CHAP, m.] GENERAL FEATURES OF NERVOUS TISSUES. 143 



Nerve cells are not normal constituents of nerves ; they are found 

 only in the central nervous system, from which the nerves issue, 

 or in the ganglia, spinal and sympathetic ganglia, through which 

 they pass or with which they make connections ; (we may omit 

 for the present the nerve cells in which, or in connection with 

 which certain nerve fibres end at the periphery). Hence all the 

 nerve fibres of the body are processes of nerve cells situated in 

 the central nervous system, or in the spinal ganglia and corre- 

 sponding ganglia on certain cranial nerves, or in sympathetic 

 ganglia. 



89. In the central nervous system, the nerve cells are 

 found in the so called ' grey matter.' The ' white matter,' putting 

 aside certain exceptions, consists so far as its nervous elements 

 are concerned exclusively of nerve fibres. Confining ourselves 

 for the present to the spinal cord, we find that the fibres of the 

 anterior root, efferent fibres, are processes of cells, prolongations of 

 the axis cylinder processes of cells, lying in the grey matter of the 

 spinal cord, but that the fibres of the posterior root are (putting aside 

 certain minor exceptions) processes of cells lying in the ganglion 

 of that root. The cell whose axis cylinder process becomes an 

 efferent fibre of an anterior root has other processes, which do not 

 become axis cylinders of nerve fibres but end by a division more 

 or less distinctly arborescent in the grey matter. The fibre of a 

 posterior root whose axis cylinder is a process of a cell in the spi- 

 nal ganglion running and dividing in the spinal cord, in a manner 

 of which we shall have to speak later on, finally ends in the grey 

 matter also in a more or less distinctly arborescent fashion. 

 The grey matter also contains nerve cells whose axis cylinder and 

 other processes both begin and end in the grey matter ; that is to 

 say the body of the nerve cell lies in the grey matter, and all its- 

 processes finally end, also in a more or less distinctly arborescent 

 fashion, in the grey matter, without leaving the spinal cord or at. 

 least the central nervous system, though the fibre which its axis 

 cylinder becomes (and there may be more than one such) may 

 traverse for a while the white matter. 



The grey matter of the spinal cord (and the same is true 

 though in a much more complex way of the grey matter of the 

 brain) may therefore be considered as a centre or a number of 

 centres, nervous centres, connected on the one hand with afferent 

 and on the other hand with efferent fibres. In some cases the 

 connection between the afferent and the efferent fibres may be 

 simple and direct ; the terminations of the afferent fibre may come 

 into direct connection with the nerve cell, a process of which is 

 the efferent fibre. In other cases the connection may be an indirect 

 one ; between the two intervenes a cell, with certain processes of 

 which the afferent fibre is connected, and another process (or other 

 processes) of which is connected with the cell whose process is the 

 efferent fibre ; or more than one such cell may intervene. And recent 

 inquiry shews that the usual mode of connection of one cell with 



