292 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



true for all the groups of cells. Hence the cells would, by reason of this fact, 

 have the greatest capability for work in the middle period. Between child- 

 hood and old age there is, however, this difference — that while in the former 

 the non-available substances in the cell are developing, not yet having ma- 

 tured, those in the latter have in some way become permanently useless. The 

 degree to which the blood-supply can be controlled varies with age. and the 

 amounts of substance capable of yielding energy at various periods of life are 

 different; so that, considering these factors alone, though there are probably 

 others, it may be easily appreciated that the sleep of childhood, maturity, and 

 old age should be quite distinguishable. 



Cause of Sleep. — It is recognized that local exercise is capable of pro- 

 ducing general fatigue, and the fatigued portions give rise to afferent impulses 

 which, reaching the central system, cause some of the sensations of fatigue; 

 moreover, the active tissue- (nerve-cells and muscles) yield as the result of 

 their activity some by-product which is carried by the blood through the cen- 

 tral system and becomes the chief cause of sleep. It has been shown by 

 Mosso that if a dog be thoroughly fatigued, giving all the signs of exhaustion, 

 and the blood from this dog be transfused to one that has been at rest, then 

 after the transfusion, the dog which has received the blood from the exhausted 

 animal will exhibit the symptoms of fatigue in full force. The inference is 

 that from the tired animal certain by-products have thus been transferred, 

 and that these are responsible for the reactions. We know, further, that we 

 can distinguish in ourselves different forms of the feeling of fatigue, and that 

 the sensations which follow the prolonged exercise of the muscular system 

 differ from those following the exercise of the higher nerve-centres. 



Two things appear as highly probable: First, that there is a wide individual 

 variation in the condition designated as normal sleep. Second, that normal 

 sleep is the result of several sets of influences which need not necessarily be 

 active to the same degree during each period of sleep. Excluding the factor 

 represented by diminution of the external stimuli, sleep has been attributed 

 more or less exclusively to one of the three following influences: 



1. Chemical Influences. — The theories emphasizing the chemical factor 

 point out that during the normal activity of the body there are formed and 

 taken up by the blood substances which may directly diminish the activity 

 of the nerve-cells and directly or reflexly affect the circulation so as to 

 diminish the supply of blood to the brain, and especially to the cerebral 

 cortex. 



•J. Circulatory Influences. — The vaso-motor theories look upon the changes 

 in the blood-supply as :i prime cause of sleep; these changes to be referred 

 in the last instance to the fatigue of the vaso-motor centre in the bulb. 



.'5. Histological Influences. — These are made dependent on the shrinkage 

 of nerve-cells during fatigue, the retraction of the dendrites of the cortical 

 cells interrupting the nerve-pathways, or the mechanical separation of the 

 nerve-elements through the intrusion of the neuroglia-cells between them 

 (Cajal). The vaso-motor and chemical theories combined are at present most 



