280 RESPIRATION. 



ingly, as they are actually performed, in infancy and childhood, during 

 sleep, and for the greater part of the waking condition, when the atten- 

 tion is not directed to them, are purely automatic in character, like the 

 pulsations of the heart, and do not require the expenditure of an} T 

 voluntary exertion. 



Quantity of Air used in Respiration. Like all the quantitative esti- 

 mates connected with respiration, that of the volume of air habitually 

 inspired and expired with the breath, varies considerably as given by 

 different observers. The differences incident to the different individuals 

 subjected to observation, and to the conditions of rest or activity, pre- 

 vent our arriving at an absolutely invariable standard. The average 

 result, however, which most nearly conforms to the truth, as derived 

 from several of the most trustworthy experimenters, as well as from 

 our own observations, is that which gives the amount of air taken into 

 and expelled from the lungs with each inspiration and expiration as 320 

 cubic centimetres. It is certain that this estimate is not above the 

 reality. If we take, accordingly, eighteen respirations per minute as 

 the mean rapidity between the sleeping and waking hours, this would 

 amount to 5760 cubic centimetres of inspired air per minute, 345,600 

 per hour, and 8,294,400 cubic centimetres, or 8294.4 litres per day. But 

 as the breathing is increased, both in rapidity and extent, by every 

 muscular exertion, the entire quantity of air daily used in respiration is 

 not less than 10,000 litres, or a little over 350 cubic feet. 



The quantity of air daily used in respiration is sometimes employed 

 as a basis for calculating the air-space necessary to allow for each in- 

 mate of a hospital or school-room. This estimate alone, however, can 

 never give sufficient data for the purpose. The successful ventilation 

 of a room depends not so much on the quantity of air which it contains 

 at any one time as upon the quantity of fresh air introduced, and of 

 vitiated air expelled, within a certain period. The air of a small room 

 which is thoroughly ventilated may be amply sufficient for respiration, 

 while that of a large room, if allowed to remain stagnant, will gradually 

 become unfit for use. A large air-space, in any occupied apartment, 

 will render ventilation more easy of accomplishment by ordinarj- 

 methods, because the air will not be so rapidly vitiated by the same 

 number of persons as if it were in smaller volume ; but the air must 

 still be changed with a rapidity proportionate to that of its contamina- 

 tion, in order to maintain the apartment in a wholesome condition. 



Changes in the Air by Respiration. 



The atmospheric air, as it is drawn into the cavity of the lungs, is a 

 mixture of oxygen and nitrogen in the proportion, by volume, of about 

 21 parts of oxj^gen to 79 parts of nitrogen. It also contains about .05 

 per cent, of carbonic acid, a varying quantity of watery vapor, and 

 some traces of ammonia. The last named ingredients, so far as animal 

 respiration is concerned, are quite insignificant in comparison with the 

 oxygen and nitrogen which form the principal part of its mass. 



