THE SPINAE COED IH MAN, :MAJVIMALIA. AND BIEDS. 
923 
sheath or envelope, which is prolonged on to the processes and connected with the inter- 
vening reticular tissue. It is beyond all doubt (for it was distinctly seen in preparations 
of this foetus) that many, at least, of the processes, by repeated subdivision, become con- 
tinuous with the fine network of this intervening tissue. See fig. 17, Plate XL VII.* 
In the same foetus the tr actus intermedio-lateralis, or tract of grey substance (t, figs. 41, 
42 & 43, Plate XLVIII.) which projects into the lateral column between the anterior 
and posterior cornua, was in a nearly complete state of formation. 
The epithelium, also, surrounding the canal was completely developed, and on no 
occasion in the human cord have I seen it so perfect and beautiful. In the adult 
human cord it is very difficult to obtain a good view of the exact form and arrangement 
of its constituent elements, which are often, especially in the cervical region, confusedly 
heaped together in a mass that entirely fills up the canalf. In ihefcetal cord, however, 
the canal is larger, the epithelial layer is deeper, and its elements in general are more 
uniformly arranged. In the case now under consideration these elements were more 
than usually distinct. In some regions of the cord they were exactly alike throughout 
the whole circumference of the layer. In such regions they consisted partly of oval, and 
partly of still more elongated nuclei or cells, which gave off a process from each extre- 
mity, and were arranged with their longer axes at right angles to the axis of the canal. 
The nuclei, however, were placed at different distances from the canal, and were so dis- 
posed as to lie in nearly close apposition, and form a compact stratum. Their central 
processes, which reached the inner margin of the layer, were consequently of different 
lengths ; and the length of these processes, in general, was inversely proportional to their 
thickness. Here and there between the rest of the nuclei, narrow interspaces were occu- 
pied by remarkably slender and fusiform bodies, of which the tapering ends reached the 
verge of the canal, without the intervention, apparently, of any distinct processes. At 
their peripheral or outer ends all these nuclei tapered into fine fibres Avhich crossed each 
other in every direction, and frequently dhided into smaller branches, to be continuous 
with the network surrounding the epithelial layer. In the lumbar region of the cord 
the nuclei were not exactly alike around the whole circumference of the canal. In the 
anterior third of the layer they were in every respect similar to those which I have just 
described ; and at the front of the canal, the fibres proceeding from their peripheral ends 
were seen to cross the commissure, and become directly continuous with the process of 
pia mater within the anterior fissure. Frequently this process consisted almost entirely 
of blood-vessels containing numerous well-preserved globules, and at the bottom of the 
* This statement is confirmatory of the descriptions which I first gave of the ramifications of the pro- 
cesses of the nerve-cells of the spinal cord. — Philosophical Transactions, 1851, p. 614. 
t On a former occasion (Phil. Trans. 1859, Part I. p. 455, and Plate XXII. fig. 55) I showed that the 
canal in the human cord is sometimes double, or rather that two secondary canals, as it were, are hollowed 
through the mass of epithelium just mentioned. The same fact has since been made the subject of a paper 
by Dr. JoH. 'Wagnee, in Eeicheet-Dubois’s Archiv fiir Anat. &c. 1861, p. 735. Even in the fourth ven- 
tricle, at the calamus scriptorius, in hlan I have freq[uently found on the surface a kind of short supple- 
mentary canal formed by a double layer of epithelium enclosing a narrow space. 
