Hence the form by itself alone reveals the infantile character of the cerebral volume, which, in relation to the bulk of the body is of far greater dimensions than in the adult. In fact, if a child simply increased in volume and its growth was not the sum total of a morphological evolution, the adult man would become a monster; his macrocephaly would be so exaggerated that his neck could not sustain the weight of the head (If the relations between the proportions in infancy were maintained through life the adult man would have a head with a perimeter of 130 centimetres, = 4 ft. 3 in.).
Aside from its mechanical relations to the volume, the form has characteristics dependent upon biological factors, such as the sex and the race. The female cranium in fact has a straighter forehead than the male and the orbital arches are absolutely wanting, while the entire surface of the cranium is smoother and more rounded.
Similarly, the different races exhibit forms determined by biological factors and not by mechanical causes—for instance, the degree of dolichocephaly (elongated cranium) and of brachycephaly (short cranium).
Hence the form is life's manifestation not only of the characteristics proper to the species, but also of the mechanical adaptations demanded by the material composing the body.
It may be said that the volume and the form of the cranium are dependent upon two different biological potentialities: the volume is mainly determined by the cerebral mass; the form, on the contrary, is mainly determined by the bony structure—no matter how completely form and volume coincide in their reciprocal mechanical relations.
That is, the attainment of a given volume of head depends upon the development of the brain; the bone follows this development passively, is the index of it, the skeletal representation of it, but never the determining factor.
At one time it was thought, on the contrary, that a precocious ossification of the cranial cavity would arrest the development of the brain; microcephaly was believed to be caused by a precocious closing of the sutures of the cranial bones; and there was a certain period when the surgical treatment of microcephaly consisted in the removal of a portion of the cranial bone, in order to allow the brain to develop freely.
Fig. 41. Fig. 42. Dividing line in human skull, as compared with that of gorilla.
