624 NERVOUS SYSTEM 



" fatigue-products " ; but the characters of these products have not been 

 ascertained. Nevertheless, they probably exist in the blood and affect 

 the nerve-cells. 



When fatigue is followed by sleep, it is certain that the supply of 

 blood to the brain is considerably diminished. The only explanation of 

 this is that in some way the somniferous agent in the blood operates 

 through the vasomotor system. The fatigue-product or products induce 

 a desire for sleep ; but the necessity for sleep is represented by certain 

 structural changes that have taken place, probably in the nerve-cells of 

 the brain. These changes are degenerative and their repair requires 

 a more or less extended period of functional rest. Studies of these 

 changes by Hodge, Mann, Vas, Lambert and others have afforded a 

 reasonable physical basis of sleep. 



Following nervous cell-activity or repeated stimulation of nerve-cells 

 through the nerves, the cells undergo marked changes in size and con- 

 figuration. The cells become shrunken, vacuolated and crenated ; and 

 the nuclei are even more shrunken and irregular in form, the diminu- 

 tion in size sometimes amounting to fifty per cent. These changes have 

 been observed in cats, dogs, birds, and other animals lower in the scale. 

 That they may occur in the human subject is inferred. After a num- 

 ber of hours of repose about equal to the daily period of sleep in the 

 human subject the cells and nuclei are found to have returned to their 

 original condition. 



In addition to the changes just indicated, it is more than probable 

 that alterations occur in the Nissl bodies. If the view can be accepted 

 that these bodies contain stored-up nervous energy that is consumed in 

 the production of nerve-impulses and may be restored by rest, it may 

 be assumed that the necessity for sleep is due in part, at least to 

 destruction of chromatoplasm by active chromatolysis. There is, how- 

 ever, what may be called a passive chromatolysis in " disuse atrophy " 

 of nerve-cells. This has been shown in the cell-degeneration that 

 occurs after nerve-fibres attached to cells have been divided. 



While the views of physiologists in regard to the office of Nissl 

 bodies are to some extent speculative, it is certain that nerve-cells 

 undergo degenerative changes during wakefulness and activity and that 

 during sleep they are gradually restored to their normal condition and 

 resume their normal appearance, calling normal the condition following 

 repose. By these restorative processes the nervous system becomes 

 capable of renewed activity. It is to be remembered, however, that 

 this restoration may be in a measure produced by rest without sleep, 

 and that sleep may follow a period of nervous and muscular inactivity 

 as nearly complete as is possible in a waking condition. 



