768 NERVE 



being about the normal rate. This is greater than the speed of the 

 fastest train in the world. According to Piper's recent measurements 

 the velocity in human medullated nerve is even greater than Helmholtz 

 concluded, about 120 metres a second under ordinary conditions. The 

 rate is independent of the intensity of the excitation. The velocity 

 with which the negative variation is propagated (p. 800) is the same as 

 that of the nerve-impulse. 



In sensory nerves there is no reason to believe that the velocity of the 

 nerve-impulse differs from that in motor nerves, but experiments on 

 man 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 stimulation 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 only one 

 link in the chain of events of which the signal marks the end. The 

 impulse has first to be transformed 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 probably fairly constant, the 

 time spent in the intermediate psychical processes is very variable. 



SECTION II. CHEMISTRY, DEGENERATION, AND REGENERATION OF 



NERVE. 



Chemistry of Nerve. Our knowledge of this subject is still scanty; 

 and most of what we do know has been obtained from analyses, 

 not of the peripheral nerves, but of the white matter of the centra.! 

 nervous system 



Proteins are present, especially in the axis-cylinder. The proteins of 

 nervous tissue include two globulins, one coagulated by heat at 47 C., 

 the other at 70 to 75 C., and a nucleo-protein coagulating at 56 to 

 60 C. 



Very important constituents are certain substances soluble in organic 

 solvents, like benzol and ether, and comprising cholesterin, certain 

 phosphatides (kephalin and lecithin), and certain cerebrins or cerebrosides . 

 The cerebrins are glucosides containing nitrogen, but no phosphorus, 

 and they yield a reducing sugar (galactose) on hydrolysis. In the 

 nervous tissue there is also present, according to some authorities, a 

 compound called protagon. Others consider it a mere mixture of phos- 

 phatides and cerebrosides. The lipoids of nerve-fibres belong largely to 

 the medullary sheath, but they are not confined to it, since non-medul- 

 lated nerves also yield a considerable quantity of lipoids (11*5 per cent, 

 of the solids as against 46-6 per cent, for medullated nerves). Non- 

 medullated nerves (splenic nerves of the ox) are distinguished from 

 medullated nerves (human sciatic) by the high proportion of their total 

 lipoids constituted by the phosphatides (kephalin and lecithin) and 

 cholesterin. Thus, in non-medullated fibres 47 per cent, of the lipoid 



