26 A TEXTBOOK OF PHYSIOLOGY 



the cell, are dissolved by solutions of cholesterin and of lecithin, 

 whereas sulphone dyes, to which the cells are impermeable, are 

 but sparingly soluble in these media. This hypothesis of the 

 lipoid nature of cell membranes is widely accepted at the present 

 day, but it is not altogether satisfactory, and has been subjected 

 to adverse criticism. It fails, for instance, to explain reasonably 

 vfhy cells are so readily permeable to water. It is also stated that 

 there are dyes readily soluble in these lipoids which are quite incapable 

 of penetrating into the living cell; while there are also dyes insoluble 

 in cholesterin which readily pass through the plasmatic membrane 

 of the cell. Moreover, certain inorganic salts insoluble in fat 

 penetrate into the cell. The sap in the plant, for instance, supplies 

 salts by some means to the cells. 



From a physiological point of view, then, a purely physical theory 

 of permeability is not altogether adequate. The red corpuscle is 

 rich in potassium and phosphate, yet the medium (plasma) in which 

 it floats is poor in these substances, but rich in sodium and chlo- 

 ride, in which the corpuscle is poor. Yet, as the cell receives its 

 nutriment from the plasma, the membrane of the corpuscle cannot 

 be wholly impermeable to potassium salts. If this be so, their retention 

 in the cell is opposed to osmotic force. Apparently there is some 

 specific intervention of the membrane or some special affinity of the 

 cell substance for potassium salts. So, too, in the case of the bo;lily 

 secretions. We shall see that it is difficult to understand, for instance, 

 how urea can be passed by purely osmotic agency from the blood, 

 in which it is in weak concentration, to the urine, where its concen- 

 tration is much greater. There appears, therefore, to be a physio- 

 logical as well as a physical permeability of the cell. This is further 

 shown by the following interesting experiments: If tadpoles be im- 

 mersed in a 5 to 6 per cent, solution of cane-sugar they are unaffected. 

 If they be transferred to an 8 per cent, solution, they shrink, owing 

 to loss of water. But immersion in a solution less than 6 per cent, 

 (hypo tonic) is not followed by an intake of w r ater and swelling of the 

 tadpoles, as might be expected. Therefore the epithelial membranes 

 of the tadpole are apparently permeable to water in one direction only. 

 A bag made of toad's lung, if placed in effervescing soda water, rapidly 

 fills with gas and floats. If. however, the lung be tinned inside out, 

 it does not fill with gas. The experiment succeeds no less if the lung 

 in each case is filled with water. Each cell must be regarded as the 

 seat of active chemical action, where concentrations of dissolved 

 substances are constantly altering. Other phenomena, e.g., imbibi- 

 tion, play an important part. 



The direct determination of the osmotic pressure of a solution is a 

 matter of difficulty. Therefore it is usual to ascertain it by some 

 indirect method by other properties of solutions quantitatively related 

 to osmosic pressure such as the lowering of the vapour pressure 

 of the solvent when the dissolved substance is non-volatile, or the 

 raising of the boiling-point of the solvent. The method, however, 

 most generally employed for physiological solutions is the lowering 



