Page:WHR Rivers - Studies in Neurology - Vol 1.djvu/342

328 back of the hand, but the impulses evoked are prevented from reaching the highest centres by the effects of coincident stimulation of the thermal end-organs. As the temperatures rise, these potentially painful impulses increase in strength, until they can no longer be inhibited; they then form the basis of a sensation of pain. In this case, consciousness is not disturbed, until impulses are produced, not only in themselves of adequate strength to evoke a sensation, but able to overcome the inhibitory effect of the activity of other specific end-organs.

In a similar way, the return of epicritic sensibility reduces the amount of pain caused by cutaneous stimuli, without at the same time raising the threshold. Radiation and reference are inhibited, and the pain produced by a prick is restricted to the immediate neighbourhood of the spot stimulated. This diminution in extent reduces the amount of pain suffered by the patient, although the measured threshold for painful sensations may be actually lower than during the preceding protopathic stage.

So long as a part of the body is innervated by the end-organs of the deep and protopathic systems, two incompatible forms of localisation are possible. Painless pressure will be localised in the neighbourhood of the spot to which it is applied; but the sensation evoked by purely cutaneous stimuli will radiate widely, and be referred into some remote part. Both forms of localisation may be present in consciousness together. When a cold test-tube is applied to the permanently protopathic area on the back of H.'s hand, the pressure of the tube is localised in the neighbourhood of the point of contact, but the cold sensation is said to lie mainly in the thumb. Thus, the existence of comparatively accurate tactile localisation, due to the deep afferent system, does not seem to inhibit or control the impulses produced by stimulation of protopathic end-organs. But, when once a part of the body is endowed with epicritic sensibility, reference ceases entirely.

It has been suggested (von Frey [39]) that protopathic sensibility is due to anatomical changes which have taken place within the central nervous system, in consequence of the abnormal state of the injured nerve. On the other hand, we beheve that this condition is due to the uncontrolled passage of a set of impulses, which normally undergo modification or inhibition before they reach the highest centres. This view is supported by the existence of a normal protopathic surface, such as that of the glans penis.

Most of the characteristic reactions obtained from a part in a condition of protopathic sensibility undergo modification with the return of epicritic impulses; reference alone is completely abolished. It may be asked why a function apparently so useless remains, though in a condition of permanent suppression. The answer to this question is given by the existence of referred pain in disease of the internal organs. These parts are probably innervated, like the glans penis, from the deep and protopathic systems. But, unlike the glans, their sensibility is extremely low; heat- and cold-spots must be scanty or even absent from most parts of the stomach and intestines. More over