THE CENTRAL NERVOUS SYSTEM 875 



mere diminution of the chromatic substance to complete dis- 

 appearance of it and such disintegration of the cell as must have 

 precluded its recovery had the animal been allowed to live. Many, 

 and indeed most, of the cortical cells were quite unaffected. Histo- 

 logical alterations may also be caused in sympathetic ganglion cells 

 by prolonged artificial stimulation of the nerves connected with 

 the ganglia. Experiments on fatigue changes in the cells of the 

 spinal ganglia after electrical excitation of the posterior root-fibres 

 are less decisive, some observers having obtained positive, others 

 negative, results (p. 809). 



Theories of the Causation of Sleep. (i) Some have suggested that 

 sleep is induced by the using up of substances necessary for the 

 functional activity of the neurons e.g., the stored-up or intra- 

 molecular oxygen or by the action of the waste products of the 

 tissues, and especially lactic acid, when they accumulate beyond a 

 certain amount in the blood, or in the nervous elements themselves. 



(2) Others have looked for an explanation to vascular changes in 

 the brain, but so far are the possible causes of such changes from 

 being understood, that it is even yet a question whether in sleep the 

 brain is congested or anaemic. Certain writers have settled this 

 question by the summary statement that when the brain rests the 

 quantity of blood in it must be supposed to be diminished, as in 

 other resting organs. But this is a fallacious argument. For when 

 the whole body rests, as it does in sleep, it has as much blood in it 

 as when it works ; in sleep, therefore, if some resting organs have 

 less blood than in waking life, other resting organs must have more ; 

 and it is the province of experiment to decide which are congested 

 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. Statements 

 to the contrary may be due to neglecting the influence of difference 

 of position in the sleeping and waking states. In sleeping children 

 the fontanelle sinks in, an indication 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 anczmia. 



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 peri- 

 pheral bloodvessels, and that the condition of the cortical nerve- 



