Skin. Coloured scanning electron micrograph (SEM) of skin from the tip of a human thumb. The top layer of the skin, the stratum corneum, is composed of dead keratinised cells that are continuously sloughed off and replaced with new cells from below. In the bottom right corner is a pore that leads down to a sweat gland beneath. Sweat glands on the hands are eccrine glands that secrete a very dilute solution of salts, lactic acid and urea. Their function is to regulate body temperature. Their greatest density is found on the hands and feet, with 600-700 glands per square centimetre. Magnification: x40 when printed at 10 centimetres wide.