Page:WHR Rivers - Studies in Neurology - Vol 1.djvu/29

Rh preserves an idea or mental picture of the limb. Many patients with cerebral lesions complain that when they wake at night they do not know where the arm is lying, and it sometimes seems as if part of the limb, such as the hand, had disappeared.

In cases of injury to the spinal cord the patient not uncommonly has an idea that his legs are in some definite position, although he is entirety insensitive below the waist. This illusory posture may not be constant, but may come and go, or change its direction at different periods in the course of the illness.

(a) Light touch is always examined first by applying a wisp of fine cotton wool gently to the skin, so that it does not produce gross pressure or deformation of structure. But this test must be used with extreme caution. Many brands of cotton wool, when rolled into a wisp, form so stiff a mass that sensations of pressure are evoked; or, on the contrary, the finer quality of cotton wool may fail to act as a stimulus to the horny palm of a workman, or even to some parts of a normal well-kept hand.

Over hair-clad parts cotton wool is not a specific stimulus, but excites both protopathic and epicritic sensibility. After complete division and suture of a peripheral nerve, the affected area, if covered with hair, not uncommonly regains its sensibility to contacts with cotton wool in a few weeks. But when the hairs are removed by shaving, the skin is found to be insensitive, and may remain so for many months. This double tactile innervation of the skin of hair-clad parts is particularly liable to lead to fallacious conclusions in cases such as injury to the ulnar nerve; it may seem as if sensibility to light touch had returned to the dorsal aspect of the hand, and yet after careful shaving this area is found to be entirely insensitive to cotton wool. This factor also played a great part in the sensations I experienced during the recovery of my arm, which are fully described on p. 272.

In cases of thalamic over-reaction cotton wool produces over hair-clad parts a peculiar sensation which has nothing to do with the sensory activities of the cortex. It is an affective response, which may take the form, on the one hand, of pleasurable "tickling," or, on the other, of uncomfortable "itching."

We have measured the sensibility to light touch by means of von Frey's graduated hairs; these depend on the fact that a constant pressure is exerted by the tip of a hair when sufficient force is used to bend it. We can arrive at the amount of this pressure per unit area if the force exerted in bending the hair, measured on a balance, is divided by its total area in mm.$2$; the result expressed in grm./mm.$2$ represents the pressure per unit area.

But von Frey contends ([36] pp. 223-9), and we believe rightly, that this is not a correct method of comparing the value of different hairs as a