A human experiment in nerve division/Chapter 3
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 antesthetic. 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 question to a direct issue. Accidental lesions usually divide mixed nerves containing both the fibres to the skin and to the muscles. By chance it may happen that one group has regenerated whilst the other group remains divided (as in Case 28, p. 321, Head and Sherren). But it is impossible to say how far the characters manifested under these conditions by deep sensibility are normal, and how far they are due to incomplete regeneration.
We therefore determined in our experiment on nerve division to bring this question to a direct issue. A large area of skin was to be robbed of its sensory functions entirely, but the sensibility of the deep parts was to remain undisturbed. By this means, we hoped to have the opportunity of discovering the nature of the sensibility subserved by the afferent fibres of the nerves to the muscles and other deep structures. The result far exceeded our expectation. We found that deep sensibility is an important factor in the sum of afferent impulses which pass into the central nervous system. For the impulses conducted by the afferent fibres of these deep nerves underlie our sensations of tactile and painful presure, of the locality of deep touch, and of the position of the parts in space.
All these sensory qualities are associated with some aspect of pressure, and all are undisturbed by the denervation of the skin. They can therefore be united into one group under the name of " deep sensibility," which will form the subject of the present chapter.
The observations upon which the statements in this chapter are based were made between April 26 and July 20, 1903, when cutaneous sensation began to return to the back of the hand. During this period the sittings occupied twenty-two separate days; so that we had ample opportunity for studying the peculiar behaviour of a part where the skin had been rendered totally insensitive.
(1) Tactile Pressure.
As soon as H. regained consciousness after the operation, he was delighted to find that no part' of the back of the hand was insensitive to a touch with the finger or similar hard object. The next day he was carefully tested by Mr. Sherren, who found that over the area of cutaneous insensibility on the back of the hand a pressure-touch with the rounded end of a pencil was not only appreciated every time, but was localized to within \ in. (l-25 cm.) of the spot touched.
Nine days after the operation, Dr. Rivers summed up the results of an extended series of observations in the following words :—
" The most striking features of the examination of May 4 were that slight pressure on the abnormal area of the skin was appreciated and could be localized, whilst touches with cotton wool or deformations of the skin, caused by drawing a hair outwards, produced absolutely no sensation."
Whether a sensation was or was not elicited, when a thick camel's hair brush was applied to the dorsum of the hand, depended largely on the way in which the brush was used. . If applied suddenly and vertically to the skin so as to cause a jar, a slight sensation of touch was produced; but when the pressure was made more gradually, no sensation was appreciated until distinct deformation of the brush occurred. Even in this case slight pressure only was necessary to evoke a sensation. In the same way, stroking the part gently with a wisp of cotton wool was entirely unperceived, in spite of the thick growth of hair on the back of H.'s hand. But cotton wool, balled together into a " swab," such as is used for sponging a wound, caused a sensation if pressed upon the affected area. Slight pressure with such a ball of cotton wool might be perceived when it was put on or taken off only, and it was possible to place it on the skin with so slight a pressure that it was not appreciated at all.
The more gradually contact was established between the stimulating object and the affected part, and the smaller the pressure applied, the less likely was it that a sensation would result. All our observations showed that parts endowed with deep sensibility only are especially sensitive to jarring impact.
Not even stimulation with No. 8 of von Frey's hairs could be recognized, when applied to the affected area on the back of the hand. When the skin was gently raised between the finger and the thumb so as to form a loose fold, it was found to be entirely insensitive, even to grave pressure.
Similarly, no sensation was experienced when considerable traction was exerted on a hair; the skin could be elevated to the maximum extent until the hair was pulled out, and no sensation was evoked. But a scarcely visible deformation of the skin by pressure was at once appreciated.
On several occasions, when the back of the hand was deeply frozen with ethyl chloride, touches were distinctly appreciated on the frozen area. On one occasion, H. stated that he could recognize no difference between the touches on the frozen portion and those on the surrounding parts. In this observation, not only was any possible sensibility of the skin excluded, but also that of the more superficial layers of the subcutaneous tissues, and it shows that they are endowed with sensibility at a considerable depth.
Several introspective observations were made on the character of the sensations of deep touch. Oh one occasion, H. recorded that without careful attention, no difference in quality would have been noticed between touches produced by pressure on the anaesthetic and normal areas. Pressure with the head of a pin on the normal skin produced a tactile sensation, together with sensations of slight cold and of hairstimulation. On the affected area, the latter elements were no longer present, but the fundamental quality of the sensation seemed to remain the same, so that when the head of the pin was passed across the border separating the normal and affected areas, a considerable effort of attention was necessary to detect when the change occurred. The differences in quality were of such a kind that it is very improbable they would be noticed by any but a trained observer. On a later occasion, observations were made by stimulating the normal right hand covered with a thin rubber glove, when H. noted that the quality of the touches resembled that experienced from the abnormal area.
The most extended observations were made on the back of the affected hand, because of the rapidity with which sensation began to return to the skin of the forearm. But during the short time at our disposal, the anesthetic parts of the forearm were found to behave exactly in the same way as the insensitive area on the dorsum of the hand.
In conclusion, there is no doubt that the back of the hand was so sensitive to contact, that most observers would have said the sensation of touch was unaffected.