876 A MANUAL OF PHYSIOLOGY 



cells which we call sleep is directly produced by the lack of blood. 

 But there does not appear to be any good reason for believing that 

 the vaso-motor centre is more susceptible of fatigue than the higher 

 cerebral centres. On the contrary, it is probable that the bulbar 

 centres are less delicately organized and more resistant than the 

 higher centres. In any case, if the cerebral nerve-cells ' go to sleep ' 

 because their blood-supply is diminished, ought we not to look for a 

 similar cause for diminished activity of the vaso-motor centre ? Or 

 if the answer is made that the activity of the vaso-motor cells is 

 directly lessened by fatigue, or by the cessation of external stimuli, 

 why should not this be the case also for the cortical cells ? It can 

 be shown by means of the sphygmomanometer (p. 106) that the fall 

 of arterial pressure is not essentially connected with sleep, but is 

 produced by the bodily rest and warmth which accompany it. 

 Further, even a great diminution in the supply of blood going to the 

 brain is not necessarily followed by sleep. For example, both 

 carotids and both vertebral arteries may frequently be tied in dogs 

 at the same time without producing any symptoms, the anastomosis 

 of the superior intercostal arteries with the anterior spinal artery 

 providing a sufficient channel for the blood absolutely required by 

 the brain. Monkeys after ligation of both carotids may be most 

 alert and active. To produce sopor in animals the cortical circula- 

 tion must be reduced almost to the vanishing-point, and to a far 

 greater degree than ever occurs in sleep (Hill). We must, there- 

 fore, conclude that although sleep is normally associated with some 

 ancBmia of the brain, 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, is a mere theory, unsupported by any real evidence. The 

 same is true of the notion that the fibrils of the neuroglia insinuate 

 themselves 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 

 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 through- 

 out the greater part of his long and active life. 



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



