CHAP, i.] DIGESTION. 309 



vessels at large even after a meal containing large quantities 

 of proteids. Of course the quantity of peptone passing into the 

 portal blood at any moment might be small, and yet a considerable 

 quantity might so pass during the hours of digestion. We may 

 suppose moreover that that which does pass is immediately con- 

 verted, possibly by some ferment action, into one or other of the 

 natural proteids of the blood, or otherwise disposed of; and indeed 

 peptone injected carefully into a vein disappears from the blood, 

 though little or even none passes out by the kidney. And the 

 view that peptone is so changed, possibly in the very act of 

 absorption, is supported not only by the fact that peptone may be 

 found in the walls of the intestine even when it appears to be 

 absent from the blood, but also and especially by the following 

 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 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 peptones 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 into the capillaries. 



But the argument that the absence of peptone from the blood 

 is no proof that peptones are not absorbed into the blood may also 

 be applied to the chyle. We have however an indirect proof that 

 peptones do not pass into the chyle. We shall see hereafter that 

 the quantity of urea passing by the kidney may, with certain pre- 

 cautions, be taken as a measure of the quantity of proteid material 

 taken into the body. Now when a cannula is placed in the thoracic 

 duct of a dog so that all the chyle passes away and is lost to the 

 blood, the amount of urea leaving the body by the kidney does 

 not materially differ from the amount which, with the same food, 

 is passed, when all the chyle flows into the blood. Did any 

 large quantity of peptone (or proteid) pass by the chyle we 

 should expect to find the urea much diminished Hence except 

 on the very improbable view that proteids absorbed into the 

 lacteals of the villi escape from the lymphatic system before they 

 reach the thoracic duct, we must accept the view which seems to 

 follow legitimately from the results of artificial digestion, that 

 proteid food is converted into peptone and so passes from the 

 alimentary canal into the blood. And we know that artificially- 

 formed peptone is available for nutrition ; for dogs fed on peptone 

 and non-nitrogenous food may actually put on flesh and gain in 

 weight. 



