III. The palm and sole system.
The method which it is the purpose of this paper to advocate and briefly explain is closely allied to that of Mr. Galton and is, in fact, an extension of his system to the palmar and plantar surfaces, which are covered with the same sort of ridges as are the finger tips and in which the variation is greater and the details larger and more obvious. A moment's inspection of a human hand and foot will show that the entire ventral surface of each, including that of the digits, is covered by a peculiar sort of skin, very different from that found elsewhere, and that along the sides of the palm and the digits, and just above the sole upon the foot, there are definite lines of separation between them and the normal skin, which in the hand corresponds in general position to the seam in a glove which unites the upper and under surfaces. This palmar and plantar skin differs from that of the rest of the body in many ways. It is absolutely hairless and at no time during embryonic life shows indication of either hairs or hair follicles. It consequently has no power of forming goose-flesh when chilled, although the back of the hand and the surface of the forearm, in the immediate vicinity of the palm, are favorite places for the display of this phenomenon. It is also very slightly, or not at all, pigmented as is readily seen by inspection of the palms and soles of a negro, and consequently does not tan or freckle, a distinction often made very obvious by a comparison of the back of the hand with the palm. The most obvious character, however, and the one which directly concerns both Galton's system and the one advocated here, is that of the small but distinct epidermic ridges, which cover the surfaces in question. These may be said to run in a general way parallel to one another and diagonally across the palm or sole, although in certain regions their direction is altered and at more or less definite places they form curiously disposed patterns, usually in the form of loops or spirals. With a moderate lens these ridges give the skin an appearance much like that of corduroy and there may be seen running along the middle of each ridge a row of minute indentations or pores, at about equal distance from one another, the orifices of the perspiratory glands. Running over and across these ridges in directions which bear no relation to them are the wrinkles or rugæ, more abundant in the hand than in the foot, and caused by the various motions of the digits and of the other movable parts of the member. Those seem at first especially obvious and interfere more or less with the study of the ridges, but a little practice will enable one to ignore them altogether. In printed impressions, which are used for purpose of study far more than are the actual surfaces, most of these are pressed out of existence while the remainder appear merely as narrow white streaks which do not affect the investigation (see Figs. 1 and 2).
