The ability to perceive pain constitutes a special sensitivity that the body has evolved as a defence system, enabling it to preserve itself by avoiding damaging conditions.
There are various types of pain. The most important is that caused by injury of the skin which contains special nerve-fibres for the conduction of painful impressions up the spinal cord. Internal parts of the body are less sensitive than the skin, and diseases in them usually give rise to a different sensation. These parts are not endowed with the capacity of feeling pain due to sudden injury, but inflammatory changes in these structures, and disturbances in their functions, are capable of influencing the brain so as to produce extremely severe pain. For example, any source of irritation on the course of a nerve is apt to produce the severe pain of neuralgia. Ordinary sensations of all sorts become painful when they are excessive, and thus liable to damage the organ in question.
Pain perception is not simply a function of the amount of physical damage alone. There is evidence that the intensity and quality of pain are also determined by past experience, expectation, anxiety, and the significance of the situation in which injury occurs. There is also physiological evidence that nerve impulses from the skin may be inhibited or modified during transmission through the spinal cord. During a life-threatening situation, feeling pain might get in the way of survival, so the body reacts by shutting off the pain response. Anecdotally, this has been reported by men in combat who did not realize that they had been shot until after they made their way back to safety.
In Chinese medicine, the differing qualities of pain are a sign of the disharmony in the body, thus: pain diminished by heat signifies disharmony of cold; diminished by cold signifies disharmony of heat; relieved /aggravated by touch or pressure signifies deficiency / excess; diminishes / increases after eating signifies deficiency / excess; increases in humid weather or accompanied by sensation of heaviness signifies dampness; accompanied by bloating or sense of fullness signifies stagnant qi; sharp or stabbing signifies congealed blood; moves from place to place signifies wind or stagnant qi; slight or accompanies by fatigue signifies disharmony of qi or dampness.
As an indication, the market for pain killers (including analgesics and headache tablets) in the UK in 1986 was £102 million.