604 A MANUAL OF PHYSIOLOGY 



In sensory nerves there is no reason to believe that the 

 velocity of the nerve-impulse differs from that in motor nerves, 

 but experiments really free from objection are as yet wanting. 



The usual method is to stimulate the skin first at a point distant 

 from the brain, and then at a much nearer point. The person 

 experimented on, as soon as he feels the stimulation, makes a signal, 

 say, by closing or opening with the hand a current connected with 

 an electric time-marker, writing on a moving surface. There is, of 

 course, a measurable interval between the excitation and the signal, 

 and this being in general longer the more remote the point of stimu- 

 lation is from the brain, it is assumed that the excess represents the 

 time taken by the nerve-impulse to pass over a length of sensory 

 nerve equal to the difference in the length of the path. But there is 

 this difficulty, that the propagation of the impulse from the point of 

 stimulation to the brain is onty one link in the chain of events of 

 which the signal marks the end. The impulse has first to be trans- 

 formed into a sensation, and then the will has to be called into action, 

 and an impulse sent down the motor nerves to the hand. And while 

 the time taken by the excitation in travelling up and down the 

 peripheral nerve-fibres is, perhaps, fairly constant, the time spent in 

 the intermediate psychical processes is very variable. 



Chemistry of Nerve. Our knowledge of this subject is 

 scanty in the extreme ; and most of what we do know has 

 been obtained from analyses, not of the peripheral nerves, 

 but of the white matter of the central nervous system. 



Proteids are present, especially in the axis cylinder. They include 

 a globulin and a nucleo-proteid. 



Substances soluble in ether include cholesterin, lecithin, and cere- 

 brin. The cholesterin and lecithin, at least, belong chiefly to the 

 medullary sheath, which further contains a kind of network of a 

 peculiar resistant substance called neurokeratin (Kiihne). 



The neuritemma consists of substances insoluble in dilute sodium 

 hydrate. 



Gelatin is obtained from the connective tissue which binds the 

 nerve-fibres together. There may also be ordinary fat in the meshes 

 of the epineurium connecting the bundles. Small quantities of 

 xanthin, hypoxanthin, and other extractives, can also be obtained 

 from nerve. 



The composition of the white matter of the brain is as follows : 

 Water - 68 per cent. 



Proteids- - 8^ 



Cholesterin - - 16 



Lecithin 3 



Cerebrin - 3 



Salts - - 0-5 



Other substances - - i 5, 



Solids 



32 per cent. 



