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 

 siven 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 neurilernrna. 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 concerned, or descending and efferent. 

 No fibre takes an isolated course on its way through the cord ; practi- 

 cally 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. 



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