6. The cranial capacity is obtained by adding together these five measurements, the antero-posterior diameter, maximum transverse diameter, antero-posterior curve, transverse curve, and maximum circumference. For a normal male the capacity is generally 92 inches (1500 c.c).
7. The cephalic index is obtained by multiplying the maximum width by 100 and dividing the product by the maximum length, according to the following formula:
If the longitudinal diameter is 200 and the transverse diameter 100, the cephalic index is 10,000 divided by 200 = 50.
The cephalic indices of degenerates, like their height, have only a relative importance; that is, when they are compared with the mean cephalic index prevalent in the regions of which the subject is a native. The cephalic index of Italians varies between 77.5 (Sardinians) and 85.9 (Piedmontese).
Skulls are classified according to the cephalic index, in the following manner:
Hyperdolichocephalic under 66 Dolichocephalic 66-75 Subdolichocephalic 75-77 Mesaticephalic 77-80 Subbrachycephalic 80-83 Brachycephalic 83-90 Hyperbrachycephalic above 90
We shall find among criminals frequent instances of microcephaly, macrocephaly, and asymmetry, one side of the head being larger than the other. Sometimes the skull is pointed in the bregmatic region (hypsicephaly), sometimes it is narrow in the frontal region in correlation to the insertion of the temporal muscles and the excessive development of the zygomatic arches (stenocrotaphy, see Fig. 5, Part I., Chapter I.), or depression of the bregmatic region (cymbocephaly).
_Face._ We have already remarked on the excessive size of the face compared with the brain-case, owing chiefly to the high cheek-bones, which are one of the most salient characteristics of criminals, and to the enormous development of the jaws, which gives them the appearance of ferocious animals (see Fig. 5). To these peculiarities may be added progeneismus, the projection of the lower jaw beyond the upper, a characteristic found only in 10% of normal persons, receding forehead as in apes, and the lemurian apophysis already mentioned.
_Arms and Hands._ With the exception of the excessive length as compared with the stature, anomalies in the arms are rare, but the hands show some interesting characteristics, which have already been described in the first chapter of Part I, an increase or decrease in the number of fingers and syndactylism or palmate fingers. Also the lines in the palm and those on the palmar surfaces of the finger-tips show deviations from the normal type resembling characteristics of apes.