440 



DEVELOPMENT OF THE NAILS AND HAIR. 



from the belly of a frog ; place it in a porcelain capsule as for the murexidc test ; add concen- 

 trated nitric acid, and heat to dryness, when a yellow residue is obtained ; on adding a drop of 

 caustic soda a red colour is struck. The yellow residue gives no reaction with ammonia. If to 

 the fluid more water be added, and it be then heated, distributed over the surface of the cap- 

 sule, and cooled by blowing upon it, various shades of purple and violet are obtained.] 



The nails (specific gravity 1*19) consist of numerous layers of solid, horny, homogeneous, 

 epidermal, or nail-cells, which may be isolated with a solution of caustic alkali, when they 

 swell up and exhibit the remains of an elongated nucleus (fig. 294, re, ire). The whole under 

 surface of the nail rests upon the nail-bed ; the lateral and posterior edges lie in a deep groove, 

 the nail-groove (fig. 296, e). The chorium under the nail is covered throughout its entire 

 extent by longitudinal rows of papilla (fig. 296, d). Above this there lies, as in the skin, many 



layers of prickle cells like those in the rete 

 i\ Malpighii (fig. 294, d), and above this again 



u! a * s substance of the nail (fig. 296, a). 



[The stratum granulosum is rudimentary in 

 the nail-bed. The substance of the nail re- 

 presents the stratum lucidum, there being 

 no stratum corneum (Klein).] The poste- 

 rior part of the nail-groove and the half 

 moon, brighter part or lunule, form the root 

 of the nail. They are, at the same time, 

 the matrix, from which growth of the nail 

 takes place. The lunule is present in an 

 Fig 295 isolated nail, and is due to diminished trans- 



Papill* of the skin, epide _j Lb, ood- ^%*$SX*&E2 IrttttA 



Growth of the Nail. According to Unna, the matrix extends to the front part of the lunule. 

 The nail grows continually from behind forwards, and is formed by layers secreted or formed 

 by the matrix. These layers run parallel to the surface of the matrix. They run obliquely 



from above and behind, downwards 

 fo t d and forwards, through the thick- 



ness of the substance of the nail. 

 The nail is of the same thickness 

 from the anterior margin of the 

 lunule forwards to its free margin. 

 Thus the nail does not grow in 

 thickness in this region. In the 

 course of a year the fingers produce 

 about 2 grms. of nail substance, 

 and relatively more in summer than 

 in winter. 



Development. 1. From the 

 second to the eighth month of 

 foetal life, the position of the nail is 

 indicated by a partial but marked 

 horny condition of the epidermis on 

 the back of the first phalanx, the 

 m - . ' e ' ., .11* "eponychium." The remainder 



Transverse section of one-half of a nail, a nail-substance ; of this substance is represented 

 b more open layer of cells of the nail-bed ; c, stratum duri Mfe b the normal f y formed 

 Malpighii of the nail-bed ; d, transversely divided pa- epidermal layer, which separates 

 pill* ; e, nail-groove ; /, horny layer of c projecting over tne future ^ from the sn ; face of 

 the nail ; g, papilla' of the skin on the back of the hnger. the furrow> 2 The future nail ig 



formed under the eponychium, with its first nail-cells still in front of the nail-groove ; then the 

 nail grows and pushes forward towards the groove. At the seventh month, the nail (itself 

 covered by the eponychium) covers the whole extent of the nail-bed. 3. When, at a later 

 period, the eponychium splits off, the nail is uncovered. After birth the papilla? are formed on 

 the bed of the nail, while simultaneously the matrix passes backwards to the most posterior 

 part of the groove ( Unna). 



Absence of Hairs. The whole of the skin, with the exception of the palmar surface of the 

 hand, sole of the foot, dorsal surface of the third phalanx of the fingers and toes, outer 

 surface of the eyelids, glans penis, inner surface of the prepuce, and part of the labia is covered 

 with hairs, which may be strong or fine (lanugo). 



A Hair (specific gravity 1*26) is fixed by its lower extremity (root) in a depression of the 

 skin or a hair-follicle (fig. 294, I, p) which passes obliquely through the thickness of the skin, 



