The skin, the largest sense organ of the body, is the interface between the organism and its environment. It must ensure that the organism is able to perceive all environmental changes, both pleasurable ones and those that threaten its existence. Thctile receptors are spread over the roughly two square meters of the body surface, whereas all of the other major sensory organs are crowded into the head. Cutaneous nerves contain sensory and sympathetic (autonomic) nerve fibers. The sympathetic motor fibers, mixed with the sensory fibers in the dermis, eventually send branches to the sweat glands, blood vessels, and arrectores pilorum muscles. The sensory fibers and their specialized corpuscular end organs are receptors for touch, pain, temperature, itch, and physical and chemical stimuli. A large portion of the human sensory cortex receives sensory messages from the skin of the face and the hands, areas that are especially well supplied with receptor organs.