144 THE INTEGUMENT 



the shaft of the hair; further toward the base of the folHcle, a 

 special epithelial layer, the inner root sheath, surrounds the root, 

 and separates it from the external root sheath. At the enlarged 

 base of the follicle the sheaths blend with the root into a wall of 

 indifferent epithelial cells above and around the papilla, and from 

 these cells the hair continues to develop. It is e^'ident that cross- 

 sections made through a hair follicle at different \eye\s will show 

 distinctly different compositions. 



Hairs are set obliquely in the skin, and on the acute angle side 

 there is a small strand of smooth muscle, the arrector pili muscle, 

 which extends through the dermis from the side of the follicle 

 diagonally upward toward the epidermis. When this muscle con- 

 tracts the hair is pulled more erect to form a temporary ridge in 

 the skin adjacent to the hair. When such contractions occur 

 generally over the body they produce the condition of the skin 

 known as "goose flesh." In man there is a constant falling-out of 

 hair and growth of new hairs, quite independent of the seasons; 

 in most mammals the shedding of hair takes place annually, in late 

 spring or early sununer. 



Sebaceous Glands. — Between the arrector pili muscle and the hair 

 follicle there is usually a sebaceous gland which is simple branched, 

 alveolar in type. The gland is composed of large, clear cells derived 

 from the epithelia of the outer root sheath, and is surrounded by a 

 capsule of connective tissue derived from the follicle sheath. The 

 excretory duct is relatively wide and has a layer of stratified squa- 

 mous epithelium. (Fig. 86.) 



The alveoli are filled with stratified cells which are relatively 

 small and mitotically active basally. Toward the center of the 

 alveolus, one can follow the accumulation of fat globules in the 

 cytoplasm with concomitant increase in the size of the cell. Ulti- 

 mately the entire cell disintegrates and the secretion, together 

 with the disintegrating products of the cell itself, is liberated at 

 the side of the hair near the skin surface as an oily compound, 

 the sebiun. After such disintegration of the cells and the discharge 

 of the sebum, regeneration of new epithelium of the gland takes 

 place from the basal cells. Possibly the muscular contraction of 

 the arrector ])iH muscle aids in the discharge of the secretion at the 

 neck of the hair follicle. 



Meibomian glands are modified sebaceous glands which occur 

 in mammals along the edge of the eyelids. Tlicy prochice a 



