GENERAL FEATURES OF NERVOUS TISSUES. 127 



At some variable distance from the cell the nerve fibre bears the first 

 node, and either at this or some early succeeding node the fibre divides into 

 two ; as we have seen, division of a medullated nerve fibre always takes 

 place at a node. The two divisions thus arising run in opposite directions, 

 forming in this way a T-piece ; and while one division runs in one direction 

 toward the posterior root, the other runs in an opposite direction toward the 

 nerve trunk. The nerve cell is thus, as it were, a side piece attached to a 

 fibre passing through the ganglion on its way from the posterior root to the 

 nerve trunk. It cannot be said that in any one ganglion this connection has 

 been traced in the case of every nerve cell of the ganglion ; but the more 

 care is taken, and the more successful the preparation, the greater is the 

 number of cells which may be isolated with their respective T-pieces ; so 

 that we may conclude that, normally, every cell of a ganglion is connected 

 on the one hand with a fibre of the posterior root, and on the other hand 

 with a fibre of the nerve trunk. We have reasons further to believe that 

 every fibre of the posterior root in passing through the ganglion on its way 

 to the mixed nerve trunk is thus connected with a nerve cell ; but this has 

 been called in question. In certain animals for instance, certain fishes 

 the cells of the spinal ganglia are not pear-shaped, but oval or fusiform, and 

 each narrow end is prolonged into a nerve fibre, one end thus being con- 

 nected with the posterior root and the other with the nerve trunk. In such 

 a case the nerve cell is simply a direct enlargement of the axis-cylinder, with 

 a nucleus placed in the enlargement. The nerve cells above described are 

 similar enlargements, also bearing nuclei, placed not directly in the course 

 of the axis-cylinder, but on one side, and connected with the axis-cylinder 

 by the cross-piece of the T-piece. Hence the ordinary ganglion cell is 

 spoken of as being unipolar, those of fishes being called bipolar. 



In examining spinal ganglia cells are sometimes found which bear no 

 trace of any process connecting them with a nerve fibre. Such cells are 

 spoken of as apolar. It is possible that such a cell may be a young cell 

 which has not yet developed its nerve process, or an old cell which has by 

 degeneration lost its nerve process. 



94. The ganglia of the splanchnic system, like the spinal ganglia, con- 

 sist of nerve cells and fibres imbedded in connective tissue, which, however, 

 is of a looser and less compact nature in them than in the spinal ganglia. 

 As far as the characters of their nuclei, the nature of their cell substance, 

 and the possession of a sheath, are concerned, what has been said concerning 

 the nerve cells of spinal ganglia holds, in general, good for those of splanch- 

 nic ganglia; and, indeed, in certain ganglia of the splanchnic system con- 

 nected with the cranial nerves the nerve cells appear to be wholly like those 

 of spinal ganglia. In most splanchnic ganglia, however, in those which are 

 generally called sympathetic ganglia, two important differences may be ob- 

 served between what we may call the characteristic nerve cell of the splanch- 

 nic ganglion and the cell of the spinal ganglion. 



In the first place, while the nerve cell of the spinal ganglia has one 

 process only, the nerve cell of the splanchnic ganglia may have, and fre- 

 quently has, two, three, or even four or five processes ; it is a multipolar 

 cell. 



In the second place, while these processes of the splanchnic ganglion 

 cell are continued on as nerve fibres, as is the single process of the spinal 

 ganglion cell, the nerve fibres so formed are, in the case of most of the pro- 

 cesses of a cell, and sometimes in the case of all the processes, non-medul- 

 lated fibres, and remain non-medullated as far as they can be traced. In 

 some instances one process becomes at a little distance from the cell a 

 medullated fibre, while the other processes become non-medullated fibres; 



