SLEEP 177 



When we court sleep we seek quiet and darkness. By 

 lying down we reduce to a minimum the inflowing im- 

 pulses from muscles, tendons, and joints. We avoid ex- 

 tremes of temperature. We interrupt, so far as we may, 

 the more vivid trains of thought. Thus, in favorable cases, 

 we withdraw from the vasomotor center the nerve-currents 

 which prompt it to react upon the blood-vessels, and we 

 are rewarded by a welcome change in the character of our 

 consciousness, if not by its actual suspension. 



The fact that one can be awakened at any time by 

 stimuli supports the theory that the immediate cause of 

 sleep is the slackening of the blood-flow through the brain. 

 A state of general poisoning could not be done away with 

 on the instant by the stimulation of afferent paths. 

 Nevertheless, the acceptance of the vasomotor hypothesis 

 leaves a place for the toxemia theories. It is the accumu- 

 lation of chemical compounds, fatigue substances, in the 

 body fluids and perhaps in the protoplasm itself that leads 

 up to the central failure. We have, therefore, to distin- 

 guish between the cause of the approach of sleep, which is 

 metabolic, and that of its onset, which is mechanical. 

 It seems to be the distribution of the fatigue substances 

 rather than their actual amount which makes sleep a 

 pressing necessity. We know that we may be very tired 

 and find sleep impossible. 



When the ties between the outer world and the indi- 

 vidual consciousness are severed as one sinks to sleep 

 there is an interesting and fairly definite order in the 

 process. Muscular control is lost before sensation; there 

 is an interval of delicious relaxation of which one is 

 pleasantly aware. Shortly the cutaneous sensibility is 

 gone, while the knowledge of the position of the extremi- 

 ties lingers for a little. Under such circumstances one 

 still realizes the posture, but not the contact with the 

 bed. Havelock Ellis 1 has suggested that this apparent 

 absence of any support may be explained by the dreaming 

 consciousness as the experience of floating in space or of 

 1 "The World of Dreams," Houghton, Mifflin & Co., Boston, 1911. 

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