CHAP, in.] GENERAL FEATURES OF NERVOUS TISSUES. 181 



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, thejrhave 

 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 

 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 dis- 

 tribution for impulses of the same kind. What further modifica- 

 tions of the impulses thus passing through them these ganglia may 

 bring about we do not know. 



It is only in some few instances that we have any indications, 

 and those of a very doubtful character, that the ganglia of the 

 splanchnic system can carry out either of the two great functions 

 belonging to what is physiologically called a nerve centre, namely, 

 the function of starting nervous impulses anew from within itself, 

 the function of an automatic centre so-called, and the function 

 of being so affected by the advent of afferent impulses as to send 

 forth in response efferent impulses, of converting as it were 

 afferent into efferent impulses, the function of a reflex centre 

 so-called. 



It is the central nervous system, the brain with the spinal cord, 

 which supplies the nervous centres for automatic actions and for 

 reflex actions ; indeed all the processes taking place in the central 

 nervous system (at least all such as come within the province of 

 physiology) fall into or may be considered as forming part of one 

 or the other of these two categories. 



