There are also certain forms of cranium having the frontal region more restricted than the parietal region, or slanting down from a much elevated vertex, which have been proved to be normal forms; while still another error previously made was that of trying to judge the forehead on the criterion of a single model, deviations from which were much too readily relegated to the category of abnormalities. The most regular and beautiful forms, and the ones that are commonest in our racial stocks are the ellipsoid, ovoid and sphenoid. In my work on the women of Latium, precisely one of the points that I noted was the frequent occurrence of certain sub-varieties of the ellipsoid and the sphenoid.
In order to recognise the forms of the cranium, a certain training is necessary which each one must acquire for himself. Observations of the cranium will make it easier to judge of the form in relation to the head, at least, when the latter is not too much hidden by the hair, as often happens in the case of young children.
A knowledge of the normal forms of the cranium will also guide us in our judgment of many abnormal forms, which very often present the appearance of exaggerations of normal types.
Thus, for example, the acrocephalic cranium (much raised in the parieto-lambdoideal region and sloping forward toward the brow, while the occipito-lambdoideal region is flattened) recalls the trapezoid; and the clinocephalic cranium (in which the coronal suture forms a slight girdle-like indenture and divides the contour of the cranium, when observed along the vertical norm, in two curves, a lesser anterior and a greater posterior curve, resembling a figure of 8) recalls certain varieties of ovoid cranium described by Sergi. This brings us to a principle that is very interesting to establish, namely, that frequently anomalies represent exaggerations of the racial or family type.
The Cephalic Index
Retzius was the first to take the cranium under consideration as a basis for a classification of the human races; and he attempted to determine a concept of its form by means of a numerical formula expressing the relation between the length and width of the cranium (cephalic index). Thus he distinguished the races into brachycephalics, or those having a short head; and dolichocephalics, or those having a long head. Following Retzius, who may be regarded as the founder of craniology, Broca adopted, completed and expanded this method, deriving from the cranium, or rather from the particular character given by the cephalic index, a key, as it were, suited to unlocking the intricate mysteries of hybridism among the human races. Consequently the cephalic index was not confined, as regards its importance, within the same limits as all the other indexes, but was raised by the French school, warmly seconded by Italian anthropologists, to the dignity of a fundamental determinant of the ethnic type, as definitely as, for example, the vertebral column serves as basis for a classification including all species of vertebrates.
The Germans refused to accept the cephalic index as determining the classification of races; but while seeking to prove themselves independent of it, they continued to regard the form of the cranium as a basis of classification (Rütimeyer, von Höller, and to-day Virchow), but without ever having identified, as Sergi has now done, existing forms as normal types of race.
The cephalic index is obtained by the well-known formula expressing the relation between the maximum transverse diameter of the skull (see "Technique") and the maximum longitudinal diameter reduced to 100, and is expressed as follows: Ci = 100×d/D (the cephalic index is equal to a hundred times the lesser diameter divided by the greater; in the present case the lesser diameter is the transverse).
This proportion between linear measurements cannot properly sum up the form of the cranium. We can, for example, conceive of a microcephalic cranium having a normal cephalic index, since the relation between the two maximum diameters necessary for deducing the index, does not tell us, for example, either the dimension of the cranium or the form of the forehead.
If, for instance, we should imagine a photograph of a cranium enlarged a hundred diameters, the reciprocal relations between the length and the width would still remain unchanged.