526 PROCESSES INFERRED FROM INDIRECT OBSERVATION 



that is, the concentration of the yellow modification in the lipoidal 

 layer (ethyl acetate) is 100 times its concentration in the watery layer 

 and, provided f (b) were a linear function, it would be the same con- 

 centration as that which would be produced in the watery layer by 100 

 times the concentration of hydroxyl ions. In other words the sensi- 

 tiveness of the indicator is multiplied by the distribution-coefficient 

 of the lipoid-soluble modification between the two immiscible solvents. 

 In addition to this there is, as has been stated, an apparent or " physio- 

 logical" increase in the sensitiveness of the indicator due to the physical 

 separation of the two colors. 



Two frogs may be taken and a powerful stimulus applied to the skin 

 of one of them by means of an induced current for a prolonged period 

 (half an hour) while the other is left undisturbed. The brains of both 

 animals are then rapidly removed, divided longitudinally and the two 

 parts of each placed in a two per cent, neutral aqueous solution of 

 neutral red for from four to five minutes. The two brains are then 

 removed from the neutral red solution at exactly the same moment 

 and dropped into neutral ethyl acetate. 



Within five or ten minutes there is seen to be a distinct difference 

 between the colors of the cut surfaces of the two brains. The cut 

 surface of the brain which has been stimulated remains deep red, but 

 the indicator diffuses out of the unstimulated brain, and the depth of 

 color diminishes until it is only pink. The differences in color increase 

 for some time, and in some instances after the lapse of an hour the 

 unstimulated brain may be almost colorless, owing to extraction of the 

 dye by the ethyl acetate, while the stimulated brain retains a reddish 

 pink hue. Evidently the stimulated brain behaves like a faintly acid 

 aqueous layer, the unstimulated brain like a faintly alkaline aqueous 

 layer. The development of acid as a fatigue-product of nerve-centers 

 may thus be clearly inferred. 



It might be imagined that in this experiment the increased acidity 

 of the brain may be apparent and not real, being due to acids carried 

 to the brain by the blood from the tetanically contracting muscles of 

 the stimulated frog. It has been shown by Gobau, however, that pre- 

 cisely the same result is obtained if the frog employed for stimulation 

 is previously curarized, in which case the muscles are immobile. 



Acids are therefore produced in the brain in consequence of its 

 activity and in the respiratory center, if we may take this area as repre- 

 sentative of the whole, certain specific acids accelerate the passage of 

 impulses through it. We have thus experimental verification of the 

 view that central nervous phenomena are self-catalyzed. The cata- 

 lyzer which is responsible for the formation of Memory-traces, however, 

 is not probably any substance so simple as lactic or carbonic acids, 

 which as we have seen, are stimulants of the respiratory center. These 

 substances are so soluble in water that they would very rapidly be 

 washed out of the nervous tissues and the persistence of memory- 

 traces would be inexplicable. It is more likely that we have here to 



