NAILS. 



[ 582 ] 



NAILS. 



eyes; mouth terminal; setaa bifid; length 

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

 respiratory chamber of the Lymnjeida}. 



See TuBiFKX. 



BiBL, Schmidt, Miiller's ArcJiiv, 1846, 

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

 Johnston, Cat. non-;)arasit. Worms ; Doyere, 

 3Ie?)i. Linn. Soc. Norniandti, x. ; Claparede, 

 Rcch. 1861. 



NAILS. — These organs, which consist of 



modified epidermic formations, are imbedded 

 posteiiorly and laterally in depressions, or 

 are covtn-ed at these parts by a fold of the 

 skin. The posterior depression (fig. 505 J) 

 is much deeper than the lateival depressions 

 (fig. 506 c). 



The nail itself consists of the root (fig, 

 505 1), the body (k), and the free end (/«). 

 The root extends over that part of the 

 matrix furnished with the ridges, and is 



Fi?. 505. 



e 



/ 



.//- 



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

 of the finger; 6, rete muco^um of the point of the finger; c, that of the nail ; d, tliat 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 ; h, eindermis of the back of the finger; i, its termination at the upper surface of the root of the nail; 

 Ic, 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 imcovered except at the sides, which are 

 overlapped by the lateral folds oi the skin. 



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

 which the under sm'face of the nail, except 

 that of the anterior free portion, is attached 

 — the matrix or bed — is covered with ridges 

 (fig. 506 a) extending from the posterior 

 part or root of the nail to the convex mar- 

 gin of the white crescentic portion called 

 the lunale, 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 papillfe. 

 The anterior portion of the matrix of the 

 nail is very vascular. 



Tlie 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 r/, 506 r, 

 507 B), corresponding to and directly con- 

 tinuous with the rete mucosum of tlie skin, 

 and the upper horny layer forming the true 

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

 surface of the latter is furnished witli small 



ridges (fig. 507 o), which occupy con'espond- 

 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 h). 



The horny portion, or proper nail, con- 

 sists of epidermic cells, flattened and aggre- 

 gated into plates or lamiufB (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. 



Tlie blood-vessels of the bed of the nail 

 form a coarse plexus in the coriuni of the 

 matrix, from which loops are given off to 

 the papillit! ; and the proper bed of the nail 

 has a mucli finer plexus and loops ascending 

 to the ridges. 



Numerous moduUatcd nerve-fibres lie in 

 the subcutaneous tissue of the nail-bed, and 



