60 . PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



through the cell-wall or the walls of capillaries, etc. These organic 

 tissues, which we have seen to be always filled with liquid, offer very little 

 resistance to the passage of a gas. The animal fluids communicate 

 through these delicate moist tissues almost directly with the gases of the 

 atmosphere. 



Gases formed by cells, or gases which pass from the exterior to the 

 interior of cells, or even the passage of gases through the membranes of 

 the lungs or gills of animals, are not governed by the law of the dif- 

 fusion of gases given above, but their transfer through an animal mem- 

 brane is governed by the co-efficient of absorption of that tissue for the 

 different gases. This may be illustrated by a very simple experiment 

 devised by Draper, who does not appear, however, to have appreciated 

 its application to gaseous interchange in the animal body. The law of 

 the diffusion of gases through porous partitions is that the rapidity of 

 the diffusion is inversely as the square root of the density of the gas. 

 If the finger be dipped in soap-water and then rapidly passed over- the 

 mouth of an empty bottle so as to leave a horizontal film, and the bottle 

 then placed under a bell-jar filled with carbon dioxide, the film soon 

 swells up into an almost hemispherical dome. Or, if the bottle be filled 

 with carbon dioxide, and then exposed to the atmosphere after its mouth 

 has been covered with a soap-film as before, the film is promptly 

 depressed into a deep concavity and bursts. Now, if the film is regarded 

 as a porous partition, the air, being of many times less density than the 

 C0 2 , should diffuse much more rapidly, according to Graham's law. The 

 reverse, however, is the case. Water, however, of which the film consists, 

 has a much higher co-efficient of absorption for C0 2 than it has for oxygen 

 or nitrogen. The C0 2 is therefore absorbed more rapidly by the film 

 than the gases of the atmosphere, and from its solution in the film dif- 

 fuses rapidly into the atmosphere. The state of affairs is similar in the 

 case of gaseous interchange in animal cells. 



The membranes of cell are not porous partitions, but are tissues 

 whose molecular interspaces are filled with liquids. That a gas may pass 

 through such a membrane it is necessary, therefore, that the gas be first 

 absorbed by the liquid in the cell-membrane. The readily -absorbed gases, 

 such as C0 2 , will thus diffuse through cell-membranes more rapidly than 

 those with a lower "co-efficient of absorption, such as N, H, or 0, the 

 rapidity of absorption being further governed, not only by the co-eflicient 

 of absorption, but by the gaseous tension and the temperature. After 

 having passed through the cell-membrane gases will, of course, diffuse 

 into the liquid or gaseous media surrounding those cells, according to the 

 tension of those gases already present. 



In the case of terrestrial animals this medium is the atmosphere, 

 which is composed of 21-volume per cent, of and 79 per cent, of N, 



