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 laminae 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 laminae 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 onlv 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, 



