7 6 4 A MANUAL OF PHYSIOLOGY 



it is not directly caused by it. The cortical centres go to sleep* 

 because they are * tired,' or because the stimuli which usually excite 

 them have ceased, and not because their blood-supply is diminished. 



(3) The idea that the dendrites are contractile, and by pulling 

 themselves in, and thus breaking certain nervous chains, cause sleep, 

 still remains a mere theory, unsupported by any real evidence. The 

 same is true of the notion that the fibrils of the neuroglia insinuatethem- 

 selves into the 'joints,' by which one neuron comes into contact with 

 another, and acting as insulating material, block the nerve-impulses. 



In general, the depth of sleep, as measured by the intensity of 

 sound needed to awaken the sleeper, increases rapidly in the first 

 hour, falls abruptly in the second, and then slowly creeps down to- 

 its minimum, which it reaches just before the person awakens. As 

 to the amount of sleep required, no precise rules can be laid down. 

 It varies with age, occupation, and perhaps climate. An infant, 

 whose main business is to grow, spends, or ought to spend, if mothers 

 were wise and feeding-bottles clean, the greater part of its time in 

 sleep. The man, whose main business it is to work with his hands 

 or brain, requires his full tale of eight hours' sleep, but not usually 

 more. The dry and exhilarating air of some of the inland portions 

 of North America, and perhaps the plains of Victoria and New k 

 South Wales, incites, and possibly enables a new-comer to live for a 

 considerable period with less than his ordinary amount of sleep. 

 Idiosyncrasy, and perhaps to a still greater extent habit, have also a 

 marked influence. The great Napoleon, in his heyday, never slept 

 more than four or five hours in the twenty-four. Five or six hours 

 or less was the usual allowance of Frederick of Prussia throughout 

 the greater part of his long and active life. 



Hypnosis is a condition in some respects allied to natural slumber; 

 but instead of the activity of the whole brain or perhaps we should 

 rather say, the whole activity of the brain being in abeyance, the 

 susceptibility to external impressions remains as great as in waking 

 life, or may be even increased, while the critical faculty, which 

 normally sits in judgment on them, is lulled to sleep. The condi- 

 tion can be induced in many ways by asking the subject to look 

 fixedly at a bright object, by closing his eyes, by occupying his atten- 

 tion, by a sudden loud sound or a flash of light, etc. The essential 

 condition is that the person should have the idea of going to sleep, 

 and that he should surrender his will to the operator. In the hypnotic 

 condition the subject is extremely open to suggestions made by the 

 operator with whom he is e?i rapport. He adopts and acts upon 

 them without criticism. If, for example, the hypnotizer raises the 

 subject's arm above his head, and suggests that he cannot bring it 

 down again, it stays fixed in that position for a long time without any 

 appearance of fatigue ; or the whole body may be thrown, on a mere 

 hint, into some unnatural pose in which it remains rigid as a statue. 

 Suggested hemiplegia or hemiancesthesia, or paralysis of motion; 

 and sensation together or apart in limited areas, can also be realized ; 

 and surgical operations have been actually performed on hypnotized 

 persons without any appearance of suffering. If, on the other 



