80 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 sec- 
tions, will be found to consist of laminz deposited one upon 
another, surface to surface. This laminated arrangement, how- 
ever, 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 lamine of the nail are not structureless, but pro- 
duced by the junction of little scales resembling those of 
epithelium. When treated with acetic or concentrated sul- 
phuric 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 polyhe- 
dral cells, which are much smaller than the scales, are found 
in that situation. Now it is a well known fact that the nail in- 
creases 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 con- 
nexion with the organized skin. The nail would certainly be 
pushed forwards by the extension of the superficies of those cells, 
