532 PROCESSES INFERRED FROM INDIRECT OBSERVATION 



shred may remain, as in a dream, to be faintly recalled or completely 

 forgotten in awakening, according to whether or not our customary 

 waking perceptions (traces) traverse the point of union of the dream- 

 shred with the whole fabric of the reawakened consciousness. 



That the onset of sleep is in reality due to the accumulation of 

 Fatigue-products which are washed out during the period of quiescence 

 by the circulatory fluids, has been very strikingly demonstrated by 

 Pieron. This observer has shown that if the blood-serum or Cerebro- 

 spinal Fluid of a dog which has been kept awake for an abnormal 

 period be injected directly into the fourth ventricle of the brain of a 

 normal dog, even if this latter animal has recently slept, it falls at 

 once into a profound slumber. The effect is much greater if cerebro- 

 spinal fluid or the fluid from the ventricles of the brain is employed than 

 if blood-serum be used. Frequently with blood-serum nothing more 

 than a moderate somnolence is elicited, whereas when cerebrospinal 

 fluid is employed the slumber which is induced may be so profound 

 that the animal will remain asleep in any attitude in which he may be 

 placed. Pieron has made many interesting observations upon the 

 chemical nature of this sleep-inducing substance, or Hypnotoxin as 

 he designates it. He finds that it is destroyed by heating to 65 C. and 

 by oxidation, is precipitable or coagulable by alcohol and is non- 

 diffusible. It is evidently, therefore, a colloidal substance of some 

 complexity, and chemically unstable. 



THE FADING OF MEMORY-TRACES. 



It is a matter of common experience, and a fact which has been 

 experimentally verified, that a person who has been deprived of sleep 

 beyond the normal period of wakefulness does not require the full sum 

 of the periods of sleep which he has lost in order completely to recover 

 from his desire to sleep. We must therefore conclude that not only do 

 Fatigue-products disappear from the brain during sleep but, furthermore, 

 that they disappear the more rapidly the greater their concentration. 

 We have seen that the initial effect of the fatigue-products of the cen- 

 tral nervous tissues is to cause facilitation of the passage of nervous 

 impulses and the formation of Memory-traces. The phenomenon of 

 forgetting must therefore be essentially of the same nature as the phe- 

 nomenon of refreshment by sleep, i. e., it must consist in (or depend 

 upon) the disappearance of the products of their functional activity 

 from certain nerve-tracts. 



Ebbinghaus has carried out a number of excessively painstaking 

 investigations upon the rate at which meaningless syllables which have 

 once been learned by heart are forgotten. Ebbinghaus was his own 

 subject. Series, each consisting of thirteen meaningless syllables, were 

 read and reread in such a manner that each syllable was presented 

 to the senses for a period of 0.41 seconds at each repetition. When 

 it was found just possible to completely recall the series correctly, 

 the total time (= ti) consumed in memorizing the series was noted. 



