THE SPINAE CORD IN MAN, MAMMALIA, AND BIRDS. 
915 
spending to the chorda dorsalis of fishes and reptiles, was at least five times as large 
in diameter as that of the Sheep, and somewhat different in the arrangement of its 
elementary parts. It consisted of a loose honeycomb structure with large vacant spaces, 
fonned by a coarse network of fibres, with a nucleus at each point of their junction (see 
fig, 45, a, Plate XLVIII.). Around the cylinder, elongated nuclei giving off fibres in a 
circular direction were arranged in concentric layers. 
In a foetal sheep a little larger than that first examined, and measuring exactly 1 inch 
in length, a section of the spinal cord from the same region as before (the upper part 
of the lumbar enlargement) presented the appearances seen in fig. 3, Plate XLV. 
The canal, as a sword-shaped slit, extended behind-forward through nearly the whole of 
the grey substance. Immediately surrounding it was a bulbous or club-shaped layer of 
nuclei, with its bulbous end {1) posterior and reaching the surface in the middle line, 
but partially covered on each side by a portion of the posterior column (c). At this 
latter part it was less clearly defined, and gave off a process of grey substance, which 
extended outward and forward beneath the rest of the white column (c), and terminated 
in a rounded but imperfectly circumscribed mass (5). Prom about its middle to its 
anterior extremity the narrower portion of the club-shaped layer consisted of a nucleated 
meshwork of epithelium with an outwardly-radiating tendency ; but on passing back- 
ward to the bulbous end, this appearance was confined to the wall of the canal, and 
moreover became less distinct, while at the same time it was gradually lost amongst the 
densely aggregated nuclei in the lateral part of the bulb. 
The anterior grey substance presented nearly the same aspect as in the section repre- 
sented in fig. I. There was a very evident decussation, or crossing of fibres from the 
opposite roots of the nerves, in front of the canal ; but, as in the last section, there was 
no anterior median fissure. 
In the elements of the grey substance there was scarcely any perceptible advance in 
development. Some of the nuclei in the anterior masses seemed in a trifiing degree to 
have increased in size, become more irregular in shape, and to be connected by a network 
which had rather more of the sponge-like arrangement. 
The antero-lateral columns liad somewhat increased in depth. Behind, where they 
joined the posterior columns, they were much shallower than elsewhere, and formed, 
on each side, the so-called lateral column (/f), which was sunk in a depression between 
the anterior and posterior grey substance. 
The posterior white columns (c, c) were also deeper than in the previous sections. 
Behind, they were bevelled on to the exposed surface of the central club-shaped layer ; 
and in front, w'here they joined and overlapped the lateral columns, they terminated in 
prominent but rounded edges, to which the posterior roots of the nerves [d) were 
attached. 
There is every reason to believe that the fibres of the white columns are developed 
from the grey substance as prolongations of the network hy which its nuclei are con- 
nected. 
