164 SPECIAL HISTOLOGY. 



polygonal elements all, without exception, the deep as well as the 

 superficial, possess nuclei of O0080-0-0046 of a line in length and 

 breadth, and 0-002 of a line in thickness, which, according as they 

 turn their surfaces or their edges to the observer, appear as rounded, 

 very pale, and finely-granulated discs, or as long, narrow, dark-con- 

 toured rods ; it deserves further to be noted, that together with these, 

 large and very pale nuclei of 0'006-0'01 of a line, and more, occur in 

 considerable numbers, probably owing their existence to the excessive 

 action of the reagent which has 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 sub- 

 sequently in the flat and hard upper elements. By treating the nail 

 with cold sulphuric and nitric acids, and also by boiling with hydro- 

 chloric 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 lamellse; each lamella being composed 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 O'012-O-OIG of a line, 

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

 of sulphuric acid, which 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 ap- 

 plies 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 continuously, 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 itself with its lamellae parallel upon the 

 nail, partly abuts upon it at various oblique angles. At the root, the 

 horny layer passes more or less deeply into the fold of the nail, and at 



