326 ABSORPTION. 



Different liquids possess widely different degrees of diffusibility ; and, as a rule, in 

 saline solutions, the rate of diffusion increases in proportion to the strength of the solu- 

 tion, at least when the quantity of salt dissolved does not exceed four or five per cent. 

 It follows from this that the activity of the endosmotic current toward any saline solution 

 will be greatest at the beginning of the experiment and will progressively diminish as the 

 currents continue and the two liquids assume a more nearly uniform density. 



The rate of diffusion of different solutions is generally increased by a moderate eleva- 

 tion of temperature. 



Bearing in mind these general laws, and remembering that they are applicable to 

 diffusion as it takes place through animal membranes, we can easily understand how 

 different liquids and solutions, in an endosmometer, will attract with different degrees of 

 intensity any given liquid, such as pure water ; and how this attractive force, which is 

 measured by the rapidity and extent of the rise of liquid in an endosmometer, may be 

 modified by the concentration of the solution, differences in temperature, and other con- 

 ditions. The influence which the membrane exerts upon the relative intensity of these 

 currents is dependent to a certain extent upon the diffusion which takes place when the 

 two liquids come together in its substance. 



As a rule to which there are not many exceptions, pure water will penetrate ani- 

 mal membranes more readily than any other liquid ; and it is consequently from the 

 water to the liquid contained in the endosmometer that the principal current generally 

 takes place. Liquids like alcohol, saline solutions, etc., which have this property, are 

 said to be positively osmotic ; while those with which the current takes place in the 

 opposite direction, such as oxalic acid, weak hydrochloric acid, bichloride of platinum, 

 etc., are said to be negatively osmotic. In a series of experiments with different liquids, 

 if the endosmometer be always the same and if all the liquids used be exposed to the action 

 of pure water, in a given time a definite change in the quantity of fluid in the endosmom- 

 eter will be produced, which will be indicated by a certain amount of elevation or de- 

 pression in its level. 



Applications of Physical Laws to the Function of Absorption. 



In no experiments performed out of the body, can the conditions favorable to the 

 passage of liquids through membranes in accordance with purely physical laws be realized 

 as they exist in the living organism. The vast extent of the absorbing surfaces ; the great 

 delicacy and permeability of the membranes ; the rapidity with which principles are car- 

 ried on by the torrent of the circulation, as soon as they pass through these membranes ; 

 the uniformity of the pressure, notwithstanding the penetration of liquids ; all these favor 

 the physical phenomena of absorption in a way which cannot be imitated in artificially- 

 constructed apparatus. It is not necessary to invoke the vital properties of tissues to 

 explain the ordinary phenomena of absorption. Enough has been learned of the laws 

 which regulated endosmosis and exosmosis to enable us to explain most of these phe- 

 nomena upon physical principles. This fact has been apparent in studying these princi- 

 ples in their relation to absorption in the living body ; but it is an important question to 

 determine whether this be applicable to all the varied phenomena of physiological ab- 

 sorption. In other words, are there any modifications in this function which cannot, as 

 yet, be explained by physical laws ? 



Admitting the fact that the general process of absorption takes place in accordance 

 with the laws of endosmosis, we shall now consider some of the phenomena which ap- 

 pear to be in opposition to known physical principles, or in which the application of 

 these principles seems to be imperfectly understood. 



It is not easy to understand how particles of emulsified fat find their way through 

 the walls of the lacteals and blood-vessels, unless we admit the existence of orifices, 

 such as have been described by recent anatomists. The experiments of Matteucci with 

 alkaline emulsions seem to show that alkalinity is a condition necessary to the penetra- 



