CHAPTER XIV 

 THE NERVOUS SYSTEM* 



Section i. 



Classification of Nerves.- — The first step towards a classification 

 of nerves was taken when the discovery was made that some 

 nerves are exclusively sensory and others exclusively motor in 

 function ; prior to this it was believed that a nerve could carry 

 either impulse indifferently. It was next found that a nerve in 

 the body could only carry impulses in one direction ; for instance, 

 a sensory nerve could only convey impulses from the periphery 

 of the body towards a centre, a motor nerve could only convey 

 impulses from a centre to the periphery. This is the real basis of 

 classification, notwithstanding the fact that the impulses travel- 

 ling to a centre, though spoken of as sensory, may be widely 

 different in nature, and those from a centre to the periphery, 

 though motor, may only be so in an opposite sense. The terms 

 ' sensory ' and ' motor ' prove very misleading in speaking of 

 the function of a nerve, and for long it has been recognised that 

 the two great divisions of nerve fibres are afferent, or centri- 

 petal, and efferent, or centrifugal. An afferent nerve is one 

 conveying impulses from the periphery to the centre, no matter 

 what the nature of these impulses may be. An efferent nerve is 

 one conveying impulses from a centre to the periphery, notwith- 

 standing that these impulses may be of widely different characters. 

 The determining factor of whether a nerve is afferent or efferent 

 does not lie in its structure ; there is no microscopical difference 

 between the two. The real explanation lies in the nature of the 

 nerve-ending in the tissues, and this is proved by the fact that one 

 kind of efferent nerve may be made experimentally to take the 

 place of another. 



Afferent nerves, as we have seen, are those engaged in conveying 

 impulses towards a centre ; the centres are the brain, spinal cord, 



* I am indebted to Professor Sherrington, F.R.S., for kindly reading 

 this chapter and supplying that portion dealing with the ' scratch, ' and 

 ' step ' reflex. 



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