d. Subcutaneous-Adipose. This layer is composed of loose connective tissue
combined with adipose (fatty) tissue. The subcutaneous layer of skin has several
(1) Storehouse for water and particularly for fat. Much of the fat in an
overweight person is in this layer.
Layer of insulation protecting the body from heat loss.
(3) Pads the body giving the body form and shape and cushioning and
protecting the body from blows.
Provides a pathway for nerves and blood vessels.
APPENDAGES ASSOCIATED WITH THE SKIN
a. Hair. See figure 1-3.
(1) General information. Hairs, growths of the epidermis, are distributed
over various parts of the body. The primary function of hair is to protect. Hair on the
head protects the scalp from injury and sunburn. Eyelashes and eyebrows protect eyes
from foreign matter. Hair in the nostrils and external ear canal protects from insects and
dust. Hair is a keratinized structure that grows out of a tubular canal called a hair
(2) Distribution. Nearly all parts of the body have hair. An average adult
probably has about five million hairs, about 100,000 of these on the scalp. Straight
hairs are oval or cylindrical in cross section while a cross section of curly hair shows it
flattened. Straight hairs are stronger than curly hairs.
Structure. A shaft and a root make up each hair.
(a) Shaft. The hair shaft is the superficial part of the hair that sticks out
above the skin surface. A course hair shaft has three parts: the inner medulla, the
middle cortex, and the outermost layer, which is the hair cuticle. Polyhedral cells with
granules of eleidin and air spaces make up the medulla. (The medulla may or may not
be present in fine hair.) The cortex is the major part of the hair shaft and has cells that
contain pigment in dark hair. These cells are mostly air in white hair. The cuticle, the
outermost layer of the shaft, is composed of one layer of thin, flat, scale-like cells that
are heavily keratinized.
(b) Root. The hair root is the portion below the skin surface. The root
is embedded in the epidermis and penetrates the dermis and the subcutaneous layers
of skin. The root also contains a medulla, cortex, and cuticle. The hair follicle,
surrounding the root, is made up of an external root sheath and an internal root sheath
(both composed of epithelium).