SECTION X 

 THE SPINAL CORD AS A CONDUCTOR 



THE nervous system is built up of chains of neurons which subserve reactions 

 of varying complexity. The complexity increases with the interference of 

 the higher parts of the brain in the reactions and becomes therefore more 

 and more marked as we ascend the animal scale. Whatever the course taken 

 by the impulses in the central nervous system they must all finally make use 

 of the motor common path, represented by the anterior spinal roots and by 

 the motor roots of the cranial nerves. 



The co-operation in any co-ordinated movement of widely separated 

 portions of the central nervous system necessitates the existence of long 

 paths, i.e. the axons of certain nerve cells must extend through a considerable 

 distance in the central nervous system before they arrive at the next relay 

 in the chain of which they form part. During this course the axons run in 

 the white matter of the central nervous system and are surrounded by 

 medullary sheaths. The white matter of the cord consists almost exclusively 

 of medullated nerve fibres running for the most part longitudinally. These* 

 are of various sizes, some of the smaller fibres being collaterals, which have 

 been given off from the larger ones and which will shortly turn into the grey 

 matter. In section they resemble closely the fibres of an ordinary peripheral 

 nerve, but differ from these in that they have no primitive sheath or neuri- 

 lemma. Each consists of an axis cylinder surrounded by a thick sheath of 

 myelin, the whole embedded' in a tube formed by the neuroglia. 



Of these fibres part belong to the spinal cord, the proprio-spinal or 

 internuncial fibres, which we have studied previously. The greater number 

 serve to establish connection between the grey matter of the cord or the 

 afferent roots entering the cord and the different levels of the brain, and these 

 fibres may carry impulses either up towards the brain or down towards the 

 spinal cord ; they may be ascending or afferent, so far as the brain is con- 

 cerned, or descending and efferent. No fibre takes an isolated course on its 

 way through the cord ; practically every one sends off fine branches or 

 collaterals, which run into the grey matter at various levels, there making ' 

 connection or having synapses with the local reflex mechanisms contained in 

 each segment. 



On inspection the white matter is seen to be divided by the anterior 

 and posterior fissures of the cord into two symmetrical halves, and the 

 nerve roots divide each half into anterior or ventral, lateral, and posterior 



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