Page:Popular Science Monthly Volume 62.djvu/49

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an eye, and was only afterward made to retrovert into such rudimentary conditions.

This explanation is, of course, the merest guess-work, resulting from a fancied resemblance between an eye and the shape of a pattern, but by a singular chance, the theory comes nearer the truth than one would at first suspect, since these scrolls and loops, not only of the fingers, but those of the palm as well, are in reality rudiments, not of eyes, but of walking pads; a conclusion which, although it may seem at first a little fanciful, is reached by a simple course of reasoning and rests upon the comparison of easily available data, namely an inspection of the volar, or lower surfaces of the paws of various mammals.

The most typical, that is, the least modified mammalian paw is one with five toes and in which the entire volar surface, from the tips of the digits back to the wrist or heel, comes in contact with the ground during the act of walking. In such paws, as, for example, those found in most rodents and in many of the carnivora, it will be seen that the weight is borne by a series of ten projecting pads or permanent callosities, five of which are situated at the ends of the digits, while the remaining five are placed upon the surface of the palm' or sole, and possess the definite arrangement shown in the accompanying diagram, (Fig. 2, a) namely, one each for the thenar and hypothenar areas, the raised portions associated respectively with the inner and outer margins, and three placed in a transverse row at the bases of the four fingers and corresponding to the intervals between them. In accordance with their positions, these pads may be conveniently named the thenar, the hypothenar and the first, second and third palmar (or plantar, in the case of the hind foot). Those at the ends of the digits may be designated as apical and numbered consecutively from one to five.

In actual cases, owing to the various modifications necessitated by habit and environment, such a diagrammatic arrangement is seldom completely realized, but often in an embryo, before the special modification characteristic of the adult form has been introduced, the condition closely approximates the typical one (Fig. 2, b). How these modifications may affect the original plan may be well seen by a comparison of the fore-paws of the mink and the common cat (Fig, 2, c and d), two carnivores representing different stages in the coalescence of the three palmar pads to form the characteristic cushion adapted for silent progression. In the mink these pads are still semi-distinct and the middle one exceeds the others in size, suggesting that, in the cat, this latter element forms the main bulk of the cushion, a conclusion proved to be the fact from the study of kitten embryos, as,