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



When a section is made through a hardened ganglion the plane 

 of the section passes through the stalks of a few only of the cells, 

 and that rarely for any great distance along the stalk, since in the 

 case of many of the cells the stalk is more or less curved and 

 consequently runs out of the plane of section ; but in properly 

 isolated cells we can see that in many cases the stalk of the cell is 

 as we have said continued on into a nerve fibre, and we have reason 

 to believe that it is so in all cases. As the cell-body narrows into 

 the stalk several nuclei make their appearance, lodged on it ; 

 these are small granular nuclei, wholly unlike the nucleus of the 

 cell-body itself, and more like, though not quite like, the nuclei of 

 the neurilemma of a nerve. They are probably of the same 

 nature as the latter ; and indeed as we trace the narrowing stalk 

 downwards a fine delicate sheath which, if present, is at least not 

 obvious over the cell-body, makes its appearance, and a little 

 farther on between this sheath, which is now clearly a neurilemma, 

 and the stalk of the cell-body, which has by this time become a 

 cylinder of uniform width and is now obviously an axis-cylinder, a 

 layer of medulla, very fine at first but rapidly thickening, is 

 established. The stalk of the nerve cell thus becomes an ordinary 

 medullated nerve fibre. The sheath of the cell is continued also 

 on to the nerve fibre, not as was once thought as the neurilemma, 

 but as that special sheath of connective tissue, of which we have 

 already spoken ( 69) as Henle's sheath, and which ultimately 

 becomes fused with the connective tissue of the nerve. 



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 

 j- piece ; and while one division runs in one direction towards 

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

 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 

 I 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 pro- 



F. 12 



