530 PROCESSES INFERRED FROM INDIRECT OBSERVATION 



The view that the formation of the Memory-trace is due to an auto- 

 catalyzed chemical reaction, therefore, not only enables us to interpret 

 some of the most striking qualitative phenomena of intellectual proc- 

 esses, but also to predict their quantitative alteration with successive 

 repetition. The quantitative data obtained by Ebbinghaus are among 

 the most readily interpretable and at the same time accurate measure- 

 ments of this kind which we possess, but a variety of measurements 

 which have been made on the rate of learning by telegraph-operators, 

 typists, and so forth, all yield " curves of learning" which very strikingly 

 resemble the curve which represents the progress of an autocatalyzed 

 chemical reaction, and in some cases, it appears, two or more of such 

 curves may be superimposed to yield "cycles of learning" just as we 

 have cycles of growth in a growing organism. 



SLEEP. 



The various theories of sleep which have been proposed are no less 

 numerous than those which have been propounded to account for 

 the phenomenon of memory. A Vasomotor Theory of Sleep has been 

 advanced by Howell, who considers that it is attributable to cerebral 

 anemia, due to a diminished blood-supply to the brain, following the 

 general fall of arterial pressure which accompanies sleep. While this 

 may very possibly be a contributing factor to the phenomenon of sleep, 

 yet, on the other hand, it is at least equally conceivable that the vaso- 

 motor-phenomena which accompany sleep are merely secondary mani- 

 festations of the processes which induce sleep, and that the actual 

 onset of sleep is due primarily to other factors. The close connection 

 of sleep with Fatigue on the one hand, and with the absence or monot- 

 ony of Sensory Stimulation on the other, indicates very clearly that a 

 condition of the nervous tissues consequent upon prolonged activity 

 is a potent factor predisposing the central nervous system to the 

 relatively suspended activity of sleep. 



The accumulation of Fatigue Products in the brain, when it has 

 exceeded the amount which causes maximal facilitation of the passage 

 of nervous impulses, begins to retard the passage of impulses, and this 

 retardation increases with the degree of accumulation. With continued 

 wakefulness, as many observers have pointed out, the Threshold of 

 Sensory Stimulation rises. A stronger stimulus than usual is required 

 to traverse the clogged and overloaded channels, and consequently the 

 environment, by exclusion of the countless slight fluctuating impres- 

 sions which lend variety to our surroundings, becomes more and more 

 monotonous, fewer and fewer "channels" of the brain are traversed 

 by impulses, larger and larger areas become quiescent through lack 

 of traversing stimuli, until finally sleep supervenes, and the whole of 

 the brain except those portions, chiefly in the medulla, which are 

 vital to the maintenance of the circulation and respiration, and some 



