528 



THE NAILS AND THE HAIR. 



FIG. 180. Transverse Section of One-half of a Nail, through the True 

 Nail-bed (after Biesiadecki) : a, nail-substance; b, subjacent loose 

 horny layer; c, mucous layer; d, nail-ridge divided transversely; e, 

 nail-fold without papillae; /, the horny layer of the nail-fold, which 

 has pushed itself over the nail; g, papillae of the skin of the dorsum 

 of the finger. 



the nail is provided throughout the entire extent of the nail-bed with longitudinal 

 rows or bands of papillae (Fig. 180, d). Immediately above these, as upon the 

 skin in other situations, is the laminated, prickle-cell layer of the Malpighian 



mucous network (Fig. 180, 

 c). Over this the nail is 

 spread, thus representing 

 the horny layer of the nail- 

 bed (Fig. 1 80, a). The pos- 

 terior nail-fold and the 

 semilunar, brighter portion 

 of the nail, the lunula, con- 

 stitute the root of the nail. 

 With the exception of a 

 small surrounding area, 

 they form at the same 

 time the matrix, from which 

 the growth of the nail takes 

 place. The whitish cres- 

 cent, present also on iso- 

 lated nails, is due to the 

 lessened translucence of this 

 posterior portion of the nail, 

 and this is a result of the 

 special thickness and the 

 uniform distribution of the 

 cells of the mucous layer in 

 this situation. 



Growth and Develop- 

 ment. According to Unna, working under Waldeyer, the matrix of the nail is 

 formed only by the floor and not also by the roof of the fold up to the anterior 

 border of the lunula. The nail grows continuously 

 from behind forward, and it is formed in layers 

 by separation of the matrix. These layers are 

 parallel with the surface of the matrix, though 

 not with that of the nail. They pass obliquely 

 from above and behind, downward and forward, 

 through the thickness of "the nail-structure. The 

 nail is of uniform thickness from the anterior 

 border of the lunula to the free margin. It, there- 

 fore, no longer grows in thickness in this area, ex- 

 cept by the deposition of new cornified layers of 

 cells from the mucous layer on the under surface of 

 the nail. In the course of a year, the fingers yield 

 about 2 grams, the hands and feet, 3.43 grams of 

 nail-substance in the summer relatively more 

 than in the winter. 



In the development of the nail, Unna observed 

 the following stages: (i) Between the second and 

 the eighth month of fetal life, the situation of 

 the nail is occupied by a partial increase of the 

 cornification of the epidermis on the dorsal aspect 

 of the terminal phalanx the eponychium. As 

 the remains of this, there persists throughout the 

 whole of life the normally formed, epidermal, 

 horny layer that separates the subsequently de- 

 veloped, definitive nail from the roof of the fold. 

 (2) The definitive nail develops in the fourth 

 month beneath the eponychium. The base of the 

 nail is situated, at first, at the extremity of the 

 terminal phalanx, and subsequently moves fur- 

 ther toward the dorsum. In the seventh month, 

 the actual thin nail, itself still covered with 

 eponychium, covers the entire extent of the nail- 

 bed, and in the eighth month it penetrates the 

 fold wholly. (3) When, subsequently, the eponychium is exfoliated, the nail is 

 disclosed. After birth, the papillae develop upon the nail-bed, and, at the same 

 time, the matrix extends to the most posterior portion of the fold. 



FIG. 181. Transverse Section of a Hair 

 below the Neck of the Hair-follicle: 

 a, external sheath of the hair-follicle, 

 with (b) blood-vessels in transverse 

 section; c, internal sheath of the hair- 

 follicle; d, vitreous layer of a hair- 

 follicle; e, external, g,' internal root- 

 sheath; /, external layer (Henle's 

 sheath); g, inner layer of the latter 

 (Huxley's sheath); h, cuticula; /, 

 hair. 



