As a matter of fact, our bilateral symmetry is an ideal standard rather than an absolutely attainable reality; we are all of us a little larger on one side and a little smaller on the other, but to so slight a degree as to escape superficial observation, so that in general we have apparently a bilateral symmetry—that is, we appear to be symmetrical according to the testimony of our senses; but a more delicate examination proves that this is not true. Plagiocephaly therefore represents an exaggerated case of a normal fact. Plagiocephaly may be simple or compound; it is simple when the asymmetry is partial; namely, when it is confined to the anterior or posterior portion; it is compound when it is total; and in such case we find a complete diagonal correspondence: for instance, if the right nodule in the frontal region is more prominent, the left nodule is more prominent in the left occipital region, or vice versa. In general it may be said that the various forms of plagiocephaly are produced by asymmetry of the nodules or of the flattened surfaces of the cranium. Even in the case of microcephaly and of macrocephaly, which are substantially anomalies of volume, we find corresponding characteristic abnormalities of form. The microcephalic cranium is of inferior type, suggesting that of the ape—in other words, it is a cranium which has mechanically adapted itself to a brain of inferior volume: the macrocephalic cranium, especially if the abnormality is due to rickets or to hydrocephaly, calls to mind the infantile type of cranium; it has the characteristic bulging forehead, while mechanical adaptation frequently renders it very round (pathological brachycephaly). We will take up this question again when we come to speak in particular of malformations and to describe the technical methods of cranioscopy. What more particularly concerns us now is a consideration of the normal form of the cranium and its morphological evolution.
Fig. 49.—Cranium of new-born child (lateral norm).
Fig. 50.—Cranium of new-born child (vertical norm).
Fig. 51. Fig. 52.
Scaphocephalic cranium.
Fig. 53.—Cranium of new-born child seen from above, showing polyhedric contour due to nodules of ossification; fontanelle of the bregma; and suture dividing the two frontal bones.