HOOEFS. 81 
and by their flattening in reference to its thickness, but the 
more the cells become flattened, the thinner must the anterior 
part of the nail become. ‘This probably is compensated for, 
by a formation of epithelium-scales upon the under surface of 
the nail, and especially at its posterior part. If, for example, 
an epithelium-scale become attached to the most posterior part 
of its under surface, it will be advanced somewhat forwards by 
the flattening of the cells above, and the formation of new 
cells at the end of the nail. At that part, however, a new 
scale is next formed, and laid upon the former one, and as the 
advance forwards goes on, a third and fourth are formed, and 
so on, so that, by this means, a thickening of the nail must take 
place proportionate to its advance from behind forwards. I 
consider, therefore, that this thickening of the nail, in conse- 
quence of growth from the under surface, and the thinning 
consequent upon the flattening of the cells, compensate each 
other, and that the almost uniform thickness of the nail is 
produced by this means. The superficial laminz of that part 
of the nail which lies external to the fold of skin at all events 
do not continue to grow. I marked several nails with two 
points, by boring them with a needle and colouring the spot 
with nitrate of silver; the marks were made at the root of the 
nail, some in the longitudinal, others in the transverse direc- 
‘tion. In the course of two or three months they had advanced 
to the point of the nail, but their distance from each other had 
not altered in the least. 
4. Hoofs. The horny tissue of hoofs, in the foetus at least, 
consists entirely of the most beautiful vegetable-like cells. If 
a thin transverse lamella be cut off from the hoof of a large 
foetal pig, the preparation will present the exact appearance of 
vegetable cellular tissue. The following facts prove that the 
cells are not flat: in the first place, when the side walls do 
not stand quite perpendicular, they may be traced down below 
the level of the section, and the depth to which they go may 
be estimated ; and secondly, longitudinal sections of the horny 
tissue of hoofs present a similar appearance to those made in 
the transverse direction. They are, therefore, polyhedral cells, 
and some of them, at least, contain a distinct nucleus. 
When the tissue is quite fresh, it is not possible to distin- 
6 
