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NAILS. contact. The fibres issuing from the cells often become very minute in the last part of their course, from which we learn that the delicacy of fibres does not preclude their being hollow.

3. Nails.—In order to investigate the structure of the nail we should make use of that of a child immediately after birth, or, what is better, that of a mature, unborn, human foetus ; such an one, when divided into delicate longitudinal sections, will be found to consist of laminae deposited one upon another, surface to surface. This laminated arrangement, however, becomes more and more indistinct upon the under surface of the nail which lies upon the skin, the nearer we approach to that portion contained in the fold of skin at the root, and the posterior half of the part which is embedded in that fold exhibits no laminated structure whatever, but consists of small polyhedral cells, many of which present perfectly distinct cell-nuclei. When a small portion is cut or torn off from the surface of such a nail, the form of the margins, which present smooth angular projections, leads at once to the supposition that the laminae of the nail are not structureless, but produced by the junction of little scales resembling those of epithelium. When treated with acetic or concentrated sulphuric acid, the scales separate more readily, and in some rare instances an indistinct nucleus may be recognized in them. No such scales can be seen in the root of the nail after the adherent lamella of epidermis has been scraped off, but polyhedral cells, which are much smaller than the scales, are found in that situation. Now it is a well known fact that the nail increases from its root, and is constantly pushed forwards. The polyhedral cells of the root must thus, therefore, become transformed into those scales by flattening and extension of their superficies, a process which the independent vitality of the cells renders easily conceivable. The cells of the nail already formed increase in size from the same cause, and the growth of the nail by no means depends upon a mere apposition at its root, although it is probable that the formation of new cells takes place in that situation only where the nail is in connexion with the organized skin. The nail would certainly be pushed forwards by the extension of the superficies of those cells,