[AS. haer]

1. A keratinized, threadlike outgrowth from the skin of mammals.
2. Collectively, the threadlike outgrowths that form the fur of animals or that grow on the human body.
A hair is a thin, flexible shaft of cornified cells that develops from a cylindrical invagination of the epidermis, the hair follicle. Each consists of a free portion or shaft (scapus pili) and a root (radix pili) embedded within the follicle. The shaft consists of three layers of cells: the cuticle or outermost layer; the cortex, forming the main horny portion of the hair; and the medulla, the central axis. Hair color is due to pigment in the cortex.

Figure: HAIR AND ADJACENT STRUCTURES OF CROSS-SECTION OF SKIN




Hair in each part of the body has a definite period of growth, after which it is shed. In the adult human there is a constant gradual loss and replacement of hair. Hair of the eyebrows lasts only 3 to 5 months; that of the scalp 2 to 5 years. Baldness or alopecia results when replacement fails to keep up with hair loss. It may be hereditary or due to pathologic conditions such as infections or irradiation injury. Cytotoxic agents used in cancer chemotherapy may cause temporary loss of hair.
See: alopecia

Subentries:
auditory hair
bamboo hair
beaded hair
burrowing hair
gustatory hair
ingrown hair
kinky hair
lanugo hair
moniliform hair
moth-eaten hair
hair papilla
pubic hair
sensory hair
tactile hair
taste hair
terminal hair
twisted hair