264 DIGESTION. 



relations in the alimentary canal, more especially to the so-called 

 digestive juices. Those substances which have already undergone 

 special molecular displacements before their transition into the 

 mass of the juices, must be conveyed by a different channel from 

 that by which unchanged or only slightly modified substances 

 pass into the blood. We must presume that it depends upon cer- 

 tain more or less prominent properties whether a substance passes 

 directly and without change into the blood, or whether it only 

 reaches its destination by indirect channels and after undergoing 

 various changes. It cannot be owing to accident that those sub- 

 stances which undergo no essential alteration in the intestinal canal 

 from the action of the digestive juices, should be especially qualified 

 for direct absorption through the blood-vessels; the one quali- 

 fication does, and must undoubtedly stand in some definite relation 

 to others, although we do not clearly understand its nature. It is 

 not only their saline character which renders the alkaline salts so 

 easy of resorption ; for there are many other salts which are not 

 resorbed by the capillaries, whilst on the other hand, urea, alcohol, 

 and certain poisons, pass with equal or perhaps greater rapidity 

 into the mass of the juices than many of these salts ; nor is it the 

 mere solubility of a substance which influences this easy transition ; 

 but it is the combination of several qualities depending upon the 

 fundamental relations of each individual substance, which induces 

 the capacity for absorption in the same manner as these funda- 

 mental relations also influence the resistance against the action of 

 the digestive fluids. When, therefore, we find some poisons 

 rapidly absorbed in the intestinal canal, while others are not taken 

 up, we should seek for the reason of these facts, not in a certain and 

 definitely limited instinct in the absorbing organs, but in certain 

 definite, although, unfortunately, not perfectly elucidated funda- 

 mental relations in these substances. It, therefore, seems most 

 appropriate to divide the objects to be digested into groups, which, 

 instead of being based upon their nutrient power (and this belongs 

 to the theory of dietetics and nutrition), or with regard to their 

 utility or their injuriousness to the living organism, should be 

 arranged according to their digestibility, that is to say, their greater 

 or lesser capacity for being absorbed. 



Before we consider those matters individually, whose ab- 

 sorption is solely or principally effected by the capillaries, we 

 must clearly ascertain in what manner a proof can be adduced 

 that a substance does not reach the general mass of the juices 

 through the lymphatics, but passes directly through the capillaries. 



