The characters that may be observed in the skull are very numerous, and may be divided into descriptive characters, which give an account of the conformation of the bony structure of the head and its parts, and craniometrical characters, which give the dimensions of these parts by exact measurements taken by means of special apparatus or instruments. These two orders of characters are complementary to each other. The cranial characters vary according to race, but within the limits of each race there are other lesser variations according to age and sex.
The general form of the cranium, as also the number, the consistence, and structure of the different parts which compose it are modified as the individual develops and grows older. Formed of a single cartilaginous and membranous substance at the beginning of embryonic life, the cranium is composed in the last fœtal state of a great number of points of ossification of various texture. At birth the number of these points has considerably diminished; they have united for the most part to form the different parts of the bones of the cranium or brain case and the bones of the face; as the child grows, these points grow and end by being contiguous; about the age of eighteen or twenty years they form bones separated by sutures. There are twenty-one separated bones described in classic treatises on anatomy. Later on these bones begin to unite, the sutures which separate them disappear, and in extreme old age the cranium is formed of a bony mass almost as continuous and homogeneous as was the cranial cartilaginous and membranous mass in the embryo. According to the number of the pieces composing the cranium, and also according to their position, structure, and conformation, according to the degree of obliteration of the sutures and the order in which the obliteration of each suture takes place, according to the general form of the forehead, the angle of the lower jaw, according to the volume and dimensions of the skull, and lastly, according to the state of the dentition, etc., the nearly exact age of the individual to whom the skull had belonged may easily be discovered in this cycle of development. Other characters serve to distinguish the sex: the forehead is straight and rounded in woman, retreating in man; the cranial cavity is less in woman than in man in any given race; the orbital edges are sharper in woman, the impress of the muscles less marked, the weight of the skull in general less than that of the masculine skull, etc.[56] Lastly, the characters of race are numerous and special. I shall proceed briefly to enumerate some of them. First in order of importance comes cranial capacity, or the volume of the cavity of the brain-case, which gives an idea of the volume of the brain, and approximately of its weight.
FIG. 10.—Dolichocephalic skull
of an islander of Torres Straits. Cephalic index, 61.9.
(After O. Thomas.)
Cranial capacity may vary to the extent of double the minimum figure (from 1100 cubic centimetres to 2200 cubic centimetres) among normal individuals in the human race. The average capacity for the races of Europe is from 1500 to 1600 cubic centimetres; that of the skulls of Asiatic races appears to be very nearly the same; that of the Negro races and Oceanians a little smaller, perhaps from 1400 to 1500 cubic centimetres on an average. That of the Australians, the Bushmen, and the Andamanese is still less, from 1250 to 1350 cubic centimetres. But it must not be forgotten that the volume of the head, as with its other dimensions, has a certain relation to the height of the individual, and, as a matter of fact, Bushmen and Andamanese are very short in stature; Australians, however, are of average height. Partly, too, to their disproportion of height must, probably, be attributed the difference between the volume of the cranium in man and in woman. According to the series examined, this sexual difference may extend from 100 to 200 cubic centimetres, and even beyond, in favour of man. The cranial capacity of woman represents from eighty-five to ninety-five of the cranial capacity of man.[57] The cranial capacity of lunatics, of certain criminals, and especially of celebrated or distinguished men, scholars, artists, statesmen, etc., appears to be slightly superior to the average of their race. We shall revert later to the question of cranial capacity in connection with weight of brain.
FIG. 11.—Brachycephalic skull
of a Ladin of Pufels (Tyrol). (After Holl.)
The general form of the brain-case is an oval, but this oval may be more or less rounded, quite globular (Fig. [11]), or more or less elongated to resemble an ellipse, the major axis of which is almost double the minor (Fig. [10]). The numerical expression of the cranial form is given in anthropology by what is called the cephalic index—that is to say, by the relation of the length of the cranium (ordinarily measured from the glabella to the most prominent point of the occiput (Figs. [10] and [13], A B) to its greatest breadth (Fig. [10], C D, Fig. [12], M N)). Reducing uniformly the first of these measurements to 100, we obtain the different figures for the breadth, which expresses the cranial form; thus very round skulls (Fig. [11]) have 85, 90, and even 100 (extreme individual limit) for index, while elongated skulls (Fig. [10]) may have an index of 70, of 65, and even of 58 (extreme individual limit). According to Broca’s nomenclature, skulls having indices between 77.7 and 80 are mesaticephalic or mesocephalic; those having the indices below this figure are sub-dolichocephalic (up to 75), or dolichocephalic (beyond 75, Fig. [10]); those which have the index above 80 are sub-brachycephalic (up to 83.3), or brachycephalic (above 83.3, Fig. [11]).[58] Peoples or ethnic groups being formed of various elements, it is in most cases impossible to determine, after the examination of an isolated skull, to which population it belongs; all that can be said is that the skull is brachy- or dolicho-cephalic, orthognathous or prognathous, etc. We must have a certain number of skulls (from ten to thirty at least, according to the homogeneity of the population) to be able to discern the constituent elements of this population as far as they are manifested in the cranial characteristics. The average measurements are then deduced from a given number of skulls, by adding the individual measurements and dividing them by the number of skulls examined. But the average of any measurement whatever only gives a very general and somewhat vague idea of the actual dimensions of skulls. To determine it we must co-ordinate and seriate these skulls—that is to say, arrange them, for example, in an ascending order of figures expressing their cephalic index. In this manner we can discover one or several indices around which the skulls are grouped in the largest number. It is thus that we can often discern two or three cranial elements in the same population.[59]