A dilute solution of procaine in glucose solution was injected by slow drip into the lumbar theca of patients to cause a differential block. Vasodilatation of the skin vessels of the lower limbs always occurred before any change in sensibility. The first changes in sensibility were inability to fell painful stimuli and warm stimuli; these two forms of sensibility were usually coextensive and occurred at the same time. When this was not so, either form of sensibility could be more extensively disturbed than the other. When warmth sensibility was removed, cold sensibility was either normal or diminished or cold stimuli caused non-thermal sensations such as non-thermal wetness or pressure. It is concluded that the nerve fibres signalling warmth are the smaller delta fibres or non-myelinated fibres or both. As two groups of investigators who recorded from the non-myelinated fibres in man found no non-myelinated fibres responding to cold, it is condluded that fibres signalling cold cover the same delta range as those signalling warmth, together with a proportion of larger delta fibres. When fibres subserving pain, warmth, and cold were blocked, tactile sensibility could be normal. It is concluded that, if there are tactile fibres in man in the smaller delta and non-myelinated ranges, their contribution to tactile sensibility is insignificant. It is also concluded that the pre-galglionic B fibres are the most susceptible of all fibres of the nerve roots to local anaesthetic agents. On two occasions, piloerection occurred when the pre-ganglionic sympathetic fibres were blocked. In two patients spontaneous itching occurred when all peripheral nerve fibres were blocked except large tactile fibres.
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