY 



of air in the lungs, because (1) the blood needs more oxygen 

 for carrying on its extra activity during the running, and (2) 

 it is necessary to dispose of the extra carbon dioxid given off 

 by the muscles when they are contracting rapidly. But in- 

 creased breathing alone will not accomplish this end. A 

 more rapid circulation of the blood is needed to bring the 

 carbon dioxid to the lungs, so the heart begins to beat more 

 quickly and the blood to flow more swiftly. As soon as the 

 blood circulates more rapidly, however, and the gases are 

 carried off sufficiently, the necessity for quick breathing is in 

 part reduced. Every runner knows that after running a few 

 minutes, he gets what is called his " second wind". Breathing 

 grows less rapid and he may keep on running for a consider- 

 able time without any trouble with respiration. This "second 

 wind" appears when the circulation of the blood has become 

 rapid enough to carry off properly the gases formed. 



Evils of Indoor Life. Nature, apparently, intended that 

 man should live out of doors. But we have adopted new 

 habits of life, and shut ourselves up for many hours of the 

 day in close rooms. Hence, we are forced to breathe air 

 which has, perhaps, been breathed by other people and thus 

 rendered unwholesome. 



In spite of its advantages living indoors is unnatural, and is 

 conducive to certain diseases. People who live out of doors 

 are rarely attacked by lung diseases; while they do not entirely 

 escape them, they are affected much less often than house- 

 dwellers. Other diseases are rendered especially serious 

 because we live in more or less limited spaces. But since we 

 cannot, in the present state of civilization, pass all our time 

 out of doors, we should do our utmost to remedy these con- 

 ditions by furnishing our rooms with a proper supply of good 

 fresh air. 



Ventilation. More attention is paid to the proper ventila- 

 tion of buildings to-day than ever before. In the times when 

 rougher materials were used in their construction and when 



