RESPIRATION 41 



per cent of water. Nitrogen has no injurious effect, ex- 

 cept by excluding oxygen. Like all nonpoisonous gases 

 it may cause asphyxia if breathed pure, but would cause 

 it no more quickly than lack of oxygen if the nitrogen 

 were absent. Carbon dioxide is poisonous, but less fatal 

 than carbon monoxide which acts by combining with the 

 hemoglobin of the blood and excluding oxygen, because 

 its compound with hemoglobin is much more stable than 

 that of oxygen. The injurious effects of rarefied air, 

 like that on high mountains, is due to lack of oxygen in 

 the blood. 



Ventilation. Briefly stated the problem of venti- 

 lation is to maintain, in a closed space like a room, the 

 nearest possible approximation to atmospheric condi- 

 tions. The problem would be simple if it were not for 

 the necessity of heating. A healthy adult gives off about 

 six-tenths of a foot of C0 2 per hour, that is he changes 

 the contents of a hundred feet of air from four-tenths 

 to one foot of C0 2 . If this process be carried too far, he 

 will not only increase the actual C0 2 but, by using up 

 the oxygen, still more increase the relative content. If 

 a supply of 1,000 cubic feet of fresh air is furnished per 

 hour for each well person, the room is sufficiently ven- 

 tilated ; but in hospitals the amount required is 3,000 

 cubic feet. 



Nervous Mechanism. Nervous mechanism is almost 

 automatic. A center to control respiration is located in 

 the medulla. The normal stimulus to which it responds 

 is carbon dioxide. When the venous blood is sufficiently 

 charged with that gas, the center sends a message, which 

 is carried by the nerves to the muscles of inspiration 

 which contract and cause an inrush of air. If there be 

 an impediment to the intake, accessory muscles of in- 

 spiration are called upon until the obstacle is overcome. 



