THE CENTRAL NERVOUS SYSTEM 763 



and which anaemic. In coma, a pathological condition which in 

 some respects has analogies to profound and long-continued sleep, 

 the brain is -congested, and the proper elements of the nervous tissue 

 presumably compressed. And artificial pressure (applied by means 

 of a distensible bag introduced through a trephine hole into the 

 cranial cavity) may cause not only unconsciousness, but absolute 

 anaesthesia. But it is possible that this artificial increase of intra- 

 cranial pressure may produce its effects by rendering the brain 

 anaemic, and it has been actually observed that the retinal vessels, 

 as seen with the ophthalmoscope and the vessels of the pia mater 

 exposed to direct observation in man by disease of the bones of the 

 skull, or in animals by operation, shrink during sleep. In sleep- 

 ing children the fontanelle sinks in, an indication that the intra- 

 cranial 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 vasomotor 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 decidedly in favour of the view that sleep is associated with cerebral 

 ancemia. 



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

 has been suggested that the anaemia is produced by fatigue of 

 the vasomotor centre, which causes it to relax its grip upon the 

 peripheral bloodvessels, 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 vasomotor 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 ? Hill has indeed shown by means of the sphygmo- 

 meter (p. 104) that the fall of arterial pressure is not essentially con- 

 nected 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 carotid 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. The same is true of monkeys after ligation 

 of one carotid and one vertebral. We must, therefore, conclude 

 that although sleep is normally associated with anamia of the brain t 



