PHYSICAL PKOCESSES IN CELLS. 49 



ing temperature. Solutions of different salts pass almost unaltered 

 through membranes, though the nitrate, from the fact that the membrane 

 keeps back some of the water of solution, may be more concentrated. 

 The reverse is true in the filtration of colloids (bodies like glue) ; there 

 the filtrate contains a smaller percentage of the colloid than the original 

 fluid, — from the fact that the membrane allows more water to pass than 

 gum, albumen, etc. 



This explains the fact that colloid bodies, perhaps on account of 

 the great size of their molecules, are with great difficulty removed from 

 the pores of animal and vegetable tissues, especially since it has been 

 found that if the fluid which contains colloids in solution also contains 

 crystalloids, less colloid will filter through than if the crystalloids had 

 been absent, and the filtrate is richer in crystalloids than the fluid in the 

 filter. Crystalloids, therefore, hinder the filtration of colloids ; and as 

 protoplasm is always associated with certain saline bodies, the latter 

 prevent the loss of the albuminoids of the cell-contents. 



Many of the phenomena of filtration may lie explained under the 

 working of the laws of capillarity, especially when the filter is an inorganic, 

 porous solid. When, however, it is an organic membrane, as of course 

 is always the case in the animal or vegetable cell, there the laws of imbi- 

 bition are of prime importance. 



Cell life furnishes many examples of the process of filtration. When 

 the cell-contents increase in bulk through imbibition the pressure on the 

 interior of the cell-membrane is greater than on the exterior ; substances 

 in solution within the cell may then pass through the cell-membrane. 

 The formation of the saline constituents of the various secretions is 

 probably accomplished in this way. When the flow of blood is obstructed 

 in a vein the pressure back of the obstruction becomes greatly increased, 

 and the water and saline constituents of the blood pass through the 

 walls of the vessel. In this way oedema and dropsies are produced. In the 

 liver, so long as the pressure in the bile-ducts is low, the bile filters from 

 the liver-cells into the bile-ducts. But if the flow of bile is interfered 

 with so as to make only a slight resistance in the bile-ducts, the bile then 

 filters into the hepatic parenchyma, from there into the lymphatics, by 

 which it is carried throughout the body, and jaundice is produced. 



When the renal secretion comes under consideration it will be found 

 that the process of filtration there fills a very important role. 



When fluids pass from the interior to the exterior of cells, or the 

 reverse, they usually come into contact with other fluids. Under certain 

 circumstances these fluids will mix ; under others mixing will not occur. 

 Those conditions now demand consideration. 



7. Diffusion of Liquids. — The molecules of liquids, as has been 

 already seen, are held together by a force of attraction, or cohesion ; the 



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