Pain is the detection by the nervous system of harmful stimuli. The function of pain is to warn the individual of imminent danger: even the most minor tissue damage will cause pain, so that avoiding action can be taken at a very early stage. The level at which pain can only just be felt is the pain threshold. This threshold varies slightly between individuals, and can be raised by, for example, hypnosis, anesthetics (see anesthesia), analgesics), and the drinking of alcohol. In some psychological illnesses, especially the neuroses, it is lowered. The receptors of pain are unencapsulated nerve endings, distributed variably about the body: the back of the knee has about 230 per square centimeter, the tip of the nose about 40. Deep pain, from the internal organs, may be felt as a surface pain or in a different part of the body. This phenomenon, referred pain, is probably due to the closeness of the nerve tracts entering the spinal cord.