Textbook of normal histology: including an account of the development of the tissues and of the organs . eskin. The substance of the nail itself consists of intimately unitedlamellae of horny epithelial cells, which possess a nucleus andclosely resemble the elements of the stratum lucidum ; the olderand most superficial layers are made up of compressed horny dryscales, while those composing the last formed and hence deepest layerare softer and more regularly polyhedral, resembling the cells of thestratum Malpighii. THE SKIN AND ITS APPENDAGES. 267 THE HAIR. The hairs, together with their homol
Textbook of normal histology: including an account of the development of the tissues and of the organs . eskin. The substance of the nail itself consists of intimately unitedlamellae of horny epithelial cells, which possess a nucleus andclosely resemble the elements of the stratum lucidum ; the olderand most superficial layers are made up of compressed horny dryscales, while those composing the last formed and hence deepest layerare softer and more regularly polyhedral, resembling the cells of thestratum Malpighii. THE SKIN AND ITS APPENDAGES. 267 THE HAIR. The hairs, together with their homologues, feathers and scales ofthe lower animals, are derived entirely from the epidermis, andare therefore of ectodermic origin. These slender flexible hornythreads are distributed, with few exceptions, over the entire surface,but differ greatly as to both size and frequency in various regions;individual and race peculiarities also greatly influence the characterof the hair. In general, in straight-haired races the hairs arethicker and coarser and more cylindrical than in crisp-haired races; Fig. 303. Section of human scalp, showing hair-follicles and sebaceous glands : a, epi-dermis ; b, corium ; c, hair embraced within its hair-follicle; d, fibrous sheathof follicle ; e, glassy membrane; /, outer root-sheath ; g, inner root-sheath ; h,expanded terminal bulb of hair ; i, hair-papilla; k, mouth of follicle from whichhair-shaft (/) projects ; m, adipose tissue ; n, blood-vessel ; o, sebaceous glands ;/, arrector pili muscle ; s, portions of sweat-gland. in the negro the hairs are flattened cylinders, small and oval in sec-tion ; dark hair is usually coarser than that of light color. Every hair presents two principal divisions, the part whichprojects beyond the surface, as the shaft, and the portion embedded 268 NORMAL HISTOLOGY. Fig. 304. within the integument, the root; at its lower extremity the root ter-minates in a bulbous expansion, the hair-bulb, which at its lowestpoint is
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