CHAPTER IX 



GENERAL PHYSIOLOGICAL ACTION OF LIPOID- 

 ALTERANT AND SURFACE-ACTIVE 

 SUBSTANCES 



From the physiological point of view the reversible 

 forms of salt action are the important ones; the proper- 

 ties and activities of living protoplasm may thus be 

 modified by changing the salt-content of the medium, 

 and return to the normal when the original salt-content 

 is restored. Such reversible effects are of special 

 biological interest, since their essential conditions are in 

 all likelihood similar to those controlling the normal 

 variations of activity. Substances are continually being 

 formed in metabolism (e.g., CO2 and other acids) which 

 directly influence protoplasmic action. It is, therefore, 

 of fundamental interest to note the existence of another 

 large class of substances, many of which are chemically 

 indifferent, i.e., not readily oxidized or reduced (hydro- 

 carbons and their substitution-products), which have a 

 profound influence on protoplasm, completely reversible 

 within wide limits. These substances are those organic 

 compounds, varying widely in their chemical nature, 

 which have in common two general physical properties : 

 (i) a solvent action on, or solubiHty in, the water-insoluble 

 organic constituents of protoplasm (fats, lipoids, etc.); 

 and (2) a high degree of surface-activity, i.e., influence 

 on the surface-tension at the boundary between water 

 and non-aqueous phases. These compounds appear also 

 to produce their physiological effects by altering the 



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