THE SPINAL COED IN MAN, MAMMALIA, AND BIEDS, 
927 
apart from each other. Between these the space w, which here represents the posterior 
median fissure, is much shallower and narrower than in fig. 20, and widely separated 
from the central canal by the coalescence of the cervix {q) of each side. The loose 
and nucleated tissue (w'), however, with which it is filled is still connected, across this 
coalesced grey substance, with a number of fibres radiating from the epithelium around 
the posterior wall of the canal (o). On ascending the sacral enlargement, the posterior 
coniua become more and more deeply separated, until the division reaches the canal. 
Fig. 22 represents a transverse section about midway between sections 20 and 21. tiere 
the space n between the cornua has, in section, the form of a deep cylmder filled with 
the loose nucleated_tissue, of which the deepest portion itself constitutes the epithelium 
around the posterior wall of the canal, instead of being only connected with it, as shown 
in fig. 21, by a number of radiating fibres. As development advances, the inner part 
(p, fig. 20) of the posterior column gradually extends over the side of the cervix cornu 
(§'), and replaces a corresponding proportion of the nucleated tissue (w'), until it reaches 
the median raphe at the back of the canal. On approaching the middle of the sacral 
enlargement, the entire lateral halves of the grey substance are more widely removed 
from each other, and joined only by the anterior commissure ; for 'posterior commissure 
has no existence. At the same time the canal is lengthened in a lateral direction, and 
opens behind, through the raphe or median fissure, to form the rhomboidal sinus, which 
is filled up with the remaining nucleated tissue continuous with the pia mater of the 
smTace *. 
I have now to describe the development of the intervertebral ganglia. 
In the young foetus, the first thing that strikes the observer on looking at these 
ganglia, is their enormous size in comparison with the diameter of the spinal cord. 
Fig. 8, e^ e, Plate XLVI. represents them as seen in a transverse section of the vertebral 
column of a human foetus of 9 weeks. In this section each ganglion appears nearly as 
large as the entire lateral half of the grey substance of the cord itself. It is closely 
invested by a fibrous sheath, which is prolonged on to the nerve-roots, and is continuous 
with the surrounding connective tissue, as well as with the denser nucleated tissue that 
constitutes the lamina (z) of the vertebra. In a foetal sheep of 1 inch in length, the 
ganglion consisted of a mass of closely aggregated nuclei or cells, connected together by 
a network of fibres. Fig. 23, a, Plate XLVII. represents a small portion magnified 
420 diameters. The fibres connecting the nuclei or cells were continuous on the one 
hand with the nerve-roots which entered the ganglion, and on the other hand with the 
surrounding connective tissue. Their course was most conspicuous in an antero-posterior 
direction — that is, in the direction in which the nerve-roots spread through the ganglion. 
The nuclei or cells were about the same size as those of the grey substance of the cord 
to which they belonged, but were rather more varied in shape. Like them also, at this 
period of development, they had a plain and smooth appearance, without any traces of 
granular contents and of distinct enveloping membranes. 
* Compare my figs. SI, 35, & 36, from the adult bird, Philosophical Transactions, 1859, Plate XXIII. 
JIDCCCLXII. 6 L 
