Chap. XVIII] 



THE SKIN 



347 



LUNULA.. ^ 



Fig. 171. — Thumb-nail. 

 (Geirish.) 



THE APPENDAGES OF THE SKIN 



The appendages of the skin are the nails, the hair, the sebaceous 



glands, the ceruminous glands, and the sudoriferous or sweat-glands. 

 The nails. — The nails are composed of clear, horny cells of 



the epidermis, joined together so as to form a solid, continuous 



plate. Each nail is convex on its outer 



surface, concave on its inner side, and 



closely adherent to the underlying derma, 



which is modified to form what is called 



the bed, or matrix, of the nail. At the 



hinder part of the bed of the nail the skin 



forms a deep fold, in which is lodged the 



root of the nail. 



The growth of the nail is accomplished 



by constant multiplication of the soft cells 



in the germinative layer at the root. 



These cells are transformed into dry, hard 



scales, which unite into a solid plate, and 



the nail, constantly receiving additions from below, slides forward 

 over its bed and projects beyond the end of 

 the finger. When a nail is thrown off by 

 suppuration or torn oft' by violence, a new 

 one will grow in its place provided any of 

 the cells of the germinative layer are left. 



The hair. — The hair is a growth of the 

 epidermis, developed in little pits, the hair- 

 follicles, which extend downward into the 

 deeper part of the true skin, or even into 

 the subcutaneous tissue. The hair grows 

 from the bottom of the little pit or follicle. 

 The part which lies within the follicle is 

 known as the root, and that portion which 

 projects beyond the surface of the skin is 

 called the shaft or stem. The substance of 

 the hair is composed of coalesced, horny 

 cells, arranged in different layers, and we 



usually distinguish three parts in the stem or shaft of a hair : — 



(1) Cuticle — an outer layer of delicate, scale-like cells. 



(2) Fibrous substance — a middle, horny, thick portion, formed 



Fig. 172. — Piece of 

 Human Hair. (Highly 

 magnified.) a, cuticle ; 

 b, fibrous substance ; c, 

 medulla. 



