THE ABSORPTION OF THE REDUCED FOODSTUFFS 1027 



unit is acted upon from all sides by the neighboring molecules, and that 

 this action is equal in all four directions. At the surface, on the other 

 hand, conditions are different, because here the molecules are not 

 counterbalanced by a tension resting upon their external surfaces. 

 Hence, they are pulled inward. Now, it will be seen that if the drop is 

 surrounded by some fluid, its surface-molecules must be acted upon by 

 the molecules of the medium, depending, of course, upon the nature of 

 the latter. Obviously, this now uneven balance must give a different 

 shape to the drop as a whole. The surface-tension may also be 

 altered by changes in temperature, because heat tends to separate 

 the different molecules from one another and to counteract their 

 power of attraction. Cold, on the other hand, increases the surface- 

 tension, because it brings the molecules closer together by removing 

 from them the kinetic energy necessary for expansion. A third means 

 by which the surface-tension may be altered, is the electrical current.^ 

 Adsorption. — The phenomenon of adsorption may be illustrated 

 by exposing a solid substance in powdered form to a solution of some 

 kind. The dissolved substance then accumulates upon the surfaces 

 of the sohd particles and leaves the solution, thereby lessening the 

 concentration of the latter. This property is well displayed by the 

 colloids to which the proteins, with the exception of the peptones, be- 

 long. Consequently, since our body contains very extensive surfaces 

 which lie in relation, on the one hand, with the body-fluids and, on 

 the other, with nutritive material, most favorable conditions are 

 estabUshed for the occurrence of this phenomenon.^ 



A, ABSORPTION FROM THE INTESTINAL CANAL 



The Absorption of Water. — Water and the ordinary soluble salts 

 are absorbed unchanged, but the quantity which actually finds its 

 way into the body, depends upon the intake and how greatly the 

 system is in need of it. Since water is lost constantly, because it 

 serves as a medium for our secretions and excretions, correspondingly 

 large quantities of it must be consumed in order to make up for this 

 loss. In a way, therefore, it may be said that the body is in water- 

 equilibrium, and it makes little difference whether a man takes in one 

 liter or six, because any superfluity is soon compensated for by a 

 greater discharge, chiefly through the kidneys. Quite similarly, any 

 scarcity is equaUzed by a corresponding reduction in the quantity of 

 the secretions and excretions. In the latter case, however, a physio- 

 logical limit is soon reached, at which the phenomenon of tissue-thirst 

 arises as a means of safety. The body also possesses the power of 

 guarding itself against too large an intake, because unusually large 



1 Macallum, Ergebn. der Physiol., xi, 1911, 598; also: Traube, Pfliiger's Archiv, 

 cv, 1904, 559. 



^Hofmann, Zentralbl. fiir Physiol., xxiv, 1910, 805; Robertson, Jour. Biol. 

 Chem., iv, 1908, 35; and Van Slyke, ibid., iv, 1908, 259. 



