The following accessory organs (skin derivatives) are embedded in the skin: Hairs are elongated filaments of keratinized epithelial cells that arise and emerge from the skin of mammals. Hair is composed of the following structures: The hair shaft is the portion of the hair that is visible on the surface of the skin. The hair root is the portion of the hair that penetrates the skin (epidermis and dermis). The hair follicle is the sheath that surrounds the hair in the skin. The bulb is the base of the hair follicle. The matrix is the bottom of the hair follicle (located within the bulb). Here, cells are actively dividing, producing new hair cells. As these cells differentiate, they produce keratin and absorb melanin from nearby melanocytes. As younger cells are produced below them, the more mature cells are pushed upward, where they eventually die. The keratin they leave behind contributes to the growth of the hair. The color of the hair is determined by the pigments absorbed from the melanocytes. The arrector pili is a smooth muscle that is attached to the hair follicle. When the muscle contracts, the hair becomes erect; in humans, “goose bumps” are produced. Nails are keratinized epithelial cells. The semilunar lighter region of the nail, the lunula, is the area of new nail growth. Below the lunula, the nail matrix is actively producing nail cells, which contribute to the growth of the nail. Sudoriferous (sweat) glands secrete sweat. Sweat consists of water with various salts and other substances. There are four kinds of sudoriferous glands: Merocrine glands occur under most skin surfaces and secrete a watery solution through pores (openings at the skin surface), which serve to cool the skin as it evaporates. Apocrine glands occur under skin surfaces of the armpits and pubic regions and, beginning with puberty, secrete a solution in response to stress or sexual excitement. The solution, more viscous and more odorous than that secreted by eccrine glands, is secreted into hair follicles. Ceruminous glands secrete cerumen (earwax) into the external ear canal. Wax helps to impede the entrance of foreign bodies. Mammary glands produce milk that is secreted through the nipples of the breasts. Sebaceous (oil) glands secrete sebum, an oily substance, into hair follicles or sometimes through skin surface pores. Sebum inhibits bacterial growth and helps prevent drying of hair and skin. An accumulation of sebum in the duct of a sebaceous gland produces whiteheads, blackheads (if the sebum oxidizes), and acne (if the sebum becomes infected by bacteria).