SEC. 11. ABSORPTION FROM THE ALIMENTARY 



CANAL. 



305. We may now return to consider the absorption of the 

 products of digestion, that is to say, the passage of these bodies 

 from the interior of the alimentary canal, where they are really 

 outside the body proper, into the body itself. For simplicity's 

 sake we may consider digestion in a broad way as the conversion 

 of practically non-diffusible proteids and starch into more diffusible 

 peptone and highly diffusible sugar, and as the emulsifying, or 

 division into minute particles, of fats. We have seen reason to 

 believe that some of the sugar may be changed into lactic acid or 

 even into butyric or other acids, that some of the proteids are 

 carried beyond the peptone condition into leucin and other bodies, 

 and that some of the fat may be saponified ; and it may be that 

 some of the proteid material of the food passes into the body as 

 albumose or even as parapeptone, or in some other little changed 

 condition. But we may probably with safety, for present purposes, 

 assume 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 emulsified but 

 otherwise unchanged neutral fat ; and we may neglect the other 

 conditions of digested food as subsidiary, and as far as absorption 

 is concerned, unimportant. 



We have seen that two paths are open for these products of 

 digestion, one by the capillaries of the portal system, the other by 

 the lacteals. It cannot be a matter of indifference which course is 

 taken. For if the products pass by the lacteals they fall into the 

 general blood-current after having undergone only such changes 

 as they may experience in the lymphatic system ; while if they 

 pass into the portal vein they are subjected to certain powerful 

 influences of the liver (which we shall study in a future chapter) 

 before they find their way to the right side of the heart. We 

 may therefore consider first which of the two paths is, as a matter 

 of fact, taken by the several products, and subsequently study the 

 mechanism of absorption in the two cases. 



