VENTILATION. 223 



phere, and the oxygen tension is therefore only about eleven per 

 cent of an atmosphere in place of twenty per cent. Therefore, 

 in order to supply the needed oxygen, respiration must become 

 more rapid. This, however, by washing out the carbon dioxide, 

 serves to reduce the tension of carbon dioxide in the alveoli and 

 blood to such an extent that the action of this gas on the re- 

 spiratory center is weakened, and breathing may be very slow 

 or cease for a time, producing a condition known as apnea. The 

 lack of oxygen weakens the heart, the slightest muscular move- 

 ments are accomplished with difficulty, and the individual suf- 

 fers from nausea, vertigo, headache and general weakness. After 

 living for some time at such altitudes a person becomes accus- 

 tomed to the rarity of the atmosphere and in some manner is 

 able to compensate for the lessened oxygen in the air. 



VENTILATION. The disagreeable odor of a crowded room and 

 the symptoms which accompany it are well known and are usual- 

 ly attributed to the rebreathing of air. In support of this the 

 historical incident of the Black Hole of Calcutta, in which many 

 people perished from lack of air, is often cited. We have already 

 seen that atmospheres up to one per cent of carbon dioxide, or 

 containing less than half of the normal percentage of oxygen, 

 can be respired with no ill effects. But the percentage of carbon 

 dioxide in the worst ventilated room does not, as a rule, rise above 

 five-tenths per cent, or at most over one per cent, of an atmos- 

 phere. That this amount affects our body metabolism is impos- 

 sible, since the carbon dioxide in the alveolar air is kept at a 

 constant level of from five to six per cent by the control which the 

 respiratory center exercises on the respiratory movements. 

 Moreover perfectly normal respiration can take place in a room 

 where the oxygen content is so low that a match will not burn. 



Because of these facts it was suggested at one time that a toxic 

 substance might be present in the expired air, but this has not 

 been confirmed by subsequent investigators. In spite of the fact 

 that there is a normal percentage of oxygen and carbon dioxide, a 

 room may be unbearably close if it is too warm and the air is 

 saturated with moisture. So long as the body can radiate its heat 

 quickly into the atmosphere, the room does not feel stuffy, but 



