SECT 51.] 



NAILS. 



99 



The thickness of this part of the nail continually increases from 

 the root to near the free border, so that the body is, anteriorly, at 

 least throe times thicker (from 03'" to o - 4'") than the root; and 

 is, at the free border, again somewhat less. Also in its transverse 

 diameter, the substance of the nail is not equally thick throughout, 

 except at the posterior border of the root, but becomes considerably 

 attenuated at the lateral margins, so that the part of the nail which 

 lies in the fold measures no more than o'o6'" to 0*12'", and ends 

 with a sharp edge. 



The structure of the proper nail-substance is not easily per- 

 ceived, without the employment of re-agents. In perpendicular 

 sections there are seen, particularly on the body, nothing but 

 horizontal, fine, straight, or curved lines, lying close upon one 

 another, which might be taken for the optical expression of delicate 

 lamellae; and between these a number of elongated, horizontal 

 streaks, of a dark or peculiar reddish aspect, which are evidently 

 nuclei. At the most posterior portion of the root, and on the 

 lower surface, where it adjoins the mucous layer — and there only 

 — more or less flattened cells, furnished with nuclei and arranged 

 in Jayers, make their appearance. Horizontal sections show only 

 a pale transparent, here and there granular, substance, for the 

 most part without indication of any kind of structure, at some 

 parts with very indistinct con- Fig. «*■ 



tours of plates, resembling those 

 of the horny layer of the epi- 

 dermis. Very different ap- 

 pearances, however, are ex- 

 hibited after treatment with 

 alkalies and mineral acids. 

 By boiling the nail for a 

 short time in dilute caustic 

 soda, or moistening a fine sec- 

 tion with this re-agent, all the 

 plates are converted into nu- 

 cleated cells, of which the 

 deeper arc thicker, the super- 

 ficial more flattened. From 

 these facts, together with what 

 is discoverable in the unaltered nail, it follows that its horny layer 

 consists of closely connected, but not sharply defined, lamellae ; 

 each lamella being made up of one or several layers of nucleated, 

 polygonal, flat scales or plates, which, apart from the, nuclei, 



n 2 



Nail-plates, boiled with caustic soda; magnified 350 

 times. A. From the side; B. from the surface, it. 

 Membranes of the distended elements; b. their nuclei 

 from the surface ; c. from the side. 



