v FOODS 73 



Diffusible and Non-diffusible Foods. These four classes 

 of food materials proteids, fats, carbohydrates, and 

 minerals may be arranged in two groups according to 

 a certain physical peculiarity. If a solution of common 

 salt is placed in a vessel v/ith a bottom made of bladder, 

 called a dialyser, which is floated in a larger vessel of pure 

 water, it is found that, after a certain lapse of time, the 

 water in the outer vessel has become salt. The sodium 

 chloride has, in fact, passed by diffusion or osmosis 

 through the bladder. The same thing will happen if 

 a solution of sugar is placed in the inner vessel : salt 

 and sugar are both diffusible substances, capable of 

 passing through an animal membrane. 



On the other hand, if the inner vessel contains white 

 of egg, or oil, or starch well boiled in water, no diffusion 

 takes place. Hence proteids, fats, and starch are 

 non-diffusible foods, and are thus sharply distinguished 

 from salt and sugar, which are diffusible. 



The mucous membrane of the stomach and intestine 

 are animal membranes having the same physical proper- 

 ties as bladder. We may consequently infer that any 

 salt or sugar contained in the enteric canal will diffuse 

 through the mucous membrane and make its way, as we 

 shall see more particularly hereafter, into the blood, thus 

 serving to nourish the whole body. Proteids, fats, and 

 starch, on the other hand, will be incapable of diffusing, 

 and will, therefore, unless some change happens to 

 them, be absolutely useless as nutriment. For, since the 

 enteric canal communicates with the outer world at both 

 ends, the food, paradoxical as it may sound, is practically 

 outside the body as long as it remains in the canal : it 

 is only when it is absorbed into the blood or lymph that 

 it is actually, in the strict sense, taken into the body. 

 Thus if proteids, fats, and starch are to be of any use 

 to the frog, they must, in some way, be rendered capable 



