We should not, however, adhere solely either to the psychological theory of the hand, or to the theory of adaptation; it is necessary to consider the characteristics of the hand from several different points of view.
Dimensions.—The dimensions of the hand bear a constant relation to the stature and to certain partial dimensions of the body, while the various parts of the hand preserve constant reciprocal proportions.
As far back as in the time of Vitruvius it was known that the human hand is related to the stature in the proportion of 10 to 100. This is a very important fact to know, because the proportion varies in the inferior races and in the anthropoid apes, the descent in the scale showing a corresponding increase of length of hand relatively to the stature. Thus, for example, in the Mongolian races the proportional length of the hand is 12.50, and in the higher apes it equals 18. Consequently too long a hand is in itself an anomaly that indicates a low type of man; it is to be classed with those anomalies that were formerly regarded as atavistic reversions, phenomena of absolute retrogression in the biological scale.
Relations between the Hand and the other Dimensions of the Body.—The closed fist, taking the extreme outside measurement between the metacarpophalangeal articulations, corresponds to the breadth of the heart.
The length of the hand corresponds to the height of the visage, and also to the distance intervening between the sternal incisura and the auricular foramen; it is also equal to the distance between the two nipples, and therefore also corresponds to the depth of the chest.
There may be hands which are either excessively large or much too small, and that are really marks of degeneration. An excessive volume of these members is called megalomelia, and an excessive smallness oligomelia.
We may encounter an extremely small hand quite as often in the son of an alcoholic labourer as in the son of a degenerate aristocrat; frequently men whose parents were mentally deficient have small, delicate, almost effeminate hands.
The Proportions between the Various Segments of the Hands.—The length of the middle finger, measured from the digito-palmar plica or fold, ought to equal the length of the palm.