CHAP, in.] GENERAL FEATURES OF NERVOUS TISSUES. 179 



seems to serve as a centre for the division of nerve fibres, and also 

 for the change from medullated to non-medullated fibres. 



All the processes of a splanchnic ganglion cell, however, are 

 not continued on as nerve fibres ; sometimes the process divides 

 rapidly into a number of fine branches, which are then found to 

 twine closely round the bodies of neighbouring cells. 



In consequence of its thus possessing more than one process 

 the splanchnic ganglion cell is more or less irregular in form, 

 in contrast to the pear shape of the spinal ganglion cell. But 

 in certain situations in certain animals, for instance in the frog, 

 in many of the ganglia of the abdomen, and in the small ganglia 

 in the heart, pear-shaped splanchnic ganglion cells are met with. 

 In such cases the nucleated sheath is distinctly pear-shaped or 

 balloon-shaped, and the large conspicuous nucleus is placed, as in 

 the spinal ganglion cell, near the broad end, but the stalk of the 

 cell is made up not of a single fibre but of two fibres ; one of these 

 is straight and seems to be the direct continuation of the cell- 

 substance, while the other, which seems to be gathered up from a 

 network on the surface of the cell, is twisted spirally round the 

 straight one. The two fibres run for some distance together 

 in the same funnel-shaped prolongation of the nucleated sheath 

 of the cell but eventually separate, each fibre acquiring a sheath 

 (sheath of Henle) of its own. Generally, if not always, one fibre, 

 usually the straight one, becomes a medullated fibre, while 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. 



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 140yu-, 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 a finely granular sub- 

 stance, 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 



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