Types and Distribution of Hair

A hair is also known as a pilus (PY-lus); in the plural, pili (PY-lye). It is a slender filament of keratinized cells that grows from an oblique tube in the skin called a follicle (fig. 6.8). Hair is found everywhere on the body except the lips, nipples, parts of the genitals, palmar and plantar skin, lateral surfaces of the fingers, toes, and feet, and

Hair shaft

Apocrine sweat gland

Hair receptor

Hair matrix

Dermal papilla Blood capillaries

Hair shaft

Hair receptor

Hair matrix

Dermal papilla Blood capillaries

Dermal Papilla

Figure 6.8 Structure of a Hair and Its Follicle. (a) Anatomy of the follicle and associated structures. (b) Light micrograph of the base of a hair follicle. (c) Electron micrograph of two hairs emerging from their follicles. Note the exfoliating epidermal cells encircling the follicles like rose petals.

Figure 6.8 Structure of a Hair and Its Follicle. (a) Anatomy of the follicle and associated structures. (b) Light micrograph of the base of a hair follicle. (c) Electron micrograph of two hairs emerging from their follicles. Note the exfoliating epidermal cells encircling the follicles like rose petals.

Saladin: Anatomy & 6. The Integumentary Text © The McGraw-Hill

Physiology: The Unity of System Companies, 2003

Form and Function, Third Edition

202 Part Two Support and Movement distal segment of the fingers. The limbs and trunk have about 55 to 70 hairs per square centimeter, and the face has about 10 times as many. The scalp has about 100,000 hairs and a man's beard has about 30,000. The number of hairs in a given area does not differ much from one person to another or even between the sexes. Differences in apparent hairiness are due mainly to differences in the texture and pigmentation of the hair.

Not all hair is alike, even on one person. Over the course of our lives, we grow three kinds of hair: lanugo, vellus, and terminal hair. Lanugo19 is fine, downy, unpig-mented hair that appears on the fetus in the last 3 months of development. By the time of birth, most of it is replaced by similarly fine, unpigmented hair called vellus.20 Except for the eyebrows, eyelashes, and hair of the scalp, all of the hair of children, two-thirds of the hair of women, and one-tenth of the hair of men is vellus. Terminal hair is longer, coarser, and pigmented. It occurs on the scalp, eyebrows, and eyelashes; at puberty it replaces the vellus in the axillary and pubic regions, on the face of males (to form the beard), and to varying degrees on the trunk and limbs.

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Essentials of Human Physiology

Essentials of Human Physiology

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