308 ABSORPTION OF PROTEIDS. [BOOK n. 



on this view it is open for us to imagine that all the peptone 

 which passes through the epithelium is not intercepted by the 

 blood capillaries, but that some reaches and passes away by the 

 more centrally placed lacteal. It is difficult to imagine how 

 proteids in any other form than that of diffusible peptone can 

 pass through the walls of the blood capillaries ; though perhaps the 

 difficulty is not insurmountable, seeing that our conceptions of 

 nutrition are based on the assumption that the natural proteids of 

 the blood plasma pass from the interior of the vessels into the 

 extravascular elements of the tissues ; and we might imagine that 

 an accumulation of proteids in the same extravascular spaces 

 might cause a reversal of the proteid current, and thus lead 

 to proteids other than peptone passing through the vascular walls. 

 On the other hand it is at least open for us to ask the question, If 

 solid particles of fat can pass from the interior of the alimentary 

 canal into the lacteals, why should not various forms of proteids 

 pass in the same way into the lacteals, either in solution or even 

 as solid particles? 



It would thus seem possible for some of the proteids to pass 

 into the lacteals and so into the system in a less digested form 

 than peptone; and it is further possible that the proteids thus 

 entering into the system in different forms may play different parts 

 in the nutritive labours of the economy. 



But in all these considerations the fact must be borne in mind 

 that the intestinal walls undoubtedly possess a selective power of 

 absorption, which overrides the laws of diffusion and solubility. 

 This is shewn for instance by an observation made on a dog, in 

 which such fairly soluble and diffusible salts as sodium tauro- 

 cholate and glycocholate were found not to be absorbed by the 

 duodenum and upper jejunum even at a time when fat was being 

 rapidly absorbed in those regions, but to disappear in the ileum or 

 lower jejunum, the glycocholate apparently being absorbed by both 

 the ileum and lower jejunum, while the taurocholate passed away 

 in the ileum alone. 



We cannot judge therefore of the course taken by the proteids, 

 or of the form in which they are absorbed, by deductions based on 

 solubility and diffusion. The problems we are discussing can only 

 be satisfactorily settled by direct experiment. And here we meet 

 with difficulties. If all proteids are converted into peptone, and 

 so pass into the lacteals or into the blood capillaries, we might 

 expect to find a quantity of peptone in the chyle or in portal 

 blood or in both after a proteid meal. Now all observers are 

 agreed that peptone is absent from chyle or at least that its 

 presence cannot be satisfactorily proved, in spite of the possi- 

 bility of its entering into the lacteals together with the fat. And 

 while some have succeeded in finding peptone in the blood after 

 food, but not during fasting, many have failed to demonstrate the 

 presence of peptone in the blood either of the portal vein or of the 



