'Sm. J. L. CLARKE OK THE GREY SUBSTAKCE OE THE SPIKAL CORD. 440 
Through the remaining half of the lumbar enlargement, the posterior vesicular colurnm 
are gradually less encircled and enveloped by the processes of their cells, which also 
diminish in number, but are still traversed by the divergent fibres of the trans\’erse com- 
missure and by a plexus of the posterior roots which sweep round on their outer sides. 
In the Ox two or more of these cells are often very large and wonderfully branched 
into six or seven processes, which radiate in all directions. I think there can be no 
reason to doubt that they form a part of the posterior vesicular column which has just 
been traced from above. 
In descending through the lower portion of the lumbar enlargement, the border of the 
grey substance between the posterior cornua is gradually drawn backwards, so that in 
the same proportion the space behind the canal, containing the transverse commissure, 
becomes deeper (compare fig. 15, Plate XXI., with figs. 16 & 17, Plate XXIII.). Some 
of the posterior roots of the nerves cross the cornu from without inwards to the side of 
the antei'ior commissure, and a large number converge to form the posterior commissure 
behind the canal. From the inner side of the caput cornu several large bundles sweep 
round through the cervix, where they interlace with the others and separate in different 
directions. Some of them descend to the anterior grey substance ; others curve inwards 
to the side of the canal ; and amongst the plexus which they form with the transverse 
commissure are a few scattered cells, which may be regarded, I think, as the remains 
of tire posterior vesicular column. These cells are scattered in front as far as the border 
of a peculiar group which, near the level of the second pair of sacral nerves, begins to 
make its appearance on each side and a little behind the spinal canal. This group was 
first pointed out by myself in 1851, and considered as the commencement of the poste- 
rior vesicular coluznn. I think, however, that Stilling is right in considering it as 
a distinct group, although it is reached by the posterior roots ; but, as I shall pre- 
sently show, its connexions with the anterior roots are more direct and intimate than 
in the case of posterior vesicular column. It is more or less round or oval, but not 
entii’ely isolated from the surrounding cells, nor circumscribed by fibres. It increases 
in size through the level of the second pair of sacral nerves, below which it gradually 
disappears. The cells are oval, fusifonn, irregular in shape, or stellate, and all of the 
larger kind. Some of their processes cross over to the opposite side behind the canal ; 
others run out towards the lateral column ; some join the anterior and posterior roots, 
and others form a system of longitudinal fibres, the cut extremities of which, in a trans- 
verse section, are seen scattered amongst the group. The oval and fusiform cells are in 
part elongated transversely towards the anterior roots, but chiefly in a lateral direction, 
and a continuous layer or tract of these elongated cells may be frequently seen to extend 
from the outer side of the group to near the border of the grey substance, in company 
or in connexion with fibres which proceed in the same direction, and escape through the 
lateral column. But the principal part of the group is most intimately connected with 
the anterior roots of the nerves ; for a large number of these roots, frequently side by 
side, run directly backwards to reach it, through the middle and inner part of the 
