THE SKIS AXD ITS APPENDAGES. 



93 



surface ; those which succeed them are of a rounded or polygonal form, 

 the more superficial ones becoming broad, thin, and flattened, and so 

 closely compacted as to make the limits of each cell very indistinct. 

 It is by the successive growth of new cells at the root and under surface 

 of the body of the nail that it advances forward and maintains a due thickness, 

 whilst, at the same time, the growth of the nail in the proper direction is secured. 

 As these cells in their turn become displaced by the growth of new cells, they 

 assume a flattened form, their nuclei become indistinct, and they finally become 

 closely compacted together into a firm, dense, horny texture. In chemical com- 

 position the nails resemble epidermis. According to Mulder, they contain a 

 somewhat larger proportion of carbon and sulphur. 



The hairs are peculiar modifications of the epidermis, and consist essentially 

 of the same structure as that membrane. They are found on nearly every part of 

 the surface of the body, excepting the palms of the hands, soles of the feet, and 

 the penis. They vary much in length, thickness, and color in different parts of 

 the body and in different races of mankind. In some parts, as in the skin of the 

 eyelids, they are so short as not to project beyond the follicles containing them ; 

 in other parts, as upon the scalp, they are of considerable length : again, in other 

 parts, as the eyelashes, the hairs of the pubic region, and the male whiskers and 

 beard, they are remarkable for their thickness. The hairs generally present a 

 cylindrical or more or less flattened form and a reniform outline upon transverse 

 section. 



A hair consists of a root, the part implanted in the skin ; the shaft or stem, the 

 portion projecting from its surface: and the point. 



The root of the hair presents at its extremity a bulbous enlargement, which is 

 whiter in color and softer in texture than the shaft, and is lodged in a follicular 

 involution of the epidermis called the hair-follicle. When the hair is of consider- 

 able length the follicle extends into the subcutaneous areolar tissue. The hair- 

 follicle commences on the surface of the skin with a funnel-shaped opening, and 

 passes inward in an oblique direction to become dilated at its deep extremity, to 

 correspond with the bulbous condition of the hair which 

 it contains. It has opening into it, near its free ex- 

 tremity, .the orifices of the ducts of one or more seba- 

 ceous glands (Fig. 68). At the bottom of each hair- 

 follicle is a small conical vascular, eminence or papilla, 

 similar in every respect to those found upon the sur- 

 face of the skin : it is continuous with the dermic layer 

 of the follicle, is highly vascular and supplied with 

 nervous fibrils : this is the part through which mate- 

 rial is supplied for the production and constant growth 

 of the hair. In structure the hair-follicle consists 

 of two coats an outer or dermic, and an inner or 

 epidermic. 



The outer or dermic coat is formed mainly of fibrous 

 tissue : it is continuous with the corium, is highly 

 vascular, and supplied by numerous minute nervous 

 filaments. It consists of three layers (Fig. 69). The 

 most internal, next the cuticular lining of the follicle, 

 consists of a hyaline basement-membrane, having a 

 glassy, transparent appearance, which is well marked 

 in the larger hair-follicles, but is not very distinct in 

 the follicles of minute hairs. It is continuous with the 

 basement-membrane of the surface of the corium. 

 External to this is a layer of spindle-shaped cells, 

 arranged in a circular manner around the follicle and 

 imbedded in a somewhat fibrous matrix, but reaching only as high as the 

 entrance of the ducts of the sebaceous glands. Externally is a thick layer of 



FIG. 69. Transverse section 

 of hair-follicle. 1. Dermic coat 

 of foUicle. 2. Epidermic coat or 

 root-sheath, a. Outer layer of 

 dermic coat, with blood-vessels. 

 6,6. Vessels cut across, c. Middle 

 layer, d. Inner or hyaline layer. 

 e. Outer root-sheath." /, g. Inner 

 root-sheath, h. Cuticle of root- 

 sheath, i. Hair. (From Quain's 

 Anatomy, Biesiadecki.) 



