MECHANISM OF ABSOBPTION. 13 



animal after a time is sensibly increased. Now this increase, 

 which under favourable circumstances may reach a third of 

 the weight of the animal, can depend only on the absorption 

 of water by the surface of the body. 



If water be introduced into the stomach of a living dog, 

 and the entrance to and exit from the organ be secured with 

 ligatures, still the liquid will disappear, absorbed by the walls 

 of the stomach, and so mingle with the blood; and yet there 

 exist neither in the stomach nor external integuments any 

 pores leading directly to the vessels containing the blood. 

 The pores observable on the skin lead merely to little cavities 

 intended to secrete various humours or t6 form the hairs. 

 Thus the tissues forming these organs (the skin and stomach) 

 are permeable to liquids, and this is the case with all the 

 other structures of the body. 



In fact, in living as well as in dead bodies, the tissues 

 uniformly imbibe surrounding fluids, and are traversed by 

 them with more or less facility. 



30. Mechanism of Absorption. It is on the permea- 

 bility of the solid parts of animal bodies that the function of 

 absorption depends. The penetration of fluids into the interior 

 depends on a peculiar force or power acting on them. Capil- 

 lary attraction contributes powerfully to effect this penetra- 

 tion of external liquids, but it is not the only force or power 

 causing this phenomenon ; another was discovered a few years 

 ago by Dutrochet, and called by him endosmose. If, into a 

 little membranous sac, surmounted by a tube, water, holding 

 gum in solution, be poured, and the apparatus be then placed 

 in pure water, as in Fig. 1, the water will be found to rise in 

 the tube to a considerable height. Here is then an evident 

 absorption of water through the walls of the sac. Next 

 reverse the experiment, by filling the sac with pure water, 

 and placing the apparatus in gum-water, and the sac will 

 empty itself instead of absorbing more, the pure water pass- 

 ing through its walls in the inverse direction. Now this 

 phenomenon has the greatest analogy with what takes place 

 in living bodies, and partly explains it ; for the purer liquids 

 from without pass readily through the spongy tissues of 

 animal bodies, whilst the denser material within passes with 

 more difficulty in the opposite direction. Hence the accumu- 

 lation within, which could not take place if both passed with 

 equal facility, and established an equilibrium. Such an equi- 

 librium is prevented by the union of the purer liquid with the 



