BESPIEATION. 587 



and above this age diminishes. In females it is about half that of males, 

 although it may amount to almost as much. Weight also exerts a great 

 influence. It has been shown that there is an average weight to every 

 average height. If the weight increase above this amount by 7 per 

 cent., the vital capacity decreases one cubic inch for every pound for 

 the next thirty-five pounds above this weight. Tor every pound thus 

 gained, an inch of vital capacity volume is lost for the next thirty-five 

 pounds. 



4. The Chemical Phenomena in Respiration. — The phenomena in 

 respiration so far studied have been purely mechanical in nature, and 

 have had for their object simply the introduction of air into the lungs 

 for oxidizing the blood, and for expelling air from the lungs after its 

 object has been served. 



We have now to study the chemical changes which occur in respira- 

 tion. We find that these are of two kinds, — the respiratory changes 

 occurring in the air in the lungs and the respiratory changes occurring 

 in the blood. 



The atmosphere consists of a mechanical mixture of oxygen and 

 nitrogen in the proportion of about twenty-one of the former to seventy- 

 nine of the latter. Watery vapor is usually present in small but variable 

 amount, while carbon dioxide is present in extremely small quantity, 

 varying from about 0.03 to 0.05 per cent. During inspiration a certain 

 quantity of this gaseous mixture, in man about thirty cubic inches in 

 amount, is drawn into the trachea and upper air-passages. These air- 

 passages, as well as the more deeply located portions of the lungs, are 

 already occupied by a gaseous mixture, in which these gases are present 

 in different proportions. It has been found that expiration immediately 

 follows inspiration without any pause, and a certain amount of this 

 inspired air is, consequently, at once again expelled. It has been calcu- 

 lated that about two-thirds which remain after expiration at once 

 commences to mix by diffusion with the air already in the lungs. It has 

 been found that when gases are placed in contact with each other at the 

 same temperature and pressure they mix rapidly, until the one gas is 

 uniformly diffused throughout the other. It has been stated in a 

 previous section that this diffusion is independent of gravity, but is 

 inversely as the square root of the densities of the gases. Mixtures of 

 gases behave precisely like single gases, and diffusion will take place 

 from one gas into another, precisely as if into a vacuum. ' 



It has been stated that the object of respiration was to supply 

 oxygen to the blood and to remove carbon dioxide from it. Without 

 discussing, at present, the process by which this change takes place, it is 

 evident that if this be a fact the air in the deepest portions of the lungs 

 will have lost oxygen to the blood and will have taken up carbon dioxide- 



