Sensory Functions of Skin

However insensitive the mechanism may be, the sensation itself has a peculiarly vivid and explosive quality and differs from all other sensations in arousing in the subject of it a strong impulse to make some kind of motor response. The barely controllable flinching with which we suffer the mildest of pin-pricks is familiar evidence of this unique character of the pain sensation.

In comparison with other senses, pain then is seen to be highly anomalous. Its anomalies have been the subject of much theo retical discussion from which however no generally accepted doc trine has yet emerged.

Distribution of Sensibility.

It has long been known that the whole of a given area of skin is not equally sensitive. If its surface is explored with stimuli sufficiently fine, it is found that in some places no response is obtained to any stimulus and that sensitiveness is limited to certain small areas mostly less than a millimeter across. These spots are distinct for each kind of stimulus and respond to it only, so that there are separate spots for touch, heat, cold and pain.

The sensitiveness of a given region depends on whether the distribution of sensitive spots is thick or thin. As a general rule density of distribution varies together for all four senses so that a part sensitive to one stimulus is sensitive to all. To this how ever there are important exceptions. The skin of the face and the skin of the finger tip are both highly sensitive to touch but the latter is much the less sensitive to pain. A far more striking anomaly however is that certain regions—the cornea of the eye and a part of the glans penis—possess sensibility to pain only. It is difficult to give a functional explanation of this curious fact but it is interesting to note that these two regions have this in common that their embryological development includes a longi tudinal splitting of the skin into two layers.

The Peripheral Sensory Mechanisms.

The sensory nerve fibres of the skin terminate in certain well-marked and minute structures called end-organs which are regarded as having the power to originate under appropriate stimulation the nerve-impulse which yields the characteristic sensation. These end-organs show a great variety of structure, but they are clearly divisible into two groups. First there is a group in which the end-organ consists of a mere breaking up of the nerve fibre into many fine naked branches among the cells of the part ; this "terminal aborisation" is the end-organ of the pain nerve. Secondly there is a group the members of which though varying much have this in common— that the end of the nerve fibre is enclosed in a definite and often thick capsular structure. These end-organs are concerned with the sensations other than pain. This marked structural difference between the encapsuled or insulated and the naked or uninsulated end-organs is yet another peculiar character of the pain sense.

Into the intimate nature of the process whereby a stimulus to an end-organ causes an impulse in the nerve fibre we cannot as yet penetrate. The less exacting problem of what is the precise quality of the stimulus is as we have seen still unsolved for the pain sense. For the thermal and tactile senses the stimulus can

be reduced to the simple physical processes of heat transference and movement respectively. In the case of touch we can even define in some detail the mechanisms by which very light touches are enabled to produce the movements which are essential to adequate stimulation of the touch-spots.

Each touch-spot is found to be placed close to a hair and the latter can be seen to move when the touch-spot is pressed on and sensation aroused. It is probable that the rigid root of the hair acts as a lever which magnifies the movement set up by the actual touch. The adequacy of the mechanism is shown by the experimentally established fact that the touch-spots of a rather coarse hairy skin are slightly more sensitive than those of a fine smooth skin. When the projecting part of a hair itself is gently touched a vague tickling sensation is felt quite unlike the clear light "pat" elicited by proper stimulation of a touch-spot. The hair is too flexible to transmit the whole movement intact to the end-organ and a sub-minimal stimulus results. This sensation of "tickle" is comparable with the sensation of itching which is probably the result of sub-minimal stimulation of pain nerves. It seems likely that the great reduction in hairiness shown by man's skin in comparison with that of other animals was favoured by the precision it gave to touch through the elimination of tickling sensations.

The hair bulb is then the ordinary means by which movement is transmitted to tactile end-organs. Where the skin is hairless however, as it is in some of its most sensitive parts, other mecha nisms are necessary. The simplest substitute is to keep the sur face layers of the skin flexible by reducing the amount of horny material in them and to keep them moist. This is the condition of the red surface of the lips which is highly sensitive to touch but at the expense of a high degree of vulnerability.

No such solution of the problem is possible in the fingers which must be kept sensitive and yet allowed a thick and horny protective covering. There the thick horny layer is grown on long closely parallel ridges with grooves between ; the grooves are lines of di minished rigidity and permit the thick epidermis to remain highly flexible and therefore sensitive to touch. These ridges and grooves that by their individual uniqueness and constancy have proved so gratifying to the criminologist, are thus seen to have another and perhaps a higher function in serving the great sense of touch.

Bibliographical Note.

The sensory functions of the skin are not usually dealt with very fully in text-books of physiology. An elaborate account of the subject (with references to the work of other investigators) will be found in Sir Henry Head's "Studies in Neurology"-192o. A recent important contribution by Sir E. Sharpey-Schaf er appears in the Quarterly Journal of Experimental Physiology—August 1928.