PHYSIOLOGICAL 265 



and noise, we get rid of many of the knocks at the door of our 

 awareness; we are unable to answer the door with our usual 

 promptitude ; and we cannot normally move about without awaken- 

 ing. There is some somnolence, so to speak, throughout the body, 

 for the pulse and breathing are slower and the temperature sinks a 

 little. The mental aspect of life is very inconspicuous in deep sleep? 

 at least we infer this from so often remembering nothing of our 

 dreams. 



Yet a difficulty is raised by the fact that the rest characteristic 

 of sleep is so partial. Although the heart gets a little rest between 

 each beat, it is not easy to understand its persistent, normally 

 tireless, activity. Why should it not need the relative rest which 

 most muscular tissues seem to require, which they find in sleep? 

 Much of the brain goes out of gear in sleep, but how is it that 

 certain centres never cease to be active? What is this sleep that 

 affects some parts so profoundly, and others so lightly? 



In his recent admirable book on Sleep and Its Disorders (1929), 

 Dr. R. D. Gillespie punctures one theory after another with gleeful 

 adroitness. There is next to nothing in the view that sleep is induced 

 by the circulation through the brain becoming slowed down, 

 because the blood has become more viscous, because the hard- 

 worked nerve-cells have used up so much water. Beautiful but 

 almost baseless is the theory of Duval, that the branches of adjacent 

 nerve-cells cease at bedtime to be in contact, so that neural cir- 

 cuits are broken. Very attractive is the theory of Pieron and others 

 that prolonged loss of sleep produces a poisonous "hypnotoxin" 

 which compels sleep by inhibiting activity in nerve-cells. But how 

 often we cannot sleep when we are very tired, how often we sleep 

 soundly when we are not tired at all, how often people fall asleep 

 without apparent cause. Moreover, we have first to catch our 

 hypnotoxin. Of course, the inducing of sleep has been referred to 

 the regulatory role of the hormone-making glands; but these have 

 already a very heavy burden to bear. 



The favourite theory at present is that the brain includes a special 

 sleep-governing centre, situated, according to some, in the central 

 grey matter; according to others, in the region called the thalamus. 

 Just as there is a temperature-regulating or thermotaxic centre (in 

 the corpus striatum of the fore-brain), so there is considerable 

 evidence of the existence of a nerve-centre that regulates the com- 

 plex function and abeyance of function which we call "sleep". By 

 an ingenious combination of studies on sleeplessness and other dis- 

 orders, on soporifics, and on the labyrinthine structure of the brain, 

 it has been possible to make out a fair case for the theory that 

 ionic changes in a centre in the floor of the thalamus bring about 

 sleeping and waking again. Changes in calcium-ion concentration 

 in the blood influence the permeability of cell membranes, which 



