CHAPTER VII.

THE PHYSIOLOGY OF THE SENSES.

THERE is an impression among many that Mr. Mill's theory has the support of physiology, and this is strengthened by the anatomical and physiological details which constitute so large a portion of Mr. Bain's work. But I cannot discover that either has found a basis, or even a starting-point, for their general theory of the mind, or for their particular theory of the manner in which we reach the idea of an extended world, in any ascertained phenomena of our bodily frame. Their speculations receive no aid from physiology, and must stand or fall by their psychological merits or demerits. The physiology of the senses is still in a very uncertain condition, and, whatever it may do in ages to come, can as yet throw little light on strictly mental action, except, indeed, in the way of correcting premature hypotheses. It may be profitable to look at some of the later researches into the senses conducted by eminent physiologists, especially in Germany. We shall find that they give no sanction to the hypothesis of Mr. Mill and Mr. Bain, and seem to favour a theory

of a very different character. In the sketch that follows, I have made free use of the great works on physiology which have been published in our country, and still more particularly of the admirable historical, critical, and expository summary by Wundt, in his Beiträge zur Theorie der Sinneswahrnehmung.

TOUCH.

The scientific investigation of this sense may be said. to have commenced with the researches of J. Müller and E. H. Weber. The general result reached by Müller is, that "every point in which a nerve-fibre ends is represented in the sensorium as a space-particle" (Wundt, Theor. Sinneswahr.) There are disputes as to how the general law should be stated, but we have a fact here which has not been and cannot be set aside. The nerves of touch proper, setting out from the base of the brain, tend towards the periphery of the body. They reach the skin each at a determined point: there is a special aggregation of these points in the mid-finger and the tip of the tongue. Now, wherever the nerve terminates, there the sensation is felt thus, if we prick a nerve which reaches the mid-finger, the pain is localized at the point where the nerve terminates. If we stretch or pinch the ulnar nerve, by pushing it from side to side, or compressing it with the fingers, the shock is felt in the parts to which its ultimate branchlets are distributed, namely, in the palm and back of the hand, and in the fourth and fifth fingers. "According as the pressure

"is varied the pricking sensation is felt by turns in the "fourth finger, in the fifth, in the palm of the hand, or "in the back of the hand; and both on the palm and "on the back of the hand the situation of the pricking "sensation is different, according as the pressure on the nerve is varied; that is to say, according as different "fibres or fasciculi of fibres are more pressed upon than

others. The same will be found to be the case in "irritating the nerve in the upper arm" (Müller's Physiology, by Baly, p. 740). So strong is this tendency to localize the sensation at the extremities of the nerves, that when an arm or leg is amputated the person has still the feeling of the lost limb. Müller has collected a number of such cases (Ib., pp. 746, 747). "A student, "named Schmidts, from Aix, had his arm amputated

above the elbow thirteen years ago; he has never "ceased to have sensations as if in the fingers.

I

applied pressure to the nerves in the stump; and M. "Schmidts immediately felt the whole arm, even the "fingers, as if asleep." "A toll-keeper in the neighbourhood of Halle, whose right arm had been shattered by a cannon-ball in battle, above the elbow, twenty years ago, and afterwards amputated, has still, in "1833, at the time of changes of the weather, distinct "rheumatic pains, which seem to him to exist in the "whole arm; and though removed long ago, the lost part is at those times felt as if sensible to draughts of "air. This man also completely confirmed our state"ment, that the sense of the integrity of the limb was never lost." When there is a change made artifi

cially in the peripheral extremities of nerves, the sensations are still felt as if in the original spots. "When, in the restoration of a nose, a flap of skin is turned

'down from the forehead and made to unite with the "stump of the nose, the new nose thus formed has, as "long as the isthmus of skin by which it maintains its original connexions remains undivided, the same sensations as if it were still on the forehead; in other "words, when the nose is touched, the patient feels the impression in the forehead. This is a fact well known 'to surgeons, and was first observed by Lisfranc" (Ib., p. 748).

No doubt it is possible to ascribe all this to experience and the association of ideas. We first, it is said, find by observation that a certain sensation originates in a particular part of the body, and the same sensation ever after suggests the part. But the facts, as a whole, will not submit to this explanation. It is difficult to see how the phenomena quoted can be thus accounted for. For surely an experience of thirteen or twenty years might have been sufficient to change the associations acquired at an earlier date, and to place the persons under the influence of new ones, provided always that the original ones had not been instinctive or native. In the case of the transference of the flap of skin, Müller says, "When the communication of the nervous fibres "of the new nose with those of the forehead is cut off "by division of the isthmus of skin, the sensations are " of course no longer referred to the forehead; the sen'sibility of the nose is at first absent, but is gradually

"developed." This language implies that the old reference to the forehead ceased in spite of the old association when the isthmus was cut; and that the new reference to the nose was occasioned by the sensibility of the nerve, according to the physiological law, which makes us ascribe the sensation to the extremity of the nerve. It is not easy to see how experience could give us the ready localization of the sensation, more particularly when the feeling is within the body, and in a part which has never fallen under the senses of touch or sight. It is hard to believe that the instantaneous voluntary drawing back of a limb when wounded, and the shrinking of the frame when boiling liquid is poured down the throat, can proceed from an application of an observed law as to the seat of sensations. From a very early age, and long before they give any evidence of knowing distance beyond their bodies, or having any other acquired perceptions, children will indicate that they know at least vaguely the seat of the pain felt by them, if a child is wounded in the arm, it will not hold out its foot. But the question seems to be set at rest by a physiological fact, thus stated by Dr. Baly :— "Professor Valentin (Repertor. für Anat. und Physiol., "1836, p. 330) has observed, that individuals who are

the subjects of congenital imperfection, or absence of "the extremities, have, nevertheless, the internal sensa"tions of such limbs in their perfect state. A girl aged "nineteen years, in whom the metacarpal bones of the left hand were very short, and all the bones of the phalanges absent--a row of imperfectly organized