Stimuli and Response.
The skin over the trunk and limbs of man is exposed to stimuli of pressure, friction, heat, cold and wind in very different degrees, according to the part which it covers. I do not here refer to nocuous, or so-called noci-cipient stimuli, as being too casual in their incidence for the question in hand. Broadly the ventral surface of the neck and trunk differ much, in respect of the qualities of their epidermis, from the dorsal. The skin over the former is softer, thinner and more flexible than the latter, which is in adult life thick, hard and with larger openings of the sebaceous glands. As the two main layers of the skin are so closely united it is impossible to state any general rule as to the parts played in this manufacture by the epidermis and dermis respectively. Altogether the skin from the dorsal surfaces of mammals provides a much denser fabric than the latter, and different qualities of leather are obtained from different regions. Corresponding differences of texture are found on the extensor and flexor surfaces of the limbs, especially on the hands and feet. In the course of his long evolution from a hairy stock, whether simian as we thought yesterday, or a lower one as Professor Woods Jones suggests to-day, these dorsal surfaces of neck, trunk and extensor surfaces of limbs have been exposed through countless generations of men to vastly more stimuli of friction, pressure, and response, than those of the ventral and flexor regions. As man’s hairy covering diminished, through some mysterious and at present unrecognised cause, these stimuli became increasingly potent in producing a tissue denser than that of the more protected ventral parts where all forms of these stimuli are slight. I do not claim that this was a phenomenon that began with man, for in a measure it was present in those forms which preceded him, and in many related mammals under the cover of their hairy covering.
When we remember, or conceive what a large portion of each of his 24-hours even in his earliest form throughout life man must have spent, as he still does, in lying on his back or sides, and in sitting with his back against a supporting object, and with his gluteal and ischial regions pressed hard against whatever seat he has selected in cave or drawing-room, we need not travel far in thought to understand how great has been the preponderance of stimuli from friction and pressure on the dorsal and extensor surfaces over those on the ventral and flexor—and here comes in our familiar “total experience” with stimulus and response spread over a vast stretch of time. It must be borne in mind that from the facts of the case a very large number of individual men and women were exposed to similar, but not the same stimuli at each stage of the process involved. It is matter of common knowledge that not only on the palm and sole of man, but on regions where the skin is not specialised in that remarkable manner that is found in those regions, but also in others, that increased pressure and friction will very soon cause a harder and thicker growth of epidermis, as on the skin over a projecting bone in club-foot, over the shoulder where a weight is constantly carried, on the knuckles of many manual workers, and over the patellæ of a devout Roman Catholic, as I have often seen.
On the other hand what conditions more calculated to thin and soften the skin could exist than those operating on the ventral and flexor surfaces, axillæ, groins, external genitals and the bends of the elbow and knee-joints, where pressure, with little friction and greater warmth and moisture prevails? I need do no more than ask which is the more reasonable of the two forthcoming explanations of such phenomena, on the one hand that they are adapted for, and on the other adapted by this experience? I doubt if at any stage of the long process this slow manufacture of differing fabrics ever conferred on man any survival value or better matrimonial prospects. At any period or stage which I have supposed it can only be claimed for the results on the skin that they did not cause the animal to pass through the meshes of the sieve, and theoretically might be classed among the indifferent modifications, even if they added a little to the comfort of their possessor.