124 PHYSICAL CHEMISTRY 



negative colloids like the rest of the cell, the approach of a 

 cathode would tend to lessen the charge on the particles and 

 induce aggregation. The negative charge of cell colloids is illus- 

 trated by the author's experiments in passing a constant current 

 through plant (McClendon, 1910, b) and animal (McClendon, 

 I 9 I 4. d) cells. Chromosomes, yolk granules, and protoplasm 

 in general pass toward the anode, whereas water passes toward 

 the cathode and forms a blister on some animal cells. If the 

 plasma membrane is composed of the same colloids as the cell 

 interior it is probably negative. It may be objected that these 

 colloids become positive in an- acid medium and that this nega- 

 tivity is incompatible with the assumed acidity of cell interiors. 

 But the isoelectric point of most of these colloids is much farther 

 on the acid side than the reaction we have assumed for the in- 

 terior. Furthermore, the plasma membrane is nearer the blood 

 which is alkaline, and is probably bathed by an alkaline tissue 

 juice. The cell sap in the cells of red flowers has an acid re- 

 action. 



The rate of propagation of the excitatory impulse may be 

 1 mm per second in some plants and 12000 in mammals. It is 

 probably very slow over the bodies of nerve cells and motor 

 end plates, as there is some delay in conduction through these 

 regions. Mayer (1916) found that nerve conduction varies 

 almost as the concentration of electrolytes around the nerve. 

 Lillie (1916 b) supposed nerve conduction to vary with electric 

 conductivity but Mayer has not observed an exact parallel. 



Since glands are the seat of an emf, and the skin contains 

 glands, the skin is the seat of an emf which varies with glandular 

 activity. If a constant electric current is passed through the 

 body, the electrical resistance is found to be high. If a gal- 

 vanometer is interposed in the circuit, the needle may soon come 

 to a constant deflection, but if the subject becomes mentally ex- 

 cited, as on hearing bad news, the galvanometer deflection is 

 suddenly increased. This is called the psychogalvanic reflex, 

 and is used by clinicians and psychologists in the examination 

 of the reactivity of persons. Gildemeister (1915) in a very 

 elaborate and carefully controlled series of experiments, showed 

 that the psychogalvanic reflex is due to increased permeability 

 of the sweat glands, which have been stimulated through sym- 

 pathetic nerves. 



