may be called a visual map of the affected area. He had but to close his eyes to see a picture of his hand with the affected area marked upon it as clearly as in a photograph. As soon as a spot was stimulated, he
saw its position on this map and at once described the neighbouring landmarks. He could even give approximate measurements; for instance, he would say that the point stimulated lay in "the interosseous space about 1 in. from the head of the first metacarpal." Occasionally, he was allowed to point with the index finger of the right hand; but, since this in itself acts as a stimulus, it should be rarely permitted and should be reserved for special occasions.
Unfortunately, the affected area on the back of the hand was too small for the satisfactory determination of the relative shape of different objects applied to the skin. Squares, triangles, and circles of wax or of india-rubber which lay well within its borders could not be distinguished with certainty over similar parts of the normal hand.
Deep sensibility was not disturbed by the operation. The vibrations of a tuning-fork were appreciated from the beginning, and this test played no further part in our investigations.
Chapter III.—The Phenomena of Deep Sensibility.
That the muscles were endowed with sensory nerves was a necessary corollary to the universal acceptance of a "muscle sense." But Sherrington was the first to demonstrate afferent fibres in the nerves of skeletal muscles. By degenerative methods he traced their course to the muscles, and, in the opposite direction, showed that they entered the spinal cord by the posterior roots of those segments which gave rise to the motor fibres of the same muscles. This discovery did not receive the attention which was due to it, because it seemed to be little more than the last stone necessary to support the universally accepted hypothesis of "muscular sense."
But we long ago suspected that the existence of so many afferent fibres in the muscular nerves was not connected with the power of estimating movement only.
At the beginning of their investigation, Head and Sherren were brought face to face with the problem of "deep sensibility." They found, when all the cutaneous sensory nerves to a part were divided, that it was not of necessity totally anæsthetic. But, although they saw that the only structures which could account for the existence of this sensibility were the afferent fibres in the nerves of the muscles and tendons, accidental lesions of nerve trunks gave little opportunity for bringing this
