448 ME. J. L. CLAEKE ON THE GEEY SEBSTANCE OF THE SPINAL COED. 
(see F, Plate XX. fig. 4; Plate XXL fig. 3). Transversely its outermost or mai-ginal vesicles 
lie, for the most part, with their longer axes and processes in an antero-postenor du-ec- 
tion, as in fig. 2, Plate XX. ; while those more internal are elongated in the dmection of 
the transverse commissure, with the fibres of which, as akeady stated, them processes 
are continuous. As it ascends, however, through the cervical enlargement, it gradua ^ > 
disappears ; but, as in the dorsal region, the lateral portion of the gi’ey substance is still 
traversed, from behind forwards, by numerous fibres apparently in connexion with 
branched and with very elongated cells of all sizes, amongst which are scattered a few^ 
that resemble those of the tractus mtermedio-laieralis. Through this lateral portion of 
the grey substance the lowest roots of the spinal-accessory nerve forwards to the cell> 
of the anterior cornu. 
In the region of the upper cervical nerves, there reappears a vesiculai- tract in the same 
position as the tractus intermedio-lateralis, and composed of the same kind of cells, 
which are elongated in a lateral direction and send their processes, on the one hand, 
outwards through the lateral column, and on the other, inwards to join the fibres of the 
transverse commissure behind the central canal (see F, fig. 11, Plate XXI. ; and fig. 12 
Plate XXA'.). It is traversed by the roots of the spinal-accessory nerve (r) as the> bem 
forwards on their way to the anterior cornu. 
In descending the cord from the dorsal to the lumhar region, the posterior grey sii - 
stance undergoes a series of changes nearly similar to those which are observed in ascend- 
ing to the cervical enlargement. The posterior cornua become gradually more separate, 
or thrown aside from each other, in a direction obliquely backwards. At the upper 
part of the lumhar enlargement, the posterior vesicular columns are decidedly larger 
than in any other region of the cord (M, fig. 13, Plate XXL). In a transverse section 
each presents the appearance of a dark oval mass, lying along the whole inner liaH ot 
the cervix cornu, the border of which it renders convex against the deep strata of the 
posterior column. Its larger cells are more numerous than in the dorsal region. ^ Ihey 
do not form in the centre of the column a circumscribed group, but he scattered uTegu- 
larly through the whole inner half of the cervix, which their processes traverse in diftor- 
ent directions and envelope in different planes. Many of the oval lariety are elongated 
in the direction of the cornu, and their processes are continuous, on the one hand, with 
the transverse commissure, and on the other, with the posterior roots of the nerves. 
Some stretch along the convex border of the cervix; and sometimes a crescentm or trian- 
gular ceU, in the same situation, sends one of its processes through the posterior column 
with a bundle of radiating fibres. Part of the fibres of the transverse commissure nni 
out in front of the mass, as already stated, in company with processes or fibres wliicli 
proceed from its inner side ; of these some are continuous ivith the cells of tlie tractus 
intermedio-lateralis, while others escape through the lateral column. 
The tractus intermedio-lateralis is prominent at the border of the grey substance, 
between the anterior and posterior cornua, but its cells are not so numerous as in the 
upper dorsal region. . 
