416 PEOTEIDS. [BOOK 11. 



the portal blood or the chyle ; during its transit through the 

 epithelium of the walls of the alimentary it is transformed from 

 peptone back again into some or other of the natural proteids 

 of the blood or lymph. That peptone is so changed before it 

 can get into the blood is shewn, among other ways, by the fol- 

 lowing observation. If an artificial circulation of blood be 

 kept up in the mesenteric arteries supplying a loop of intestine 

 removed from the body, the loop may be kept alive for some 

 considerable time. During this survival a considerable quan- 

 tity of peptone placed in the cavity of the loop will disappear, 

 i.e. will be absorbed, but cannot be recovered from the blood 

 which is being used for the artificial circulation, and which 

 escapes from the veins after traversing the intestinal capillaries. 

 The disappearance is not due to any action of the blood itself, 

 for peptone introduced into the blood before it is driven through 

 the mesenteric arteries in the experiment may be recovered from 

 the blood as it escapes from the mesenteric veins. That if it 

 did pass into the chyle it would undergo a similar change by 

 some action of the epithelium is indicated by the fact that when 

 peptone is introduced into the lymph-spaces of the connective 

 tissue, its presence may soon be recognized in the lymph of the 

 thoracic duct, but that no peptone can be detected in the lymph 

 of the thoracic duct when peptone, even in large quantity, is 

 introduced into the alimentary cavity provided that the epi- 

 thelium be intact and healthy. 



We are therefore guided in deciding this question by indirect 

 evidence ; and this, though pointing to the probability that the 

 proteids pass into the portal blood and not into chyle, cannot 

 be regarded as conclusive. One argument in this direction 

 may be drawn from the fact that when the portal blood is ex- 

 perimentally diverted from the liver into the vena cava, the 

 grave troubles which result seem to be chiefly caused by proteid 

 food. 



But, if this view be provisionally accepted, it must be on the 

 understanding that it is probable only ; and it may be that pro- 

 teids do not take the same paths and are not absorbed in the same 

 condition in all animals. In carnivorous animals whose (nat- 

 ural) food contains a considerable quantity of fat, the lacteals 

 might be considered as preoccupied in the absorption of fat. 

 The food of herbivora on the other hand contains a relatively 

 small amount of fat ; and if in these animals all the proteids and 

 carbohydrates are absorbed by the blood vessels, there is com- 

 paratively little left for the lacteals to do. Yet in these ani- 

 mals the lacteals and the lymphatics are well developed. In the 

 villus of a herbivorous guinea-pig or rabbit, though the reticular 

 tissue is very scanty as compared with that present in the villus 

 of a dog, the lacteal chamber is, relatively to the diameter of 

 the villus, not merely as large as but much larger than in the 



