314 FUNCTIONS OF NUTRITION. 



But a substance like albumen, which will not pass out by exosmosis 

 toward pure water, may traverse a membrane which is in contact with 

 a solution of salt. This has been shown with the shell-membrane of 

 the fowl's egg, which, if immersed in a watery solution containing 3 or 

 4 per cent, of sodium chloride, will allow the escape of a small proportion 

 of albumen. If a mixed solution of albumen and salt be placed in a 

 dialysing apparatus, at first the salt alone will pass outward, leaving 

 the albumen behind ; but after the exterior liquid has become perceptibly 

 saline, the albumen also begins to transude in appreciable quantity. 



The continuance of endosmosis is favored by renewal of the two 

 liquids. Since the accumulation of fluid on. one side of the membrane 

 depends on the difference in composition of the liquids employed, when 

 the process has continued for some time, and the two liquids have 

 approximated each other in composition, the activity of endosmosis 

 is diminished in a corresponding degree. But if the exterior liquid be 

 replaced by pure water, and the interior solution maintained at its 

 original strength by the addition of new ingredients, transudation will 

 go on with undiminished activity so long as the membrane retains its 

 absorbent power. The effect of a continuous current in aiding endos- 

 mosis may be shown by filling the cleansed intestine of a rabbit with 

 water from a reservoir and then placing it in a shallow vessel containing 

 a dilute solution of hydrochloric acid. If the water be allowed to flow 

 through the intestine under pressure from the reservoir, that which is 

 discharged from its open extremity will in a few seconds show the 

 presence of hydrochloric acid. The acid in this case passes through 

 the coats of the intestine against the pressure of the current, which is 

 of course directed from within outward. 



Endosmosis is also regulated, in great measure, by temperature. As 

 a rule its activity is increased by moderate warmth. Dutrochet found 

 that an endosmometer, containing a solution of gum, which absorbed 

 only one volume of water at a temperature of 0, absorbed three 

 volumes at about 34 C. Variations of temperature will sometimes 

 even change the direction of the endosmotic current, particularly with 

 solutions of hydrochloric acid. In the experiments of Dutrochet, when 

 the endosmometer was filled with dilute hydrochloric acid and placed 

 in distilled water at the temperature of 10 C., endosmosis took place 

 from the acid to the water, if the density of the acid solution were less 

 than 1.020 ; but from the water to the acid, if its density were greater 

 than this. On the other hand, at the temperature of 22 C., the current 

 was from within outward when the density of the solution was below 

 1.003, and from without inward when it was above that point. 



Nature of Endosmosis and Exosmosis. 



The continued transudation of a solution through an animal mem- 

 brane and its diffusion in an exterior fluid are dependent on the simul- 

 taneous action of two different properties ; first, the absorbent capacity 

 of the membrane for the solution, and secondly, the capacity of the 



