346 The Outline of Science 



brush or tree of fine fibres, and they interlock with each other. 

 In the nerves that carry messages or commands to the muscles 

 and glands many long fibres are bound up into bundles by a 

 sheath. Inside each fibre there is a mysterious central channel, 

 the axis cylinder, probably of a liquid nature. 



What the real nature of a nerve-thrill is we do not yet know. 

 It is accompanied by electricity, but it is not itself an electric 

 wave, for such a wave would travel more than a million times 

 faster than the nerve-message does. The nerves are also peculiar 

 in the fact that they never get tired (as long as they are well 

 supplied with oxygen), and physiologists have not been able to 

 discover any definite chemical change in them. Even the pro- 

 duction of carbon dioxide is questionable. Sleeping or waking, 

 the wires are always alive, yet physiologists have not found that 

 any heat is produced in connection with their activity. 



The Brain-centre 



It is otherwise with the masses of nerve-cells which make 

 up the great brain-centres. Everbody knows that these grow 

 tired, and must have a period of rest and recovery. Sleep is, 

 however, still a puzzling phenomenon, and no theory of it can be 

 regarded as satisfactory. All that physiologists are generally 

 agreed upon is that the blood-supply to the brain is checked, and 

 this lessening of the supply of oxygen (as to which the brain is 

 particularly sensitive) lowers the vitality of the organs of con- 

 sciousness. About the end of the first hour of sleep (which is the 

 real "beauty-sleep") the brain-life is entirely suspended, and the 

 blood is busy feeding the tired muscles. Some hours later more 

 blood seems to return to the brain, and we get partial conscious- 

 ness, uncontrolled by intellect, in the form of dreams. In a few 

 individuals there may be, instead of a partial return of conscious- 

 ness, an awakening of the power of automatic response to stimu- 

 lations. They are apt to "walk in their sleep." 



Our knowledge of the brain is now a special and formidable 



