CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 543 



supplying a loop of intestine removed from the body, the loop may 

 be kept alive for some considerable time. During this survival a 

 considerable 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 capil- 

 laries. 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 re- 

 covered 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 it 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 so 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. 



Other evidence, also indirect, is afforded by the following obser- 

 vation. It has been found possible in dogs to effect a direct 

 junction between the vena portse and the inferior vena cava, so 

 that all the blood carried by the vena portse from the abdominal 

 viscera passes straight to the vena cava, and so into the general 

 circulation without passing through the liver. It is then found 

 that proteid food produces or is apt to produce grave nervous 



