740 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



Between childhood 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 matured, 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 producing 

 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 tissues (nerve-cells and muscles) yield as the result of their activity 

 some by-product which is carried by the blood through the central 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, 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 follow- 

 ing the exercise of the higher nerve-centres. 



Cessation of stimuli, decreased responsiveness of the active tissues, and a 

 change in the composition of the blood are the preliminaries to sleep. To 

 these should be added the diminution of the blood-supply to the head. 



A condition superficially resembling sleep can be induced in various ways. 

 Removal of all external stimuli, extreme cold, anaesthetics, hypnotic suggestion, 

 compression of the carotids, a blow on the head, loss of blood, all produce a 

 state of unconsciousness which, in so far, has a similitude with sleep. These 

 conditions produce this state, however, by mechanically decreasing the blood- 

 supply or cutting off the peripheral stimuli. 



Normal sleep is tested by the fact that during its progress the changes that 

 occur in the central system are recuperative, whereas this feature may be more 

 or less absent from the states which merely resemble it. 



Condition of the System during Sleep. It appears that during sleep 

 the capacity of the central system to react is never lost. Were such the case 

 it would not be possible to awaken the sleeper. Moreover, the sleeping per- 

 son is far more responsive to stimuli from without than at first might be 

 thought. The close relations between dreams and external stimuli has been 

 recognized, and plethysmographic studies show still more clearly how the matter 

 stands. 



It was found that when a subject fell asleep with the arm in a plethysmo- 

 graph various stimuli which did not waken the sleeper still served to cause a 

 diminution in the volume of the arm, which was certainly due to the with- 



