THE EVIDENCE OF THE SENSES.

The senses are the witnesses which bring in evidence from the outer world, without which that world would for us have no existence at all; but the mind sits aloft on the judgment-seat and forms its conclusions from the evidence laid before it; and these conclusions are for the most part wonderfully correct; for, though the testimony of one sense alone might lead the mind to form an erroneous opinion, this can be rectified by discovering what one or more of the other senses have to say on the same subject. When, however—as sometimes happens under peculiar circumstances—the evidence of one sense only is available, the mind may very readily arrive at a false conclusion. As an instance of this may be cited what is often observed by surgeons in cases of hip-joint disease. The patient, usually a child, complains of severe pain in the knee, which, however, has not, so far as can be ascertained, been injured in any way. Very likely, the pain is severe enough to prevent sleep at night, so that there can be no doubt about its existence, and it may perhaps have been almost continuous for some time past. Now, in such a case the surgeon will have a shrewd suspicion of what is really amiss, and very often will at once proceed to examine the hip. This he will do, too, in spite of assurances on the part of the parents that the patient always complains of the knee and of that joint only. He does not doubt that the pain feels as if it was in the knee, but he strongly suspects, nevertheless, that the disease is in the hip; and this often proves to be the case. This is an instance of what is called ‘referred sensation.’ The nerve which conveys sensation from the knee also sends a branch to the hip-joint, and it is this anatomical fact which explains the phenomenon. It might be expected that even if the pain was not felt solely in the hip, it would at least be always felt there as well as in the knee. This, however, though sometimes the case, is by no means always so. In this instance, the patient comes not unnaturally to the conclusion that where he feels the pain, there the cause of the pain must of necessity be situated. He would be quite ready to declare that there was nothing the matter with his hip, for he cannot see into the joint and discover the disease there. He has, in fact, to depend upon the evidence of one sense only, and the conclusion based upon the evidence of the single sensation of pain, is false.

Another instance in which the testimony of one sense alone may lead to a false conclusion as to the whereabouts of the cause of a pain is found in what often takes place after the amputation of a limb. Most people are aware that after part of a limb has been removed by the surgeon’s knife, the patient may still feel as though his arm or leg, as the case may be, was entire, may feel much pain in the foot when the leg has been amputated far above the ankle. Here, in recovering from the effects of the anæsthetic, were it not for the additional evidence of his eyesight, the patient might well doubt whether his limb had been removed at all. The amusing story, in Marryat’s Jacob Faithful, of the old sailor who, having two wooden legs, was accustomed at times to wrap them up in flannel on account of the rheumatic pains which he said he felt in them, is not so very extravagant after all. It is not, however, altogether correct, as it represents the man feeling these pains in his legs long after they had been amputated. As a matter of fact, the false impression passes off before very long. The explanation given by physiologists is as follows: The severed nerve in the stump is irritated and gives rise to pain; and inasmuch as irritation to this nerve-trunk has hitherto been always caused by irritation of its ultimate filaments distributed to the foot and leg, the mind continues for some time to believe that the sensation still proceeds from thence.

We may glance at another and very similar instance of referred sensations occurring also in surgical practice. Amongst the rarer operations of what is termed plastic, and, by Sir James Paget, ‘decorative’ surgery is that by which a new nose is formed by calling in the aid of the tissue of other parts of the body. This has been done by bringing a flap of skin cut from the forehead down over the nasal bones. The flap retains its connection with the deeper tissues at a point between the eyes by means of a small pedicle, and thus its blood-vessels and nerves are not all severed. This flap is not simply pulled down from the forehead—it is twisted at the pedicle, so that the raw surface lies on the bones of the nose. Now, for some time after this operation has been performed, any irritation in the nose is referred by the mind to that part of the forehead from which the flap of skin was taken; and therefore, if a fly crawls over the patient’s nose, it appears to him to be creeping across his forehead. Before the operation, whenever the nerve-ends in the flap were irritated, it was caused by something touching the forehead, and it is some time before the mind ceases to refer such irritation to that part of the face.

Leaving, now, the domain of surgery, we may notice two simple experiments mentioned by physiologists, which all can perform for themselves. They both prove that conclusions formed upon the evidence of the sense of touch alone may be quite incorrect. By crossing the second finger over the first, and then placing a marble between the tips of the fingers, we get a sensation that leads us to suppose that there must be two marbles instead of one only. This is because two points in the fingers are touched simultaneously, which in the ordinary position could only be touched at the same moment by two marbles. Judging, then, from the sense of touch alone, the mind infers that there are two round hard substances beneath the finger-tips; but the evidence of eyesight and the knowledge that we have placed but one marble in position, corrects the misapprehension. Again, if we take a pair of compasses the points of which are not sufficiently sharp to prick the skin, and separating the extremities rather more than an inch from one another, draw them across the cheek transversely from a little in front of one ear to the lips, we shall be tempted to think, from the evidence of touch alone, that the points are becoming more widely separated. By measuring the distance between the two points afterwards, we can assure ourselves that this has not been so; but whilst the compasses were being drawn along the cheek, and still more when they had reached the lips, the impression that the distance between the points increased was very strong. This delusion is said to depend upon the fact, that some parts of the cutaneous covering of the body are much more plentifully supplied with nerves than others. It is stated that the mind probably forms its idea of the distance between two points on the skin which are irritated in any way—as, for instance, by the points of a pair of compasses touching the surface—by the number of nerve-endings lying between these two points which remain unirritated. Thus, if there be fewer unirritated nerve-endings lying between the two points of the compasses when placed on the cheek, than there are when they are placed at the lips, the mind will infer that the distance between these points is smaller in the former position than in the latter.