reduced to as few as two bristles on the lips. The term "hair," however, is apt to be somewhat loosely applied; it has been made use of to describe, for example, the slender processes of the
 Fig. 1.—A, Section of human skin. Co, Dermis; D, sebaceous glands; F, fat in dermis; G, vessels in dermis; GP, vascular papillae; H, hair; N, nerves in dermis; NP, nervous papillae; Sc, horny layer of epidermis; SD, sweat gland; SD1, duct of sweat gland; SM, Malpighian layer. B, Longitudinal section through a hair (diagrammatic). Ap, Band of muscular fibres inserted into the hair-follicle; Co, corium (dermis); F, external longitudinal; F1, internal circular, fibrous layer of follicle; Ft, fatty tissue in the dermis; GH, hyaline membrane between the root-sheath and the follicle; HBD, sebaceous gland; HP, hair-papilla with vessels in its interior; M, medullary substance (pith) of the hair; O, cuticle of root-sheath; R, cortical layer; Sc, horny layer of epidermis; Sch, Hair shaft; SM, Malpighian layer of epidermis; WS, WS1, outer and inner layers of root-sheath. (From Wiedersheim's Comparative Anatomy.)
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chitinous skin of the Crustacea. It will be necessary, therefore, to enter into the microscopical structure and development of the mammalian hair. Hair is found in every mammal. The first appearance of a hair is a slight thickening of the stratum Malpighii of the epidermis, the cells taking part in this being
