CHAP, in.] GENERAL FEATURES OF NERVOUS TISSUES. 177 



the other, usually the .twisted or spiral one, is continued as a non- 

 medullated fibre. While within the common nucleated sheath 

 both fibres, especially the spiral one, bear nuclei of the same 

 character as those seen in a corresponding situation in the spinal 

 ganglion cell. It has been maintained that the straight and spiral 

 fibres take origin from different parts of the nerve cell, but this 

 has not been definitely proved. 



In the walls of the intestine, in connection with splanchnic 

 nerves, are found peculiar nerve cells forming what are known as 

 the plexuses of Meissner and Auerbach, but we shall postpone for 

 the present any description of these or of other peculiar splanchnic 

 cells. 



99. In the central nervous system nerve cells are found in the 

 so-called grey matter only, they are absent from the white matter. 

 In the grey matter of the spinal cord, in the parts spoken of as the 

 anterior cornua, we meet with remarkable nerve cells of the follow- 

 ing characters. The cells are large, varying in diameter from 50//, 

 to 140//-, and each consists of a cell body surrounding a large con- 

 spicuous refractive nucleus, in which is placed an even still more 

 conspicuous nucleolus. The nucleus resembles the nuclei of the 

 ganglion cells already described, and the cell body, like the cell 

 body of the ganglion cells, is composed of finely granular proto- 

 plasm, often fibrillated, though generally obscurely so ; frequently 

 a yellowish brown pigment is deposited in a part of the cell 

 body not far from the nucleus. The cell body is prolonged 

 sometimes into two or three only but generally into several 

 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 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 



F. 12 



