CHAP. XV.] ALIMENTATION. 175 



(3) The pancreatic juice, containing trypsin (a ferment act- 

 ing in the presence of an alkali), transforms proteids into pep- 

 tones, and, by virtue of other constituents, transforms starch 

 into sugar, and emulsifies fats or turns them into soluble soaps. 



(4) Bile, containing bile-salts and other matters, assists the 

 pancreatic juice in saponification and emulsion of fats, promotes 

 absorption of the same, and modifies putrefactive changes in the 

 intestine. 



(5) Intestinal juice, containing mucus, transforms all food- 

 stuffs in a feeble fashion, not clearly demonstrated nor un- 

 derstood. 



All material that these solvents fail to transform into a soluble 

 and absorbable condition is gradually worked downwards by the 

 peristaltic contractions of the alimentary canal, and finally 

 leaves the body as waste and useless matter. 



NOTE. For the sake of simplicity, we have considered digestion in a broad 

 way as the conversion of practically non-diffusible proteids and starch into more 

 diffusible peptones and highly diffusible sugar, and as the emulsifying of fats. 

 There is reason to believe that some of the sugar may be changed into lactic 

 acid, or even into butyric or other acids, and that some of the proteids are car- 

 ried beyond the peptone condition. But there is no doubt that the greater part 

 of the proteid is absorbed as peptone, that carbohydrates are mainly absorbed 

 as sugar, and that the greater part of the fat passes into the body as an 

 emulsion. 



Absorption. We have now to consider how the products of 

 digestion find their way out of the alimentary canal into the 

 tissues of the body; for, properly speaking, though the food may 

 be digested and ready for nutritive purposes, it is, until it 

 passes through the walls of the alimentary canal, still practi- 

 cally outside the body. 



There are two paths by means of which the products of diges- 

 tion find their way into the blood : (1) by the capillaries in the 

 walls of the stomach and intestines ; and (2) by the lacteals in 

 the walls of the small intestine. 



(1) The network of capillary blood-vessels is spread, as we 

 have seen (page 146), immediately beneath the basement mem- 

 brane of the mucous coat lining the interior of the alimentary 

 canal, and matters in solution pass readily by diffusion or 

 osmosis from the interior of the stomach and intestines into 

 the blood-vessels in their walls. All the blood from the diges- 



