CHAPTEE XXIV 

 THE NERVOUS SYSTEM {Continued) 



B. HlSTOLOGICAL MORPHOLOGY 



THE spinal cord and brain collectively form a long slender 

 mass consisting of conduction paths or fibre tracts, which are for 

 the most part axially disposed and connect more or less distant 

 groups of nerve cells. Each cell group controls the functions of a 

 limited portion of the body, to which its neuraxes pass in bundles 

 that follow definite paths. These paths are the so-called tracts, 

 and the cell group which gives origin to the fibres of any one tract 

 is its center or nucleus. 



The cerebro-spinal axis is thus composed of many series of 

 such nuclei with their connecting tracts ; the nuclei collectively 

 forming the grey, and the tracts the white matter. Except for 

 the cortex of the cerebrum and of the cerebellum the grey matter 

 is, as a rule, centrally, and the white matter peripherally disposed. 



Thus in the spinal cord the grey matter consists of a central 

 H -shaped mass extending from the filum terminale upward to the 

 medulla. Above this level the continuity of the central grey 

 mass suffers frequent interruption from the irregular and oblique 

 disposition of the fibre tracts of this region, so that above the 

 medulla the grey matter is only represented by a series of islands 

 (nuclei), each containing one or more cell groups. 



In the cerebrum and cerebellum these central nuclei are sepa- 

 rated from the grey matter of the cortex by an intervening stratum 

 of white matter ; in these locations the larger mass of grey matter 

 is found on the surface of the white medulla. 



The peculiar disposition of the grey matter of the brain is ex- 

 plained by the progress of its development. In the early embryo 

 the cerebral vesicles are at first surrounded by a cell mass only. 

 From these cells the fibre processes grow out, and in so doing the 

 excessive formation of fibres at the cephalic end cuts off the grey 

 matter of the cortex (pallium) from the more caudal portions. 

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