6go 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 experi- 

 ments 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 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 only 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 probably fairly constant, the time spent in 

 the intermediate psychical processes is very variable. 



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 central 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. 



The lipoids of nervous tissue are very important constituents. 

 They are substances soluble in organic solvents, like benzol and 

 ether, and comprise 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-medullated nerves also yield a considerable quantity of 

 lipoids (ii'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 phos- 

 phatides (kephalin and lecithin) and cholesterin. Thus, in non- 

 medullated fibres 47 per cent, of the lipoid extract consisted of 

 cholesterin, and 23^7 per cent, of kephalin ; while in the medullated 

 fibres cholesterin made up only 25 per cent, of the extract, and 

 kephalin 12*4 per cent. On the other hand, the cerebrosides are 

 present, both relatively and absolutely, in much larger quantity 

 in medullated than in non-medullated nerves. In both varieties of 

 fibres kephalin, and not lecithin, is the chief phosphorus-containing 

 body (Falk) . The medullary sheath further contains a kind of net- 

 work of a peculiar resistant substance, neurokeratin. The neurilemma 

 consists of substances insoluble in dilute sodium hydroxide. Gelatin 



