HOOFS. 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 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 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 



