986 THE CENTRAL NERVOUS SYSTEM 



to neglecting the influence of difference of position in tfie sleeping and 

 waking states. In sleeping children the fontanelle sinks in, an indica- 

 tion that the intracranial pressure is reduced. Observations with the 

 plethysmograph have shown that the arm swells in sleep, and shrinks 

 when the sleeper awakes, or even when he is subjected to sensory stimuli 

 not sufficient to arouse him e.g., a tune played by a musical-box 

 (Howell). The tone of the vaso-motor centre is therefore diminished, 

 and the arterial pressure falls during sleep. But a fall of general arterial 

 pressure is usually accompanied by a diminution of the quantity of 

 blood passing through the brain. So that the balance of evidence is in 

 favour of the view that sleep is associated with a certain degree of cerebral 

 antemia, 



As to the nature of the relation between the two conditions, it has 

 been suggested that the anaemia is produced by fatigue of the vaso- 

 motor centre, which causes it to relax its grip upon the peripheral blood- 

 vessels, and that the condition of the cortical nerve-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. 114) 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 circulation must be reduced almost to the 

 vanishing-point, and to a far greater degree than ever occurs in sleep 

 (Hill). We must, therefore, conclude that although sleep is normally 

 associated with some dn&mia 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 them- 

 selves 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 



