Chapter XXII

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The term “integumentary” comes from the Latin integumentum, meaning “protective covering,” and refers to the skin with all its layers and appendages (the same concept was thought to underlie the Latin for “skin,” pellis, which was connected with the base pell- meaning “to push back,” i.e. to repel harm from the body). The skin is composed of three layers: the , , and , named according to their relative positions ("on the skin," "skin," "under the skin"). The last layer is sometimes called the “subcutis” or “ layer,” which as you know refers to the layer “underneath the skin.”

The five strata of the epidermis in ascending order are the basale, , granulosum, lucidum, and —the “basal,” “thorny,” “granular,” “transparent,” and “horn-like” layers—although in areas where the epidermis is thinner the granulum and lucidum are sometimes not found. Most cells in the epidermis are (i.e., cells that synthesize the protein keratin). New cells are formed at the bottom of the epidermis and slowly rise to the surface, splitting and undergoing other changes. When they reach the top, they are called “” (i.e., cells in the stratum corneum). Gradually these cells lose their cohesion and fall away individually in a process known as , from the Latin meaning “process of descaling.” Also found in the epidermis are , which are the cells that give the skin its color from the production of melanin (lit. “ substance”).

Much thicker than the epidermis, the dermis, or , is a dense bed of vascular connective tissue that gives the skin its toughness and elasticity. It is divided into two layers. The first is called the “ layer” because of the nipple-like projections that interlock with the epidermal tissue above. The formation of this layer increases the contact surface area between the epidermis and dermis, which allows for a greater transmission of nutrients to the epidermis (where there are no blood vessels) as well as a stronger connection between layers. The lower layer of the dermis is called the “ layer” because of its dense, “net-like” arrangement of fibrous tissue. Also contained in the dermis are the sweat glands and the glands, which secrete sebum, an oily semifluid that helps protect the skin.

The hypodermis is primarily made up of ("fatty") tissue and is vascularized. This layer of fat helps to protect and insulate and is also the locus of the roots of hair follicles. The hairs themselves are controlled by tiny pili muscles (Lat. “lifter of the hair”), which are located in the dermis. Pilus is the Latin term for body hair, as opposed to capillus, a hair on the head (the source of the medical term “capillary”; see p. 317).