SELECTIVE ACTION OF TISSUES 329 



Thus the Red Blood-corpuscles and the Skeletal Muscles, although 

 bathed by fluids which contain a marked excess of sodium over potas- 

 sium salts, nevertheless, in themselves, contain a very marked excess 

 of potassium over sodium salts. Again, although in fresh-water streams 

 the relative content of potassium is often extremely low, the plants 

 which live in them are capable of storing up a comparatively large 

 amount of potassium in their tissues. One of the most extreme instances 

 of this selection by living tissues of components in disproportion to 

 their abundance in the surrounding medium is that afforded by the 

 presence of Iodine in considerable amounts in the tissues of the Thyroid 

 Gland in mammals and in the tissues of Marine Algae. Iodine is present 

 in normal blood only in undetectable traces and in sea-water in extra- 

 ordinarily small amounts. 



If we place within a dialyzer an excess of diffusible potassium salts 

 over diffusible sodium salts and dialyze against a solution containing 

 excess of diffusible sodium salts, the proportions of sodium to potassium 

 within and without the dialyzer sooner or later readjust themselves, 

 approaching equality. Now the surface of the living cell, although, 

 perhaps, sparingly permeable to water-soluble substances is neverthe- 

 less not absolutely impermeable to them, and in the course of time if 

 the inorganic constituents of the cell are present therein wholly in 

 diffusible forms, the concentrations of the various inorganic components 

 within and without the cell must ultimately attain equality. Even 

 the One-sided Permeability of the cell-surface would not alter the 

 proportions of the various constituents from those prevailing in the 

 external medium, although their total concentration would, in conse- 

 quence of this, be constantly maintained at a somewhat higher level 

 than that prevailing in the external medium. Hence this phenomenon 

 admits, as Loeb has pointed out, of only one- explanation, namely that 

 the inorganic constituents of a tissue which are found therein in excess 

 of the proportion in which they occur in the fluids which bathe it, 

 must exist within the tissue in the form of non-dissociated and non- 

 diffusible compounds. "If a tissue utilizes one kind of metal in this 

 way, for example K, while another metal, for example Na, is chiefly 

 used for the formation of dissociable compounds with Na as the free 

 ion, the consequence will be that the ashes of the tissue contain K and 

 Na in altogether different proportions from those in which they are 

 contained in the surrounding solution. I think we may take it for 

 granted that, at least, potassium forms a non-dissociable constituent 

 of the protoplasm of a number of tissues of animals and plants' ' 

 (Loeb.) 



The proteins are the only abundant constituents of protoplasm 

 which possesses the amphoteric property necessary for simultaneous 

 combination with acid and basic radicals. We have seen, furthermore, 

 that the compounds of proteins with inorganic bases, acids and salts, 

 do not yield any inorganic ions to the solution; they are non-dis- 

 sociable compounds in so far as the inorganic component is concerned. 



