128 GENERAL FEATURES OF NERVOUS TISSUES. 



and we are led to believe that in this case the medullated fibre is proceeding 

 to the cell on its way from the central nervous system, and that the non- 

 medullated fibres are proceeding from the cell on their way to more peri- 

 pherally placed parts ; the nerve cell seems to serve as a centre for the 

 division of nerve fibres, and also for the change from medullated to non- 

 medullated fibres. 



In consequence of its thus possessing several processes, the splanchnic 

 ganglion cell is more or less irregular and often star-like in form, in con- 

 trast 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 cell substance of 

 the cell is gathered at the stalk, not into a single fibre, but into two fibres, 

 one of which is straight and the other twisted spirally round the straight 

 one. The two fibres run for some distance togther 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. 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 Meiss- 

 ner and Auerbach, but we shall postpone for the present any description of 

 these or of other peculiar splanchnic cells. 



95. In the central nervous system nerve cells are found in the so-called 

 gray matter only ; they are absent from the white matter. In the gray matter 

 of the spinal cord, in the parts spoken of as the anterior cornua, we meet 

 with remarkable nerve cells of the following characters. The cells are large, 

 varying in diameter from 50,a to 140/jt, and each consists of a cell body sur- 

 rounding a large conspicuous 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 protoplasm, 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 unbranched band, which retains a fairly 

 uniform diameter for a considerable distance from the cell, and when suc- 

 cessfully 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 gray 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 sub- 

 stance of the gray matter. Indeed, the gray matter is partly made up of a 

 plexus of delicate filaments arising, on the one hand, from the divisions of 



