HOOFS.
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 At that part, however, a new cells at the end of the nail. 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 consequence 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 laminae 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 direction. 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-
