ME. J. L. CLAEKE ON THE GEET SUBSTANCE OF THE SPINAL COED. 455 
Of the Epithelium of the Central Canal. 
In the Ox, the epithelium which lines the central canal consists not only of colunmar 
or cylindrical cells— -as I first described them and as they have since been described 
by other observers — but also of cells which are more or fusiform. Fig. 53, Plate 
XXII. {a) is an exact representation of some of their forms, magnified 400 diameters. 
In the columnar variety (the first three on the left of a) the nucleus is large, sometimes 
spheroidal, but generally oval, and contains apparently from two to five large, brilliant, 
globular granules or nucleoli, which powerfully refract the light. Between this and the 
verge of the canal the free ciliated end of the cell is broad and short like the end of a 
cylindrical brush, and appears to be made up of granular fibres ; but in the fusifo7'm 
epithelium (the foiu’ on the right of a), the corresponding free portion consists only of 
a long and narrow kind of stalk extending from the tapering end of the cell, which 
latter in this case is similar to what forms the nucleus of the columnar variety, and 
contains several brilliant granules, which are frequently arranged in a row. Between 
these two kinds of epithelium there are difierent grades of transition. They are all 
beautifully packed in close apposition, so that the convexity of each is applied to the 
concavity of those which surround it, and by reason of its peculiar shape, it exactly 
occupies the intervening space (see fig. 53 h, Plate XXII.). In the human spinal cord, 
the canal is often completely filled up by what would appear to be the debris of the epi- 
thelium ; for nothing is to be seen but a confused heap of nuclei, which are here mostly 
large and round : but sometimes in the midst of this heap there remains a small open- 
ing or canal, which, strange to say, is still lined or surrounded at its margin by the usual 
regular layer of columnar cells ; and what is still more curious, I occasionally find, par- 
ticularly in the cervical region, two such secondary canals, each lined in the ordinary way 
(see fig. 55, Plate XXII.). 
The cilia of this epithelium are much coarser and less numerous than those in the 
larynx and trachea (fig. 53 b). In the conus medullaiis they may be very well seen 
around the entire circumference of the canal, but in other regions of the cord they are 
very sparingly found. If this be the result of accident in manipulation, it is difficult to 
say why they are always preserved in the one place and not in the others. 
The light-coloured space immediately surrounding the canal is interspersed with nuclei 
or small cells. Some of these are round or oval, finely granular, and exactly resemble 
those in the connective tissue of the white columns (see fig. 46, Plate XXII. c ) ; others 
are similar to the nuclei of the epithelium, and have the same kind of large brilliant 
granules, or nucleoli, but are themselves rather smaller. These latter nuclei, which are 
frequently elongated in different directions, are connected with the processes of the epi- 
thelium (see fig. 53, Plate XXII. b), and also with the fine fibres which surround the 
canal, and which are precisely similar to the fibres of connective tissue at the circum- 
ference of the cordf. They are not, however, confined to the neighbourhood of the 
♦ Philosophical Transactions, 1851, p. 614. 
t In 1851, I described the light-coloured part immediately surrounding the canal, as composed of fila- 
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