Hair and hair follicles

Several types of hair are distinguished: long (on the head, beard, moustache, in the axillae, pubis, genitals); bristly (eyebrows, eyelashes, the hair in the nose and external acoustic meatus); downy (on the face, trunk, and limbs).

The length, thickness of localization, rate of growth, and shedding of hair vary considerably in the different skin areas. There are 300 to 320 hairs per 1 cm² on the crown of the head, 200 to 240 on the back of the head, 44 on the chin, 24 on the dorsal surface of the forearm, and 18 per 1 cm² on the dorsal surface of the hand (Brunn). According to different authors, there are 30 000 to 150 000 hairs on the scalp. The rate of hair growth varies from 0. 1 to 0. 5 mm per 24 hours and increases or decreases depending on the general condition, the function of the nervous system and the endocrine glands, the value of nutrition, and many other factors which exert an effect on tissue trophies. The duration of the life of hairs varies from several months to four years and longer; under normal conditions, an adult loses 30 to 100 and more hairs daily.

The hair consists of a shaft rising above the skin surface (the outer or free part of the hair) and a root buried in the dermis. The hair root enclosed in membranes and a connective-tissue capsule is called the hair follicle, or sacculus. The lower expanded part of the root is known as the hair bulb, this is the site from which the hair grows. The hair papilla conveying nerve fibres and blood vessels supplying nutrients to the hair protrudes into the bulb.

The hair follicle is cylindrical in shape and terminates on the skin surface as a peculiar expansion called the infundibulum of the follicle, through which the hair shaft passes. The draining ducts of the sebaceous glands are connected with the follicle on the boundary between its upper and middle third.

Like the hair bulb, the hair follicles are surrounded by many nerve fibres. The hairs are supplied with smooth muscles (mentioned above) shaped like a band, one end of which is attached to the compact dermis layer and the other to the outer connective-tissue hair sheath slightly below the orifice of the sebaceous gland. On contracting, the muscle raises the hair which thus compresses the sebaceous gland and facilitates the excretion of the sebum.

Melanocytes migrate into the bulb and are responsible for the different colours of hair (eumelanin, brown/black; phaeomelanin and trichochromes, red). Grey or white hair is caused by low pigment production, and the filling of the cells in the hair medulla with minute air bubbles that reflect light.

NAILS

The nail (unguis) is a horny plate on the dorsal surface of the distal phalanx of the fingers and toes (nail bed). The proximal end and sides of the nail plate are covered with skin folds called the nail folds.

The nail has a body (corpus unguis), a root (radix unguis) which is its proximal part, and a distal free margin (margo liber) that freely protrudes forward. The proximal nail fold covers the proximal part of the nail plate like an arch and forms a thin horny plate of the epidermis, the nail cuticle (eponychium). The area of the nail bed covered by the nail root is called the matrix (matrix unguis); this is where the nail grows. In the proximal part of the nail body (which corresponds to the distal part of the matrix) there is a whitish crescent area called the lynula; it is particularly noticeable on the thumb and big toe.

The nail plate is formed of hard compact horny masses the outer surface of which is smooth, whereas the inner surface is rough because it has horny ridges which ensure close fitting of the nail plate to the nail bed.

The nail plate moves gradually along the nail bed and is completely renewed in 90 to 115 days.