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SPECIAL PHYSIOLOGY. 



is^ Exosmosis, Osmosis, Liquid Diffusion, and Dialysis. 



The absorption of liquids, or of substances in a state of solution, by the 

 living animal body, is either a simple filtrating process, connected with the 

 fine porosity of the tissues; or it partakes of the character of dialysis, or the 

 penetration of liquid or dissolved substances through a moist membrane per- 

 meable to such bodies, without being directly porous, like a filter ; or, lastly, 

 it may be connected with special selective or repellent actions in the living tis- 

 sues. Even in the last case, the process may be physical, i. e., either filtrating 

 or dialytic. The penetration of dissolved substances through the tissues, 

 occurs, not only in general absorption and the absorption of food, but also in 

 intrinsic absorption, in all acts of nutrition, in the reabsorption of the disinte- 

 grated materials of the body itself, likewise in the various acts of secretion and 

 excretion, in certain processes of the function of respiration, and of those of 

 taste and smell. 



Endosmosis. The action of the living tissues, in these several functions, 

 has, since the researches of Dutrochet (1827), been in part referred to the phys- 

 ical processes of so-called endosmosis and exosmosis, or the passage of fluids in 

 opposite directions through dead animal membranes (evrtov, endon, within ; 

 UO/LIOC, osmos, impulse). It was first pointed out by Parrot, of St. Petersburg 

 (1803), that if two liquids of unequal density are separated by a permeable 

 organic membrane, a mutual but unequal interchange takes place between 

 them ; but Dutrochet more fully investigated the subject. His endosmometer 

 consists of a bell-shaped glass, covered at its mouth with a thin animal mem- 

 brane, and fitted at its upper end with a graduated tube ; a colored solution of 

 sugar, gum, or some saline substance, being introduced into the glass, the 

 covered mouth is then immersed in water, when it is found that the solution 

 rises in the graduated tube, to a considerable height above the level of the 

 water around it. This phenomenon Dutrochet named endosmose. During its 

 occurrence, however, some of the dissolved substance contained in the tube, 

 passes into the water outside, and this process he named exosmose. The more 

 rapid flow, however, usually takes place from the rarer to the denser fluid ; 

 and hence, if the endosmometer be filled with water, and be dipped in the 

 solution, the more active, or so-called endosmotic current, really passes out- 

 wards through the membrane. Dutrochet pointed out that the force of endos- 

 mosis bears a certain ratio to the density of the inner fluid, and that the 

 quantity of fluid which passes, depends also on the extent of the membrane. 

 To avoid the effects of gravity, he from time to time adjusted the endosmom- 

 eter, so that the fluids inside and outside, were kept on a level. He showed 

 that capillarity, or capillary ascension, does not account for the phenomena, 

 which, he admitted, cannot be satisfactorily explained. He supposed that 

 endosmosis and exosmosis are peculiar to organic membranes, and that they 

 explain the rise of the sap in plants, many processes of the animal body, and 

 probably also the motions of various vegetable and animal fibres and cells. 



More recently, these physical phenomena have been studied by Beclard, 

 Matteucci, Graham, and others. The direction of the current through an animal 

 membrane, is not always found to be from the lighter to the denser fluid ; for 

 water passes more rapidly into alcohol, than alcohol into water. The great 

 endosmotic tendency of water has been attributed to its high specific heat, 

 which is higher than that of any other fluid. (Beclard.) But the properties 

 and qualities of the various fluid, or saline and other soluble substances, are 

 also found to influence the result. The phenomena are favored by moderately 

 high temperatures, by pressure, by the saturation of the membrane with acids or 

 alkalies, by special relations between the membrane and one of the fluids, and 

 by the constant removal of the endosmosing fluid by motion or by evaporation. 



Professor Graham has examined separately, first the tendency of different 

 liquids or solutions to mix with each other directly, and, secondly, the influ- 

 ence of a permeable membrane interposed between them. The former phe- 

 nomena constitute liquid diffusion, and the latter osmosis, or dialysis. 



Liquid Diffusion. A phial, with open mouth, is filled, nearly to the top, with 

 a given solution, and is then placed in a larger vessel, into which water is care- 

 fulty poured, so as to stand considerably above the level of the mouth of the phial; 



