COURSE OF ABSORBED MATERIAL. 109 



itself. The double condition here specified must be complied with ; the 

 material to be introduced must be dissolved in water, and must be sol- 

 uble in the blood. If the latter condition be wanting, the vessels seem 

 to manifest a selecting power, absorption not taking place, as in the case 

 of litmus, presented above as an illustration a coloring matter which, 

 though soluble in water, is not soluble in alcohol, and so can not, under 

 those circumstances, pass through a piece of bladder. 



While thus there is an introduction of digested material from the stom- 

 ach and intestine into the blood, the physical principles which are guid- 

 ing us in our explanation teach us that there must be a percolation of 

 the more watery portions of the blood in the opposite direction that is, 

 into the digestive cavity. There is every reason to believe that this 

 percolation is to a far greater amount than is generally supposed. Under 

 certain circumstances, it is a matter of ordinary observation that the wa- 

 ter discharged from the intestine is more in quantity than that which has 

 been taken as drink^ 



Turning our attention now to the course which is followed by the liq- 

 uid which has been introduced from the digestive cavity Course of the 

 into the blood-vessels, we must bear in mind that the con- absorbed mate- 

 tent of those vessels is composed of two distinct portions, "** ^dificaT" 

 the matter thus recently introduced, and the original venous tions it under- 

 blood. These together make their way through the portal goes 

 vein to the liver, a gland of double function, and, as we may say in this 

 respect, of double structure ; for, though it has a duct for the disposal 

 of the products which arise from its action on one portion of the material 

 thus brought to it, the venous blood, it is ductless as regards the other 

 portion, which has been received from the digestive cavity. This portion, 

 under the influence of the cell structure of the liver, undergoes profound 

 modification ; for instance, liver-sugar makes its appearance, though none 

 existed before. It is not necessary for us to specify these changes par- 

 ticularly here, since we shall have to examine them more in detail in a 

 subsequent chapter ; but it may be observed that the anatomical pe- 

 culiarity of the liver in this branch of its duty is, that it simply impresses 

 a change on the compounds thus brought to it, gives rise to no excretions, 

 and therefore has no channel or duct of escape, unless indeed we say, as 

 we are actually justified in doing, that the hepatic veins themselves are 

 the ducts of the liver in this respect. 



Though it does not strictly appertain to the subject of which we are 

 now speaking, absorption, we may, for the sake of completeness, describe, 

 in a superficial manner, what occurs to the other constituent of the portal 

 blood, its proper venous portion. This, brought into the liver, is acted 

 upon by that organ and decomposed into two portions, one of which, con- 

 stituting the bile, is brought back eventually through the proper bile duct 



