SLEEP 



737 



with the weakening or withdrawal of the stimuli the abeyance becomes 

 complete. The over-fatigued soldier falls asleep in a dug-out in spite 

 of the enemy's shells bursting around him. We ordinarily favour the 

 onset of sleep by seeking the warm, quiet, and dark atmosphere of our 

 beds, where the excitements of the outside world are at a minimum. 



Sleep has been attributed to a lessened circulation of blood through 

 the brain, caused by the dilatation of the vessels of the skin and of 

 the splanchnic area. Rest in an armchair in a warm room after a 

 heavy meal conduces to sleep. Exposure to cold wind repels it. 



The periodicity of sleep has been attributed to a gradual loss of 

 tone of the vaso -motor centres. When awake the vessels of the body 

 are so regulated that the blood-supply of the brain is ample, but as 

 fatigue of the vaso-motor mechanism ensues the blood-supply to the 

 brain gradually lessens, and the increasing anaemia induces the onset 

 of sleep. 



FIG. 449. COBEA HYPNOTIZED BY STROKING, AND MADE STIFF AND STRAIGHT. 



Another view of sleep is that it is due to the accumulation of waste 

 products or fatigue toxins within the body. It has also been suggested 

 that the synapses of the neurons are interrupted in sleep by an amoeboid 

 retraction which blocks the conduction of impulses. There is no 

 evidence of this. 



There is no single theory which satisfactorily explains the phe- 

 nomena of sleep. 



Narcosis. The unconsciousness induced by volatile anaesthetics 

 is due to chemical change of the nerve cells. As the result of 

 prolonged anaesthesia, degenerative changes are induced in the nerve 

 cells and elsewhere. The volatile anaesthetics easily permeate the 

 cell protoplasm. Theie is no need to evoke the lipoid nature of the 

 cell membrane as an explanation. When administration of the anaes 

 thetic ceases, the volatile anaesthetic diffuses again from the cell into 

 the blood, and the state of anaesthesia gradually passes off as the 

 drug is breathed out. 



Hypnosis. The condition of hypnosis superficially resembles that 

 of normal sleep. It may be induced in many animals merely by 

 holding them in a strange posture, as in the well-known '' experi- 

 mentum mirabile," which consists in placing a fowl on its back with 

 its beak to a line chalked upon the ground. In man it is produced 



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