CHAP. XVIII.] 



THE HAIRS. 



215 



The appendages of the skin are the nails, the hairs, the 

 sebaceous glands, and the sweat-glands. They are all devel- 

 oped as thickenings, or as down-growths, of the Malpighian 

 layer of the epidermis. 



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

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

 plate. Underneath each nail, the true skin is modified to form 

 what is called the bed or matrix of the nail. This bed is very 

 vascular, and is .raised up into numerous papillae. 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 Malpighian 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 off by violence, a new one will grow 

 in its place provided any of the cells of 

 the Malpighian layer are left. 



The average rate of growth of the nails 

 129. PIECE OF is about -fa of an inch (0.79 mm.) per week. 

 The ^rs.-The hairs are growths of 

 the epidermis, developed in little pits, the 

 hair-follicles, which extend downwards into 

 the deeper part of the true skin, or even into the subcu- 

 taneous tissue. The hair grows from the bottom of the little 

 pit or follicle, the part which lies within the follicle being 

 known as the root. 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 hairs. 

 An outer layer of delicate, scale-like cells, the cuticle ; a middle, 

 horny, thick, and coloured portion, formed of elongated cells, 

 the fibrous substance; and a central pith formed of angular cells, 

 the medulla. 



The root of the hair is enlarged at the bottom of the follicle 

 into a bulb or knob, and this bulb is composed of soft-growing 



FIG. 



6, fibrous substance 



