CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 515 



that is, mucous membrane, of the intestine removed from a dead 

 animal even when it appears to be absent from the blood, but also 

 and especially by the following observation. If an artificial circu- 

 lation 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 con- 

 siderable quantity 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. 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 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 hereafter that the 

 absorption of proteid material leads to an increase in the elimi- 

 nation 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 thoracic 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 

 capillaries 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 the proteids, transformed into 



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