180 NERVE CELLS OF SPINAL CORD. [BOOK i. 



processes, which appear more distinctly fibrillated than the more 

 central parts of the cell-body. These processes are of two kinds. 

 One process and, apparently, one only, but in the case at least of the 

 cells of the anterior cornu, always one, is prolonged as a thin un- 

 branched band, which retains a fairly uniform diameter for a 

 considerable distance from the cell, and when successfully traced 

 is found sooner or later to acquire a medulla and to become the 

 axis-cylinder of a nerve fibre ; the processes which thus pass out 

 from the grey matter of the anterior cornu through the white 

 matter form the anterior roots of the spinal nerve. Such a 

 process is accordingly called the axis-cylinder process. The 

 other processes of the cell rapidly branch, and so divide into very 

 delicate filaments, which are soon lost to view in the substance of 

 the grey matter. Indeed the grey matter is partly made up of a 

 plexus of delicate filaments arising on the one hand from the 

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

 the division of the axis-cylinders of fibres running in the grey 

 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 

 grey matter ; hence within the central nervous system the fibres, 

 whether medullated or no, 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 grey matter. 

 But in certain parts, as for instance in the posterior cornu, cells are 

 met with which appear to possess no axis-cylinder process ; all the 

 processes seem to branch out into fine filaments. Except for this 

 absence, which is probably apparent rather than 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 distinguished from the ganglion cells 

 is the remarkable way in which their processes branch off into a 

 number of delicate filaments, corresponding to the delicate fila- 

 ments or fibrilla3 in which at its termination in the tissues the axis- 

 cylinder of a nerve often ends. 



100. From the above descriptions it is obvious that in the 

 spinal cord (to which as representing the central nervous system 

 we may at present confine ourselves, leaving the brain for later 



