THE STKUCTUKAL BASIS OF THE BODY 



23 



centration than would correspond to the plasma of the blood from which the 

 corpuscles were derived, whereas if placed in weak salt solutions or distilled 

 water they swell upland burst, discharging their haemoglobin in solution into 

 the surrounding fluid. By comparison of various salts it is found that the 

 strength of each salt solution which is just necessary to cause plasmolysis 

 or haemolysis, as the case may be, is determined entirely by its molecular 

 concentration, i,e. a decinormal solution of sodium chloride will be equivalent 



1234 

 FIG. 7. Vegetable cells, showing varying degrees of plasmolysis. (DE VEIES.) 



in its effect on the cells to a decinormal solution of potassium nitrate or of 

 potassium chloride. The impermeability of the plasma skin does not apply 

 to all dissolved substances. Overton has found that, whereas this layer is 

 practically impermeable to salts, sugars, and ammo-acids, it permits the 

 easy passage of monatomic alcohols, aldehydes, alkaloids, &c. All these 

 substances are more soluble in ether, oil, and similar media than they are in 

 water. The passage of dissolved substances through a membrane wetted 

 by the solvent depends on the solubility of these substances in the membrane, 

 and Overton therefore concludes that the superficial layer of protoplasmic 

 cells must itself partake of a ' lipoid ' character, and that cholesterin and 

 lecithin probably enter largely into its composition. Thus only those aniline 

 dyes which are soluble in a mixture of melted lecithin and cholesterin have 

 the property of penetrating the living cell, and only these dyes, such as 

 methylene blue, neutral red, can be used for intra vitam staining. For the 

 same reason substances which have the power of dissolving lecithin and 

 cholesterin, such as ether or bile salts, also act as hsemolytic agents, i.e. they 

 cause a destruction of the red blood cells by dissolving the superficial layer 

 which is necessary for their preservation from the solvent effects of the 

 surrounding fluid. 



The semi-permeability of the plasma skin can be altered by changes in 

 the saline concentration or other factors of the surrounding medium. Over- 

 ton has shown that, whereas a 7 per cent, solution of saccharose produces 

 plasmolysis in living cells, no plasmolysis is observed if they are treated with 

 a solution containing 3 per cent, methyl alcohol plus 7 per cent, cane sugar. 

 The superficial layer, therefore, is able to dissolve a mixture of methyl 

 alcohol and cane sugar, although it has no solvent power on cane sugar in 



