452 ME. J. L. CLAEKE OX THE GEET SUESTAXCE OF THE SPIXAL COED. 
parts ; their larger cells, also, are gi’adually reduced in number, but other's of a smaUer kind 
are scattered through the spaces which they occupied. In the middle of this region, the 
lumbar (fig. 15, Plate XXI. M), there is still, near the centre of the cerrlx coi-nu, a some- 
what dark and irregular mass, which in a moderately thin section may be very distincth 
seen even with the naked eye. Each of these masses is interlaced and snn-onnded, a. 
usual, on its outer side by bundles of the posterior roots, and traversed by the transverse 
commissure. They present no traces of the large, strongly-marked cells which are found 
higher up, but, when examined under a sufficiently high magnifying power, they are seen 
to'be interspersed and surrounded by cells of a smaUer and more delicate kind, like those 
contained in the coiTesponding part of the cervical region. In descending through the 
conus medullaiis, the posterior cornua, by the retreat of the border between them, be- 
come gradually shortened and approach each other towards the middle line, so that the 
grey substance on the inner side of each cervix is brought nearer the centiul canal (com- 
pare figs. 16 & 17, Plate XXIII. M). This grey substance, which, I think, must be con- 
sidered as the remains of the posterior vesicular column, is still mtei-spersed with a 
multitude of small and delicate cells, which lie amongst the fibres of the transverse com- 
missure and posterior roots of the nerves. But there is scarcely any trace of the iiecuhar 
group which is found in Mammalia on each side, and a little behind, the canal. Some- 
times, however, two or three cells rather larger than the rest may be seen at this spot 
in comexion with a few fibres of the anterior roots, but they are not distinguisliable as 
a separate group. _ • i 
In my first description of the posterior vesicular columns, it was stated that in t le 
C’alf they commence small at the lower extremity of the cord, increase in size in lumbai 
and cenical enlargements, and terminate at the upper part in the medulla oblongata. 
This statement, though in many respects true, must be modified by the descriptions 
above given. It is true that they are actually largest, and contain the greatest number 
of large cells in the lumbar region, but only at its upper part; it is also true that both 
in the Calf and Sheep the large cells scattered through the inner halt of the cervix 
cornu are at least as numerous in the lower part of the cervical enlargement as in the 
dorsal region. Stilling, however, while agreeing in the main with my first description ot 
these columns and of their connexion with the posterior roots, denies their existence at 
the upper part of the cervical enlargement; for he does not consider the cells scattered 
through the inner half of the cervix cornu as part of the vesicular columns ; and in the 
two lower thirds of the lumbar enlargement, he denies the existence ot cells altogethei . 
under low powers of the microscope they are certainly not visible*. 
In discussing tlie causes wliicli have led to this difference of opinion between himself and me, he adopts 
a very singular kind of argument. 1. He assumes that I made the sections thicker at one part of tlie cord 
than at another. 2. He assumes that I sqiieezed cells, lying at different depths, into one layer, in conse 
(pience of the necessary pressure resulting from the employment of Canada balsam and thin covering glass. 
The Canada balsam which I always use is fluid; everyone covers his preparations with thin glass; and i , 
as he asserts, my preparations are already too transparent, there is no nerd ot pressure. 
