1G0 SPECIAL HISTOLOGY. 



swollen them up. Caustic soda and potass also (which has 

 a similar action upon the whole, though it acts more upon the 

 nuclei) demonstrate the important fact, that the cells of the 

 nail are flatter in the superficial, than in the deeper layers. If, 

 in fact, a fine perpendicular section be moistened with cold, or 

 better, with hot solution of soda, we see the cellular structure 

 of the nail appear almost at the very instant it becomes 

 moistened, without any obvious enlargement of its elements ; 

 and it is observable, at the same time, that its deepest cells are 

 at least as thick again as the most superficial. 



If the soda solution acts longer, the section gradually swells 

 up, in the under cells first, on account of their greater softness, 

 and only subsequently in the flat and hard upper elements. 

 By treating the nail with cold sulphuric and nitric acids, 

 and also by boiling with hydrochloric acid, its elements are 

 isolated. 



Taking these facts, together with what we see in the 

 unaltered nail, it results that its horny layer consists of closely 

 united but not sharply defined lamellae; each lamella being com- 

 posed of one or many layers of nucleated, polygonal, flat scales 

 or plates, which, excepting their nuclei, are very similar to 

 those of the horny layer of the epidermis and in their deepest 

 layers are thicker and somewhat less in circumference than in 

 the upper and uppermost layers. Those of 0*012 — O'OIG"' may 

 be regarded as of an average size, as may be seen upon the addi- 

 tion of sulphuric acid, Avhich otherwise exerts but little action, 

 and at the commencement of the operation of soda and potass. 



§ 50. 



With respect to the relation of the nail to the epidermis, I 

 must especially refer to the perpendicular and transverse 

 sections figured in figs. 58 and 60. They show, in the first 

 place, that the epidermis applies itself upon the root, the 

 posterior part of the body, and upon the margins of the nail, 

 and that it meets it under the free edge and on the anterior 

 parts of the lateral edges. This happens in such a manner, 

 that whilst the mucous layer of the epidermis passes con- 

 tinuously and without any line of demarcation, into that of the 

 nail, the horny layer is, properly speaking, never continued 

 directly into the actual substance of the nail, but partly applies 



