ABSORPTION FROM THE DIGESTIVE CAVITY. 197 



diffuse themselves through colloids. A simple experiment showing this fact 

 may be thus performed: In the centre of a mass of any colloid substance, as 

 boiled starch, lot there be placed any highly colored crystalloid, as potassium 

 bichromate; in a short time it will diffuse itself throughout the entire mass, 

 giving it a bright yellow color; or, for the bichromate, substitute a crystal of 

 potassium iodide, and in a short time upon touching the starch with a drop 

 of some concentrated mineral acid, as sulphuric, a potassium sulphate will 

 be formed, the iodine liberated, and the starch colored blue from the forma- 

 tion of the iodide of starch. If, on the other hand, a colloid, as burnt 

 sugar, or caramel, be placed in the centre of a similar mass, at the end of 

 many days it will be found to have diffused itself scarcely at all. 



In consequence of this property, when crystalloids and colloids are 

 mingled, as in the egg, the former may be caused to diffuse themselves 

 away from the latter. This fact is easily demonstrated in the egg-endos- 

 inometer, which is prepared by removing the shell from one end of an egg 

 in such a manner as to leave the membrane intact. In the opposite end a 

 small opening is made, around which is cemented a glass tube. The egg is 

 then placed in a wineglass of distilled water, with the end upon which the 

 membrane is intact downward. Soon the contents of the egg will begin to 

 appear in the tube, showing that water has passed through the animal mem- 

 brane, and caused the displacement of the egg's contents. That the diffu- 

 sion of the crystalline contents of the egg has taken place can be proved by 

 adding a drop or two of a solution of silver nitrate to the water in the wine- 

 glass, when a copious precipitate of the silver chloride will be thrown down, 

 thus demonstrating the passage of soluble chlorides from the interior of the 

 egg through the animal membrane into the water. By this experiment also, 

 the non-diffusibility of colloids may be proved, as no test whatever can de- 

 tect the presence of albumen in the water in the wineglass. It is this diffu- 

 sion of crystalloids away from colloids through an animal membrane which 

 is termed, by Prof. Graham, dialysis.] 



140. There seems to be no reason for doubting that the absorption of the va- 

 rious nutritive and alimentary materials takes place according to the ordinary 

 rules of Osmosis ; for on the one hand is the aliment always more or less per- 

 fectly reduced to the liquid state, the density of which is generally less than 

 that of the blood, in consequence of the very copious discharge of aqueous 

 fluid into the alimentary canal during the operation of digestion ; and on 

 the other is the blood or lymph in rapid movement, possessing a high spe- 

 cific gravity, at a temperature of 99 or 100 F. ; whilst between them is the 

 septum, of immense extent and great tenuity, formed by the mucous mem- 

 brane, and the walls of the blood or lymph vessels: the tendency is, there- 

 fore, in accordance with the facts already stated, in favor of the passage of 

 the aliment from the intestinal tract towards the circulating fluids. As 

 regards such substances as albumen, gum, and gelatin, which belong to the 

 "colloid" class of substances, and therefore transude with extreme difficulty, 

 it appears at first sight difficult to explain how they pass through the intes- 

 tinal mucous membrane. 1 The experiments of Funke, however, show that 

 the act of digestion essentially effects a conversion of the "colloid" group 

 into the "crystalloid;" for that as soon as the albuminous substances intro- 



1 Mialhe (Chimie appliquee a la Physiologie, 1856) broke the shell from the end 

 of an egg, leaving the membrane intact, and immersed it in water. After 5 hours 

 it had increased in weight upwards of 30 grains, and the membrane was tight and 

 prominent. The water in which the egg had been immersed became alkaline from 

 the exosmose of the salts, but no albumen had escaped. Bo Graham and Eckhard 

 (Beitrage, Bd.iii, pp *51 and 85, 186'-') state that in analogous experiments with solu- 

 tions of gum no exosmosis of that substance occurred. 



