v INTERNAL RESTITUTIVE SECRETIONS 299 



absorbed from the intestine, both into the lacteals and the blood- 

 vessels. 



As regards the penetration of sugar and protein, we cannot 

 tell why these substances should under ordinary conditions be 

 absorbed, if not exclusively at least to a predominating extent, 

 by the blood capillaries of the villus. The mechanism of this 

 penetration also is unknown to us by direct experiment. It is 

 probably a process of transudation identical with that which gives 

 rise to the formation of lymph (see Vol. I. p. 523 et seq.}. The 

 difference is that while in the formation of lymph, transudation 

 takes place from the interior of the capillaries into the lymph 

 sinuses, in the absorption of chyle it occurs from the lymph 

 sinuses to the interior of the blood capillaries of the villi. We 

 have seen that trausudation consists of two well-known physical 

 processes, diffusion and filtration. The greater concentration of the 

 food-stuffs of the chyle collected in the meshes of the reticuli of 

 the villi, as compared with that of the constituents of the blood 

 that incessantly courses through the capillary network, certainly 

 presents a condition favourable to endosmosis. On the other 

 hand, the pressure due to the contraction of the muscles of the 

 villi must favour filtration through the blood capillaries from 

 without inwards. 



VIII. We have seen that while the products of the proteolytic 

 and lipolytic processes are regenerated during absorption into 

 natural proteins and neutral fats by the synthetic activity of the 

 columnar epithelium of the intestine, the digestive products of the 

 carbohydrates which pass through the intestinal wall penetrate 

 almost entirely by the blood vessels of the villi to the portal system, 

 and are, almost without exception, carried to the liver in the form 

 of grape sugar or glucose. According to Pfliiger, part of the 

 glucose that penetrates into the blood may, instead of remaining 

 there in the free state, enter into chemical combination with the 

 proteins, or the lecithin with which it forms jecorin. We have 

 seen elsewhere (Vol. I. p. 130) that the amount of glucose 

 normally present in blood plasma varies, according to Otto, from 

 O'lO-O'lo per cent, and only exceptionally rises to 0'30 per cent, 

 or a little over. This proportion of glucose in the blood is wholly 

 independent of the nature of the food. Two facts may, however, 

 now be considered well established, from the results of numerous 

 analyses of the blood, arrived at by different observers : 



(a) After a meal rich in starchy and saccharine substances, the 

 sugar content of the portal blood reaches its maximum, w y hile in 

 the blood of the hepatic veins it remains normal. 



(&) During fasting, the sugar content of the blood in the 

 hepatic veins is somewhat higher than that of the portal blood, 

 which is minimal. 



The first fact leads us to admit that the sugar absorbed from 



