THE SPINAL COED IN MAN, MAMMALIA, AND BIEDS. 
933 
sheaths are much finer and thinner, processes from the cells are continuous by fine sub- 
divisions with the surrounding reticular structure, as shown in fig. 17, Plate XLVII. 
These observations, then, appear to throw important light on the question which I 
formerly proposed, as to whether there is any actual and essential difference between 
the connective and the true nerve-tissue, or “ whether the connective tissue of the cord 
be intermediate in its nature, passing on the one hand into nerve-thsv\.Q, and on the 
other into ^ia mater''*. We have seen that the cell-sheath or wall is the product of, and 
indeed is constituted by, the very surface of the primitive nucleus or cell, and that, 
while it ever after remains in connexion vdth its contents, it forms a part of the sur- 
rounding connective tissue, which is itself a prolongation not only of the pia mater of 
the surface, as well as the walls of the blood-vessels, but also of the processes of the 
epithelium. But although there is this uninterrupted continuity between all the con- 
stituent elements of the cord — although, perhaps, the nerve-tissue actually changes by 
insensible degrees into the tissues with which it is continuous — and although the cell- 
wall, which forms part of the surrounding reticular structure, is a product of the primi- 
tive nucleus, there is yet no ground for believing that the connective tissue, as such, can 
ever develope itself into nerve-tissue, any more than that any one of the differentiated 
parts of a fully-developed organ can reproduce the entire structure ; for the nerve-cell, 
although it developes from itself its ovm sheath, which forms part of the nucleated 
connective tissue, produces something more than this tissue, viz. the granular contents 
of the cell f . 
We know that processes of the nerve-cells constitute the axis- cylinders of the vaso- 
motor nerves distributed to parts external to the cord ; and therefore it seems probable 
that the finer processes which are lost by subdivision in the pia mater or connective 
tissue within the cord are the means of transmitting nerve-power to that tissue and to 
the coats of its blood-vessels, from which, by their uninterrupted connexion with them, 
as already shown, the nerve-cells in return receive their supply of nutriment. 
* Philosophical Transactions, 1859, Part I. p. 442. 
t There can be no doubt that a considerable proportion of the gelatinous substance and other parts of the 
posterior cornu are of the nature of pia mater ; but amongst this there are numerous small nerve-cells. As 
I have dwelt, however, on this point in another place (Phil. Trans. 1859), I need not repeat my remarks. 
