OF THE NAILS. 165 



the same time runs, in a thin layer which becomes very fine anteriorly, 

 upon the upper free part of the nail, as far as the end of the lunula or 

 the beginning of the body. Anteriorly and posteriorly, in which latter 

 region this layer not uncommonly reaches the posterior margin of the 

 root, its cells lie parallel to the upper surface of the nail ; while in the 

 middle, where it is thickest (Fig. 60 i), they are oblique or perpendicular 

 to it. At the free edge of the nail the relations of the parts are similar, 

 where the horny layer meeting the end of the under surface of the body 

 of the nail, partly with more horizontal, partly with oblique lamellae, is 

 perhaps also continued upon the commencement of the free edge. On 

 the lateral edges, again, the horny layer passes anteriorly, in horizontal 

 strata, under the nail ; more posteriorly it is arranged as upon the root, 

 or simply rests against the edge of the nail. The horny layer thus 

 forms a kind of sheath for the nail, which bears some resemblance to 

 the sheath of the hair, though it is much more imperfect. If we com- 

 pare the nail with the epidermis, we find, in the structure of its mucous 

 layer, not the slightest peculiarity of any importance, while the horny 

 layer is distinguished from that of the epidermis by its cells being 

 nucleated, harder, and chemically different ; by their flattening and in- 

 timate union. For the rest, the agreement of the two structures is so 

 close, that the proper nail may justly be considered, as it has long been, 

 a modified portion of the horny layer of the last joints of the fingers 

 and toes. 



According to the chemical investigations of Scherer and Mulder, the 

 nails agree very closely with the epidermis ; and, according to Mulder, 

 they are distinguished from it, only by their somewhat greater propor- 

 tion of sulphur and carbon. In his last essay, he considers them to be 

 composed of protein + sulphamid (6'8 per cent, of the latter). This 

 agrees with the observed action of reagents, with which the plates of the 

 nail behave almost exactly like horny plates, only they are attacked 

 with more difficulty and possess nuclei. According to Lauth, the nails 

 contain more phosphate of lime than the epidermis, whence they derive 

 their hardness : this may be correct, although, as Mulders tates ( a Phys. 

 Chemie," p. 536), both yield about the same proportion of ash (1 per 

 cent.). 



As regards the lamellar structure of the proper nail, it is to be re- 

 garded in the same light as that of the horny layer of the epidermis ; but 

 it is not so distinct, because the plates of the nail are more intimately 

 connected than the elements of the epidermis. Reagents, however, 

 render the lamellar structure very evident, and it is also clear in patho- 

 logically thickened and curved nails. 



51. G-rowth of the Nails. The nails grow continually, as long as 

 they are cut ; on the other hand, if uncut, their growth is limited. In 



