160 ANATOMY FOR NUKSES. [CHAP. XIII. 



2. However dry the external air may be, the expired air is 

 quite, or nearly, saturated with moisture. 1 



3. The expired air contains about four or five per cent less 

 oxygen, and about four per cent more carbon dioxide than the 

 external air, the quantity of nitrogen suffering but little change. 

 Thus : 



Oxygen. Nitrogen. Carbon Dioxide. 



Inspired air contains . . . 20.81 79.15 0.04 



Expired air contains . . . 16.033 79.587 4.38 



(Foster.) 



In addition the expired air contains a certain amount of effete 

 matter of a highly decomposable and impure character. The 

 quantity of water given off in twenty-four hours varies very 

 much, but may be taken on the average to be about nine ounces 

 (266 cubic centimetres). The quantity of carbon given off at the 

 same time is pretty nearly estimated by a piece of pure charcoal 

 weighing eight ounces (248 grammes). 



If a man breathing fifteen to sixteen times a minute takes in 

 thirty cubic inches (492 cubic centimetres) of air with each 

 breath, and exhales the same quantity, it follows that in twenty- 

 four hours from three hundred and fifty to four hundred cubic 

 feet (9910 to 11,326 cubic decimetres) of air will have passed 

 through his lungs. And if such a man be shut up in a close 

 room measuring seven feet (2.1 metres) each way, all the air 

 in the room will have passed through his lungs in twenty-four 

 hours. 



Since at every breath the external air loses oxygen and gains 

 carbon dioxide and other waste and poisonous matters, it is 

 imperative that some provision be made for constantly renewing 

 the atmospheric surroundings of people in dwelling houses. 

 This is accomplished by ventilation, which consists of a system 

 of mechanical contrivances, by means of which foul air is con- 

 stantly removed and fresh air as constantly supplied. 



The minimum amount of air space every individual should 

 have to himself is 800 cubic feet (22,652 cubic decimetres), 

 a room nine feet (2.7 metres) high, wide, and long contains 729 



1 This moisture evaporates from the blood. It is thought by some authorities 

 that most of the moisture is collected by the breath from the mucous membrane 

 of the respiratory tract. A certain quantity, however, evaporates from the 

 blood through the walls of the capillaries, and, escaping with the carbon dioxide 

 through the membrane of the alveoli, is carried upwards in every expiration. 



