178 NERVE CELLS OF SPINAL CORD. [BOOK i. 



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, 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 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 fibrillse 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 

 study) afferent fibres (fibres of the posterior root) are in some way by 

 means of the grey 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 different 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 connected with the nutrition, the growth and 

 repair of the nerve fibres springing from it. Besides this nutritive 



