THE CENTRAL NERVOUS SYSTEM. 49 



(1) those which run a short course in the grey matter and form 

 terminal arborisations in relation with other nerve cells in the grey 

 matter (cells of Golgi's second type), and (2) those which acquire a 

 myelin sheath, run into the white matter, divide into a short descend- 

 ing and a longer ascending branch, and enter into the formation of the 

 tracts of the white matter. 



Certain well-defined groups of nerve cells may be recognised, three 

 in the anterior horn, one in the lateral horn, the cells constituting 

 Clarke's column, and those of the posterior horn. The anterior horn 

 cells are very numerous in the cervical and lumbar regions, from which 

 arise the nerves to the limbs. 



The Nerve Roots. The anterior and posterior roots meet a short 

 distance from the lateral aspect of the spinal cord, and unite to form a 

 spinal nerve. Just before it joins the anterior root the posterior root 

 exhibits a swelling, the spinal ganglion ; the unipolar cells of the 

 ganglion are situated for the most part around the periphery of the 

 structure, and the single process of each divides in a T-shaped manner 

 into two fibres, one of which has a peripheral and the other a central 

 direction. 



The exit of the anterior root has already been described. The fibres 

 of the posterior root, consisting of the central divisions of the processes 

 of the ganglion cells, enter the spinal cord in the neighbourhood of the 

 posterior horn of grey matter. Each divides into a short descending 

 and a longer ascending branch. The longest ascending divisions run 

 upwards in the posterior column to reach the medulla oblongata. The 

 others turn into the grey matter at varying distances above their point 

 of entry to terminate by arborisations around nerve cells. The descend- 

 ing fibres end in the same manner. Both ascending and descending 

 fibres give off fine medullated collateral branches at intervals, these also 

 entering the grey matter to form terminal arborisations in relation with 

 nerve cells. 



The Collateral Fibres. Collaterals enter the grey matter from 

 white fibres in all parts of the anterior, lateral, and posterior columns. 

 Those from the long tracts of the white matter may be looked upon as 

 associational in character, serving either to distribute the impulses con- 

 veyed by the main fibres and so to promote co-ordination, or to form 

 part of a system of relays by which impulses are conveyed by short 

 tracts from segment to segment of the spinal cord. The collaterals 

 from, and terminations of, posterior root fibres, on the other hand, or 

 many of them, are concerned with the formation of reflex arcs in the 

 spinal cord itself ; in addition to the fibres of the posterior root which 

 have been described as running to the medulla oblongata in the posterior 



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