THE SPINAL COED IN MAN, MAMMALIA, AND BIEDS. 
917 
blends with the oiiter portion (c) of the posterior column previously developed. The 
inner and smaller column (p') is in general more conspicuous and distinct in the dorsal 
and cervical than in the lumbar region, as shown in figs. 5, 6, & 7. The opening of 
the original canal or slit between these additional columns constitutes the posterior 
median fissure {n\ which is now occupied by blood-vessels and pia mater in connexion 
with radiating fibres from the central epithelial layer. 
AVe see, then, that the direction taken by the developmental growth of the posterior 
grey substance is just the reverse of that which is followed by the anterior. The 
changes which occur in the former, and the consequent production of new columns, have 
much resemblance to some of those that take place in the upper part of the cord during 
its transition and development into the medulla oblongata. Tn the course of those 
changes the central canal retreats further and further to the posterior part of the 
medulla, until at length it opens on the surface in the form of the fourth ventricle. In 
a corresponding proportion the posterior cornua diverge, while from their roots on each 
side of the mesial line the posterior pyramid is developed The cases, however, are 
not exactly parallel. 
Eeturning to the interior of the grey substance, "we find that a variety of changes, 
more or less important, have occurred, — first, in the disposition and destination of some 
of its parts; secondly, in the structure and arrangement of its constituent elements. 
First, then, we may observe that the club-shaped layer surrounding the central fissure 
or canal, as represented at ?, fig. 3, has lost its defined outline and become very much 
diffused. Behind the oval canal, on each side of the mesial line (at q q, fig. 4), it forms 
the inner half of the cervix cornu posterioris, which subsequently contains the posterior 
vesicular column, or nucleus of the cervix. Flitherto, however, it consists only of a 
closely aggi’cgated mass of nuclei, which become gradually more diffused on its outer 
side, where it joins the rest of the grey substance. Passing backward and outward and 
then forward in the direction I h, as an arched layer near the posterior border of the 
comu, it terminates in the dark mass {h) within its outer angle or point. In the space 
between this arched layer and the posterior border of the cornu, the nuclei are less 
closely aggi'egated, and thus constitute, with their intervening network, a paler lamina 
(rr), which is the rudiment of the gelatinous substance. In the adult cord, as I showed 
on former occasions, the space occupied by the above-described arched layer of nuclei 
contains numerous bundles of nerve-fibres continuous with the posterior roots. On its 
inner side, near the centre of the cornu, there is a somewhat paler space, which is con- 
tinued forward and outward, in a curve, to the border of the grey substance at the 
bottom of the lateral column. This is more conspicuous in the dorsal region, as shown 
in figs. 5 & 6. 
On tracing the club-shaped layer forward along the side of the median line, we 
observe that its outer part has lost the defined outline which it possessed in section 3, 
* See my memoir on the “Medulla Oblongata,” Philosophical Transactions, 1858, Plate XII. figs. 13 
Plate XIII. fig. 16 ; Plate XIV.; and Plate XV. fig. 19. 
