CHAP, in.] GENERAL FEATURES OF NERVOUS TISSUES. 175 



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 | 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 

 |- 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 connected 

 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 

 I 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 the process 

 which it formerly possessed. 



