A small gland in mammalian skin that secretes sweat. There are two types with different structures and properties. Eccrine sweat glands are most numerous in humans and certain other primates; they occur in high concentrations over much of the body surface, especially on the palms of the hands, soles of the feet, upper torso, and forehead. The gland lies in the subcutaneous layer of the skin and discharges sweat directly to the skin surface through a narrow coiled duct. Apocrine sweat glands discharge their secretion into hair follicles. In horses and marsupials they produce a watery secretion that functions in cooling the body surface, whereas in other mammals, particularly ones with a body covered by dense hair or fur, they produce an oily secretion that mixes with sebum as it passes alongside the hair shaft to reach the skin surface. Here the odourless secretion is subject to microbial action that can produce smelly substances, some of which may act as pheromones. In humans, apocrine sweat glands are confined to the armpits, groin, and around the nipples.