SEC. 9. ABSORPTION FKOM THE ALIMENTARY 



CANAL. 



246. 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 conver- 

 sion of practically non-diffusible proteids and starch into more 

 diffusible peptone and highly diffusible sugar, and as the emul- 

 sifying, 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 leu- 

 cin 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 neu- 

 tral fat ; and we may neglect the other conditions of digested 

 food as subsidiary, and as far as absorption is concerned, unim- 

 portant. 



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. It has been possible, in the dog, so to connect 

 the portal vein with the inferior vena cava, that the portal blood 

 is diverted into the latter, and so is thrown on the general cir- 



412 



