218 STUDIES IN ADVANCED PHYSIOLOGY. 



Carbon dioxide (CO L .) is the gas which results when 

 carbon, or things which contain carbon, such as \vood or 

 coal, are burned. It is colorless, and in diluted amounts 

 tasteless and odorless, and so not readily perceived when 

 mixed with the gases of the atmosphere. In a pure form 

 carbon dioxide is familiar as the gas which rises from a 

 freshly-drawn glass of soda-water, the soda fountains being 

 charged and the pressure in them produced by this gas. 

 It is the gas which causes the effervescence when vessels 

 containing any of the so-called (( sparkling liquids, " such 

 as pop or champagne, are opened. 



THE PHENOMENA OF EXTERNAL RESPIRATION. 



1. The Supply of Oxygen. Possibly the best way to 

 arrive at a proper understanding of the exact way in which 

 the gaseous interchange in the lungs is accomplished is to 

 get a clear picture of the actual condition of things in that 

 organ. In the first place, in the alveoli is fresh air, brought 

 there by the movements of respiration. In the walls of 

 these alveoli lie the pulmonary capillaries, into which the 

 venous blood coming from the right side of the heart enters. 

 This venous blood in the capillaries is separated from the 

 air in the alveoli by the membranes which form the walls 

 of the capillaries and the linings of the alveoli. Thus there 

 seems to be at first a difficulty in having the air and the 

 blood put in direct contact. Experiments, however, show 

 that a moist membrane which may separate a liquid from a 

 gas may be for all practical purposes disregarded. Thus, 

 if a glass vessel be taken, filled with a certain liquid and a 

 moist membrane stretched carefully over it, and then the 

 vessel be put into an atmosphere of a certain gas, the result 

 will be much the same as if that membrane had been left 

 entirely off, the only difference being that the presence of 

 the membrane slightly retards the rapidity of the gaseous 

 interchange. Thus this apparent difficulty at once disap- 

 pears and we may therefore imagine the venous blood of the 

 capillaries and the air in the alveoli in immediate contact. 



