ABSORPTION FROM THE ALIMENTARY CANAL. 325 



ABSORPTION FROM THE ALIMENTARY CANAL. 



258. 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 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 condi- 

 tion 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 carbohy- 

 drates 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 prod- 

 ucts pass by the lacteals they fall into the general blood-current after hav- 

 ing 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, and if the substances 

 be prevented from reaching the liver, as when in the dog the portal vein is 

 connected with the inferior vena cava and they are thus at once transmitted 

 to the general circulation, serious results ensue. We may, therefore, con- 

 sider 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. 



The Course taken by the Several Products of Digestion. 



259. From what has already been said we have been led to regard the 

 villi as the most active organs of absorption, and the structure of a villus 

 leads us further to conclude that the diffusible peptones and sugar pass, 

 together with the water in which they are dissolved, into the superficially 

 placed capillary network of the villus and so into the portal system, while 

 the merely emulsified fat, unable to traverse the wall of the capillary, passes 

 on to the deep-seated lacteal radicle, and so finds its way into the lymphatic 

 system. And the results of observation and experiment, as far as they go, 

 support this view. 



Fats. After a meal containing fat the lymph of the lacteals contains fat, 

 and is now called chyle ; and the richer the meal in fat the more conspicuous is 

 the fat in the lymph-vessels. We cannot, however, prove that all the fat of a 

 meal absorbed from the alimentary canal is poured by the thoracic duct into 

 the venous system. If a meal containing a known quantity of fat be given 

 to a dog and the small quantity of fat present in the feces corresponding to 

 the meal be subtracted from that amount, we can determine the amount of 

 fat absorbed, for we have no evidence whatever that any appreciable amount 

 of fat undergoes a destructive decomposition in the alimentary canal. Col- 

 lecting by means of a canula inserted into the thoracic duct the whole of 



