134 PROTOPLASMIC ACTION AND NERVOUS ACTION 



or pores, (2) chemical conditions, including special 

 composition and solvent properties of constituents, (3) 

 physical conditions of other kinds, such as electrical 

 polarization at the faces of the partition or other surfaces, 

 electrical endosmose effects, filtration effects, and (4) 

 factors dependent on the Hving condition of the mem- 

 brane, including variation of permeabiHty due to meta- 

 bolic or other changes; the last-named factors are 

 apparently the ones chiefly responsible for the active 

 types of material transport in and through cells in 

 absorption and secretion (factors of ''physiological 



permeability")- 



A complete discussion of these various factors need 

 not be attempted here, especially since the whole subject 

 of permeability has recently been carefully reviewed 

 in the well-known textbooks of Bayliss and Hober. 

 Certain aspects of the problem of permeability require 

 special consideration, however, especially the dependence 

 of permeabiHty on physical conditions, and the nature 

 of the factors controlling the normal or physiological 

 variations of permeability. The most characteristic 

 pecuHarity of the Kving protoplasmic membranes, 

 especially of irritable cells, is that their properties vary 

 with both the external and the internal conditions, and 

 are subject to regulative control of a highly definite 

 kind. Such properties cannot be derived from any 

 simple static type of structure, or explained on the basis 

 of the pecuUarities of the colloidal compounds composing 

 the membrane. We must recognize that the distinctively 

 vital factors, those depending on the metaboHc activity 

 of the cell, are probably the most important of all, and 

 they are the least understood at present. The simpler 



