ME. J. L. CTLAEKE ON THE GEET SUBSTANCE OE THE SPINAE COED. 
447 
and lumbar enlargements they are joined only by a narrow band of the transverse com- 
missure, immediately behind the canal. During these changes, the posterior vesicular 
columns undergo certain modifications in form, size, and structure. As they approach 
the cenical enlargement, the cylinder or opake portion of each is gradually reduced in 
size, but the whole inner half of the ceinix is still occupied by the surrounding layer of 
cells. Sometimes the cylinder is intersected, and divided into smaller portions, by fibres 
of the transverse commissure, in continuity with the processes of some of its own cells, 
which are more or less intermixed with those of the surrounding layer ; other fibres of 
the transverse commissure, in their passage outwards, enclose it in front and behind, and 
after partly reuniting on its outer side and running backwards through the caput cornu, 
where they are also continuous with cells, they diverge and traverse the gelatinous sub- 
stance as posterior roots of the nerves. Some of those fibres which enclose it in front, 
in company with processes or fibres from the inner side of the vesicular column, proceed, 
tis already stated, with a gentle curve, or in a slightly serpentine course, to the lateral 
border of the grey substance, where they are partly continuous with the cells of the 
fradus intermedio-lateralis (F, figs. 2 & 4, Plate XX.), while the rest escape through 
the lateral column. Behind, and at the sides of, the canal, the fibres of the transverse 
commissm’e are connected ■with many small oval and fusiform cells, — sometimes with 
others of larger size, — and are separated at intennls by blood-vessels which run longitu- 
dinally and obliquely and communicate with one of considerable size, which proceeds 
transversely, with the commissure, to the lateral grey substance, where it divides into 
numerous branches. In the middle of the cervical enlargement, the circumscribed 
cylinder of the posterior vesicular column has entirely disappeared, but the whole inner 
half of the cerrix comu is filled with cells, and contains a somewhat dark and imper- 
tectly-defined mass, W'hich is traversed by the fibres of the transverse commissure and 
interlaced by the posterior roots. In the Ox many of these cells are very much branched, 
and send out their processes in all directions : some are fusiform in various degrees : 
others are apparently riband-shaped, and sometimes not much broader than the processes 
into which they gradually taper. 
In the upper part of the cervical region, near the origin of the third pair of nerves, 
a darker and more defined mass (M, fig. 12, Plate XXV.) reappears at the base of the 
cerrix cornu. It is composed of cells both large and small, and of bundles of the 
posterior roots which interlace amongst them. This mass is not distinctly circum- 
scribed like that of the posterior vesicular column in the dorsal region, but is some- 
what triangular, with one of its angles directed towards the point of the posterior cornu, 
another towards the transverse commissure, and the third obliquely forwards and out- 
wards towards the antero-lateral column. It gradually diminishes upwards, and disap- 
ixjars near the first pair of neiwes. 
I he tradus intermedio-lateralis is larger at the upper part than in the middle of the 
dorsal region. On the one hand it projects further into the lateral column, and on the 
other tapers inw’ards across the grey substance to near the front of the vesicular cylinder 
3 N 2 
