GENERAL FEATURES OF NERVOUS TISSUES. 129 



processes of the nerve cells, and on the other, from the division of the axis- 

 cylinders of fibres running in the gray matter. 



The cell is not surrounded like the ganglion cell by a distinct sheath. As 

 we shall see later on, while treating in detail of the central nervous system, 

 all the nervous elements of the spinal cord are supported by a network or 

 spongework of delicate peculiar tissue called neuroglia, analogous to and 

 serving much the same function as, but different in origin and nature from, 

 connective tissue, This neuroglia forms a sheath to the nerve cell and to its 

 processes, as well as to the nerve fibres running both in the white and the 

 gray matter ; hence within the central nervous system the fibres, whether 

 medullated or not possess no separate neurilemma ; tubular sheaths of the 

 neuroglia give the axis-cylinder and medulla all the support they need. 



All the nerve cells of the anterior cornu probably possess an axis-cylinder 

 process, and other cells similarly provided with an axis-cylinder process are 

 found in other parts of the gray matter. But in certain parts, as, for instance, 

 in the posterior cornu, many of the cells appear to possess no axis-cylinder 

 process ; in such cases all the processes appear to branch out rapidly into fine 

 filaments. Except for this absence, apparent or real, of an axis-cylinder 

 process, such cells resemble in their general features the cells of the anterior 

 cornu, though they are generally somewhat smaller. Speaking generally, 

 the great feature of the nerve cells of the central nervous system as distin- 

 guished from the ganglion cells is the remarkable way in which their pro- 

 cesses branch off into a number of delicate fi.laments, corresponding to the 

 delicate filaments or fibrillse in which at its termination in the tissues the 

 axis-cylinder of a nerve often ends. 



96. From the above description it is obvious that in the spinal cord 

 (to which as representing the central nervous system we may at present con- 

 fine ourselves, leaving the brain for later study) afferent fibres (fibres of the 

 posterior root) are in some way by means of the gray matter brought into 

 connection with efferent fibres (fibres of the anterior root) ; in other words, 

 the spinal cord is a centre uniting afferent and efferent fibres. The spinal 

 ganglia are not centres in this sense ; the nerve cells composing the ganglia 

 are simply relays on the afferent fibres of the posterior root, they have no 

 connection whatever with efferent fibres, they are connected with fibres of 

 one kind only. Concerning the ganglia of the splanchnic system we cannot 

 in all cases make at present a positive statement, but the evidence so far at 

 our disposal points to the conclusion that in them as in the spinal ganglia 

 each nerve cell belongs to fibres of one function only, that where several 

 processes of a cell are prolonged into nerve fibres these fibres have all the 

 same function, the nerve cell being, as in the spinal ganglia, a mere relay. 

 We have no satisfactory evidence that in a ganglion the fibres springing 

 from or connected with one cell join another cell so as to convert the 

 ganglion into a centre joining together cells whose nerve fibres have dif- 

 ferent functions. 



We shall have later on to bring forward evidence that the nucleated cell 

 body of a nerve cell in a ganglion or elsewhere is in some way or other con- 

 nected with the nutrition, the growth and repair of the nerve fibres springing 

 from it. Besides this nutritive function the multipolar cells of the splanchnic 

 ganglia appear to serve the purpose of multiplying the tracts along which 

 nervous impulses may pass. An impulse, for instance, reaching a multipolar 

 cell in one of the proximal sympathetic ganglia along one fibre or process 

 (the fibre in very many cases being a medullated fibre) can pass out of the 

 cell in various directions along several processes or fibres, which, in the 

 majority of cases, if not always, are non-medullated fibres. Thus these nerve 

 cells are organs of distribution for impulses of the same kind. What further 



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