310 ABSORPTION BY DIFFUSION. [BOOK n. 



Sugar. With regard to the path taken by the sugar, careful 

 inquiries shew that the percentage of sugar both in chyle and in 

 general blood is fairly constant, being to no marked extent in- 

 creased by even amylaceous meals ; but that a meal of sugar or 

 starch does temporarily increase the quantity of sugar in the 

 portal blood. From this we may infer that such portions of the 

 sugar of the intestinal contents as are absorbed as sugar pass 

 exclusively by the portal vein. But it must be remembered that 

 at present we have no accurate information as to how large a 

 proportion of the sugar resulting from a meal passes in this way 

 unchanged until it reaches the liver, and how much undergoes the 

 lactic acid or analogous fermentation. Nor do we know as yet 

 how much of the starch taken as food is removed from the 

 alimentary canal in the form not of sugar but of dextrin. 



When a solution of sugar is injected into an empty isolated 

 loop of intestine a large quantity disappears, without the contents 

 of the loop becoming acid. In such a case it may fairly be 

 inferred that the sugar is directly absorbed without undergoing 

 any change. And where sugar is introduced in large quantities 

 into the alimentary canal, the percentage of sugar in the blood 

 may be temporarily increased; to such an extent indeed that 

 sugar may appear in the urine. But neither of these facts prove 

 that the sugar of an ordinary meal, passing as it does along the 

 intestine with the other portions of the food, and products of 

 digestion, and appearing as it does in most cases in comparatively 

 small quantities at a time owing to the more or less gradual con- 

 version of the starch of the meal, is similarly absorbed unchanged ; 

 while in order that the marked acidity of the contents of the 

 lower intestine should be kept up, a considerable quantity of 

 sugar must suffer lactic acid fermentation, if the acidity be due as 

 stated to lactic acid. 



To sum up, the evidence is distinctly in favour of the fats 

 passing largely by the chyle, and of the proteids and sugar passing 

 largely by the portal vein ; but there still remains much doubt as 

 to the course and fate of a not inconsiderable portion of the fat, 

 and the question as to the exact form in which proteids and 

 carbohydrates leave the alimentary canal, cannot be answered in a 

 perfectly definite manner. 



Absorption by diffusion. It is evident, from the discussion 

 just concluded, that simple diffusion is far from explaining the 

 whole transit of the digested food from the intestine into the 

 blood. Nevertheless, it must not be supposed that the great 

 and general property of diffusion does not make itself felt in the 

 process of absorption, however much it may, in the case of various 

 substances, be subordinated and held in check by more potent 

 influences. Thus the passage of water from the alimentary cavity 

 into the blood, or from the blood into the alimentary cavity, and 



