328 THE TISSUES AND MECHANISMS OF DIGESTION. 



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 disap- 

 pearance 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 ex- 

 periment may be recovered from the blood as it escapes from the mesenteric 

 veins. It would seem as if the peptone were changed before it actually gets 

 from the interior of the intestine into the interior of the capillaries. 



But the argument that the absence of peptone from the blood is no proof 

 that the peptone is not absorbed into the blood may also be applied to the 

 chyle, and thus leaves us unable to draw a conclusion as to the path of the 

 proteids. The following indirect proof that peptone does not pass into the 

 chyle has been offered, but it too is open to objection. We shall see here- 

 after that the absorption of proteid material leads to an increase in the 

 elimination of urea by the kidneys. So marked is this increase, that unless 

 there be clearly some other causes at work leading to an increase of urea, 

 such as fever for instance, an increase of urea in the urine following upon 

 the administration of proteid food may be taken as a proof that the proteid 

 food has been digested and absorbed. Now if in a dog the thoracic duct be 

 successfully ligatured so that the chyle cannot pass as usual into the blood, 

 and the dog be fed on proteid food, as free as possible from fat, so as not 

 unnecessarily to load the obstructed lacteals, an increase in the urea of the 

 urine is observed as usual. Obviously in such a case the proteid food is 

 absorbed, and obviously also does not pass into the blood through the tho- 

 racic duct (the success of the ligature having been proved by post-mortem 

 examination). But the experiment, though as far as it goes supporting, does 

 not rigorously prove, the view that the proteids are absorbed by the capilla- 

 ries of the alimentary canal ; for the thoracic duct and lymphatics below 

 the ligature were found largely distended, and lymph and chyle appear to 

 have escaped from the vessels ; hence it is possible that some at least of the 

 proteids were absorbed by the lacteals of the intestine, but finding their 

 usual path blocked made their way into the blood stream. 



We may, therefore, say that the results of experiment, while they do not 

 definitely prove, give some support to, and at least do not contradict, the 

 view which we a little while ago put forward as probable, namely, that pro- 

 teids, transformed into diffusible peptones, pass into the bloodvessels and 

 not into the lacteals. 



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

 ing that it is probable only ; and it may be that proteids do not take the 

 same paths and are not absorbed in the same condition in all animals. The 

 experiments just related were performed on dogs, that is to say, on carnivo- 

 rous animals whose (natural) food contains a considerable quantity of fat, 

 and whose lacteals might, therefore, 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 bloodvessels, there is comparatively little 

 left for the lacteals to do. Yet in these animals the lacteals and 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 dog. It is diffi- 

 cult to suppose that this wide chamber is intended solely for the absorption 

 of the relatively small amount of fat present in vegetable food. The ques- 

 tion which we are discussing is clearly at present to be regarded as by no 

 means settled. 



