The Structural System
Lesson 14, Page 5 of 16

Excessive movement, which could dislocate a joint, is prevented by strong elastic connective tissues that cross the joint and are known as ligaments.

Skin, Hair and Nails

The Skin The skin is the largest organ of the body, providing a protective barrier and the first line of defense against would-be invaders. The skin also contains nerves and sensory organs involved with touch; and blood vessels and sweat glands that help regulate body temperature. The outer layer of skin, known as the epidermis, is several cells thick and has an external layer of dead cells that is constantly being sloughed off and replaced from below by a basal layer of living cells. The inner layer, or dermis, is composed of a network of collagen and elastic fibers, nerves, blood vessels, fat lobules, and the bases of sweat glands and hair follicles.

Each square inch of skin contains hundreds of sweat glands that are controlled by the temperature regulation area of the brain. The sweat glands secrete moisture, which cools the body surface through the process of evaporation, helping to maintain the normal body temperature. Through the sweat glands the skin acts as an excretory organ.

In certain areas of the body the outer layers of the skin are modified to produce hair and nails. The hair, nails and epidermis of the skin all contain a fibrous protein called keratin, which is responsible for their form and strength.

The rate of growth of the hair varies with age, sex, and the length of the hair. When it is short, hair grows about 3/4 inches a month (about 2 cm.) When it is a foot long its rate of growth is reduced to about half that. Women between the ages of 16 and 24 experience the fastest rate of hair growth.

Which of the following is not a function of the skin?

Temperature regulation

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