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 with 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 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 properties 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 communi- 

 cates with the outer world at both ends, the food, para- 

 doxical 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 ac- 

 tually, in the strict sense, taken into the body. Thus, 

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



