Pain sensibility in deep somatic structures.

Introduction It is well recognized that pain may be provoked by the stimulation of many deep-lying somatic structures, such as muscle, tendon, periosteum, etc. (Lewis, 1942). It was noted by Lewis (1938) that the quality of this pain is independent of the method of stimulation adopted, and that it differs markedly from the quality of the pain which can be aroused from skin. In consequence he suggested that the pain nerves of the skin form a system different from that to which those nerves supplying deeper tissues belong. This contention he supported by observing that the reflexes resulting from painful stimulation of the skin and those due to stimulation of deeplying tissues tended to be of a different character. In 1939, Lewis and Kellgren brought forward additional evidence for the specificity of reflex responses obtained from these two sources. So far, however, no anatomical evidence has been produced which would lend colour to the theory of two separate systems subserving the sensation of somatic pain. It has been shown (Weddell, 1945) that in normal skin the network of nerve endings subserving the sensation of pain is disposed in such a manner that any one area of skin is supplied by several overlapping terminals derived from different axons. Weddell, Sinclair, and Feindel (1948) have recently investigated the innervation pattern of skin from which an abnormal reaction to painful stimuli was obtained. They found that in such areas the terminals associated with painful sensation were invariably isolated from each other, and that the interweaving of adjacent terminal fibres which is characteristic of the normal skin did not occur. As a result of this investigation they suggested that alterations in the peripheral pattern of innervation of the skin may be responsible for alterations in the quality of the sensation perceived following the application of a painful stimulus. Thus they