The dermis, a deeper and thicker region than the epidermis, is composed of dense irregular connective tissue. The upper layer of the dermis has fingerlike projections called dermal papillae. Dermal papillae project into and anchor the epidermis. In the overlying epidermis, dermal papillae cause ridges, resulting in spiral and concentric patterns commonly known as fingerprints. The function of the epidermal ridges is to increase friction and thus provide a better gripping surface. Because they are unique to each person, fingerprints and foot- prints can be used for identification purposes. The dermis contains collagenous and elastic fibers. The collagenous fibers are flexible but offer great resistance to overstretching, they prevent the skin from being torn. The elastic fibers stretch to allow movement of underlying muscles and joints, but they maintain normal skin tension. The dermis also contains blood vessels that nourish the skin. Blood rushes into these vessels when a person blushes; blood is reduced in them when a person turns cyanotic, or "blue". Sometimes, blood flow to a particular area is restricted in bedridden patients, and consequently they develop decubitusulcers (bedsores). These can be prevented by changing the patients position frequently and by massaging the skin to stimulate blood flow. There are also numerous sensory nerve fibers in the dermis that take nerve impulses to and from the accessory structures of the skin.
Hypodermis, or subcutaneous tissue, lies below the dermis. From the names for this layer, we get the terms subcutaneous injection, performed with a hypodermic needle. The hypodermis is composed of loose connective tissue, including adipose (fat) tissue. Fat is an energy storage form that can be called upon when necessary to supply the body with molecules for cellular respiration. Adipose tissue also helps insulate the body. A well-developed hypodermis gives the body a rounded appearance and provides protective padding against external assaults. Excessive development of adipose tissue in the hypodermis layer results in obesity.
