496 THE WONDER OF LIFE 



points out, these suggestions cannot be solutions ; they 

 simply shunt the problem. For why this periodic change 

 in the flow of blood to the brain, why the retraction of the 

 nerve cells, why the unresponsiveness to outside stimu- 

 lation ? 



In a luminous lecture on sleep, Professor Fraser Harris 

 distinguishes four types — chemical, vascular, sensory, and 

 psychic. (1) Sleep may be due to fatigue- toxins, the 

 poisonous waste-products of exertion, just as it may be 

 induced by drugs. The nerve-cells in the brain no longer 

 exhibit their normal interhnking (or synapsis) ; there is 

 resistance to incoming sensory messages. Men fall asleep 

 in the saddle or on forced marches, and Holbein fell asleep 

 when swimming the Channel. (2) Sleep may be due to 

 diminution of the velocity of the cerebral blood flow, just 

 as may occur abnormally in some kinds of fainting. It has 

 been shown that the sleeping brain is paler, there is less 

 blood and a lower blood pressure. Mosso devised an ex- 

 periment of balancing a wide-awake man accurately on a 

 table, and showed that when he fell asleep the foot end 

 of the table sunk — ^the dip indicating the depth of sleep. 

 When the heart is excited we cannot sleep. (3) Sleep 

 may be due to sensory changes, to an increase of the 

 ' resistance ' at the interhnking (or synapsis) of the cells 

 in the sensory centres of the fore-brain, or a diminution 

 of conductivity at these interUnkings. When sensations 

 force their way in we cannot sleep. (4) Sleep may be due 

 to the absence of emotions and ideas ; thus stupid people 

 faU asleep easily. As Bergson has said, we do not go to 

 sleep if we are more interested in anything else than going 

 to sleep. When we are worried we cannot sleep. 



Another approach to the problem of sleep is in the hght 



