6. The Difference between Irritation and Release from Control.
It must not be supposed that we deny the existence of true irritative phenomena. These can be studied best in cases of injury to peripheral nerves, where they form an instructive contrast to the manifestations of protopathic release.
Take such an instance as that described on p. 111, where the anterior division of the external cutaneous nerve had been accidentally wounded in the lower part of the forearm. A considerable area became intensely tender to the point of a pin dragged lightly across the skin; but sensation was perfect to all the measured tests for prick, light touch, heat and cold, and the compass points were discriminated with equal ease on both hands. On exploration the nerve trunk was found to be irritated by inflammatory changes and the full distribution of its fibres was revealed as an area of tenderness accompanied by no coincident sensory loss.
On the contrary, protopathic over-reaction is strictly limited to parts which have been deprived of the higher forms of sensibility; after division of the ulnar nerve it extends no further than the borders of the loss to light touch. If the same nerve is irritated the tenderness may extend far beyond these limits and occupy all those parts of the radial palm which are innervated by pain fibres from the ulnar nerve. This is the area that remains sensitive to prick when the median has been completely divided. Protopathic over- reaction is one of the phenomena of dissociation due to removal of higher control and marks out the parts which have been robbed of their higher sensory functions. Irritative tenderness, on the contrary, may be accompanied by no loss of sensibility; it expresses the complete peripheral distribution of the nervous mechanism that has been subjected to excitation.
Excessive sweating is another phenomenon which may be due at one time to irritation, at another to release of spinal centres from higher control. After gross injury to the spinal cord outbursts of hyperidrosis may occur, which corresponds to the parts below the lesion; these are produced by an uncontrolled response to superficial, proprioceptive or visceral stimulation. On the other hand, the sweating may be an irritative manifestation evoked from the central portion of the injured cord.
Irritative phenomena can occur without any other disturbance of function; but release from control is always signalised by some coincident defect.
7. The Necessity for avoiding a priori Hypotheses in the Study of Sensory Phenomena.
The study of the phenomena of sensation has been much hampered by a priori hypotheses. The older psychologists assumed that the immediate consequences of stimulation corresponded categorically to the various aspects of sensation. They failed to recognise that between the impact of a
