CHAP. XL] PATHS OF ABSORPTION. 447 



rendered inactive when passing from the ileum into the colon. 

 Seeing that .the contents of the small intestine possess normally an 

 acid reaction, which (presumedly) increases from above downwards, 

 it may be assumed that the pancreatic enzymes are destroyed 

 before the ilio-caecal valve is reached. The observations which have 

 been made on the fluid obtained from fistulas of the ileum in the 

 vicinity of the caecum leave no room for doubt on this question. 



Paths of It is the intention of the Author to defer the de- 



absorption, tailed consideration of all the questions connected with 

 the absorption of the products of digestion, and he will, therefore, 

 confine himself in this place to the most rudimentary facts. 



It is in the small intestine that absorption of the dissolved organic 

 solids of the food chiefly occurs. The large surface of the mucous 

 membrane of this part <5f the alimentary canal, with its innumerable 

 villi, offers an absorbing surface of large extent, pervaded by mesh- 

 works of capillaries and by the commencement of the lymphatics, the 

 so-called 'lacteals.' In considering the extent of this surface the 

 ' valvulse conniventes ' must not be forgotten. These so-called valves 

 are crescentic folds of the mucous membrane, which is doubtless 

 arranged in this manner so as to afford, in a given area, a larger extent 

 of absorbing surface than would otherwise be possible. 



The absorption of materials from the alimentary canal takes place 

 by their passage into the blood capillaries and, in part, by their passage 

 into the lymphatics of the mucous membrane (lacteals). Water, in- 

 organic salts, sugars, fatty acids and their salts, albumoses, peptones 

 and the products of their decomposition, and emulsionized fats are the 

 principal substances present in the intestine. The paths of absorp- 

 tion followed by these various substances are not the same. Whilst 

 the fat makes its way into the lacteals, the other constituents, 

 more or less changed in their passage, find their way into the capil- 

 laries and thence directly into the blood. Even in this strictly 

 rudimentary reference to intestinal absorption, it is necessary to 

 mention that although albumoses and peptones abound in the con- 

 tents of the intestine, these bodies can neither be detected in the 

 lymph nor in the blood. Either a reconversion of albumoses and 

 peptones occurs within the alimentary canal itself (as Kronecker and 

 his school have contended), or the reconversion takes place as the 

 albumoses and peptones are brought in contact with the epithelial 

 layer covering the villi, or are subjected to the action of the leuco- 

 cytes of the adenoid tissue. These various views will be examined 

 and discussed in the next volume. 



