526 STRUCTURE OF HAIRS. 



lar, and give a deeper tint of redness to the nail. These plait-like 

 papillae of the dermis are well calculated by their form to offer an ex- 

 tensive surface both for the adhesion and formation of the nail. The 

 cyto-blasts and cells are developed on every part of their surface, 

 both in the grooves between the plaits and on their sides, and a lamina 

 of nail is formed between each pair of plaits. When the under sur- 

 face of a nail is examined, these longitudinal laminae, corresponding 

 with the longitudinal papillae of the ungueal portion of the dermis, are 

 distinctly apparent, and if the nail be forcibly detached, the laminae 

 may be seen in the act of parting from the grooves of the papillae. It 

 is this structure that gives rise to the ribbed appearance of the nail. 

 The papillary surface of the dermis which produces the nail is conti- 

 nuous around the circumference of the attached part of that organ 

 with the dermis of the surrounding skin, and the horny structure of 

 the nail is consequently continuous with that of the epidermis. 



HAIRS are horny appendages of the skin produced by the involution 

 and subsequent evolution of the epidermis ; the involution constituting 

 the follicle in which the hair is enclosed, and the evolution the shaft of 

 the hair. Hairs vary much in size and length in different parts of the 

 body ; in some they are so short as not to appear beyond the follicle ; 

 in others they grow to a great length, as on the scalp ; while along the 

 margins of the eyelids and in the whiskers and beard, they are re- 

 markable for their thickness. Hairs are generally more or less flatten- 

 ed in form, and when the extremity of a transverse section is examined, 

 it is found to possess an elliptical or reniform outline. This examina- 

 tion also demonstrates that the centre of the hair is porous and loose in 

 texture, while its circumference is dense ; thus affording some ground 

 for a division into a cortical and a medullary portion. The free extre- 

 mity of a hair is generally pointed, and sometimes split into two or 

 three filaments. Its attached extremity is implanted deeply in the in- 

 tegument extending through the dermis into the sub-cutaneous areolar 

 tissue, where it is surrounded by adipose cells. The central extremity 

 of a hair is larger than its shaft, and is called the root t>r bulb. It is 

 usually infundibular in form in the larger hairs, and conical in the 

 smaller hairs, and those of the head. 



At the bottom of each hair-follicle is a vascular and sensitive for- 

 mative substance or pulp, which is analogous to a papilla of the dermis, 

 and is the producing organ of the hair. The process of formation of 

 a hair by its pulp is identical with that of the formation of the epider- 

 mis by the papillary layer of the dermis. A stratum of plastic lymph 

 is in the first instance exuded by the capillary plexus of the pulp, the 

 lymph undergoes conversion, first into cyto-blasts, then into cells, and 

 the latter are either lengthened out or split into fibres. The cells 

 which are destined to form the surface of the hair undergo a different 

 process. They are converted into flat scales, which enclose the fibrous 

 structure of the interior. These scales, as they are successively pro- 

 duced, overlap those which precede and give rise to the prominent and 

 waving Hues which may be seen around the circumference of a hair. 



