NAILS. 



[ 532 ] 



NAILS. 



eyes; mouth terminal; setae bifid; length 

 1"; among Lemna, in ditches, and in the 

 respiratory chamber of the Lymnaeidae. 



See TUBIFEX. 



BIBL. Schmidt, Midler's Archiv, 1846, 

 406; Duges, Ann. Sc. Nat. 2 ser. xv. 319 ; 

 Johnston, Cat. non-par asit. Worms ; Doyere, 

 Mem. Linn. Soc. Normandy, x. ; Claparede, 

 Rech. 1861. 



NAILS. These organs, which consist of 



modified epidermic formations, are imbedded 

 posteriorly and laterally in depressions, or 

 are covered at these parts by a fold of the 

 skin. The posterior depression (fig. 505 d ) 

 is much deeper than the lateral depressions 

 (fig. 506 c\ 



The nail itself consists of the root (fig. 

 505 0, the body (&), and the free end (wt). 

 The root extends over that part of the 

 matrix furnished with the ridges, and is 



Fig. 505. 



Longitudinal section through the middle of the nail and its matrix, o, matrix and cutis of the back and point 

 of the finger; b, rete mucosum of the point of the finger; c, that of the nail ; d, that of the bottom of the root- 

 fold ; e, the same of the back of the finger ; f, epidermis of the point of the finger ; g, its origin beneath the margin 

 of the nail ; A, epidermia of the back of the flnger ; t, its termination at the upper surface of the root of the nail ; 

 k, body, I, root, m, free end of the proper nail. 



Magnified 8 diameters. 



either entirely lodged in the posterior de- 

 pression of the cutis, or the crescentic por- 

 tion of it is exposed. The body of the nail 

 is uncovered except at the sides, which are 

 overlapped by the lateral folds of the skin. 



The portion of the cutis (fig. 506 a) to 

 which the under surface of the nail, except 

 that of the anterior free portion, is attached 

 the matrix or bed is covered with ridges 

 (fig. 506 ) extending from the posterior 

 part or root of the nail to the convex mar- 

 gin of the white crescentic portion called 

 the lunule, where they become larger and 

 higher, forming plates which run to the end 

 of the matrix. The margins of the ridges 

 and plates are covered with short papillae. 

 The anterior portion of the matrix of the 

 nail is very vascular. 



The under surface of the root and body 

 of the nail is covered with depressions and 

 ridges to adapt itself to those of the matrix. 



Two layers are distinguishable in the 

 nails an under soft layer (figs. 505 d, 506 c, 

 5075), corresponding to and directly con- 

 tinuous with the rete mucosum of the skin, 

 and the upper horny layer forming the true 

 nail (figs. 506/, 505 k, 507 C). The lower 

 surface of the latter is furnished with small 



ridges (fig. 507 c), which occupy correspond- 

 ing furrows in the mucous layer. 



In minute structure the soft layer resem- 

 bles that of the cutaneous rete, except in 

 the deeper layer of cells being elongated 

 and arranged perpendicularly (fig. 507 6). 



The horny portion, or proper nail, con- 

 sists of epidermic cells, flattened and aggre- 

 gated into plates or laminae (fig. 507 C). 

 In the natural state, these cells are undistin- 

 guishable, except at the root and the under 

 surface, where the nail is in contact with 

 the mucous layer the remainder merely 

 exhibiting shorter or longer dark lines, re- 

 presenting the flattened nuclei, or indicating 

 the existence of the laminae. But if a 

 section of nail be treated with solution of 

 caustic potash or soda, the nucleated cells 

 swell up, and resume their proper form and 

 appearance. 



The blood-vessels of the bed of the nail 

 form a coarse plexus in the corium of the 

 matrix, from which loops are given off to 

 the papillae ; and the proper bed of the nail 

 has a much finer plexus and loops ascending 

 to the ridges. 



Numerous medullated nerve-fibres lie in 

 the subcutaneous tissue of the nail-bed, and 



