There are various curves; we shall confine ourselves to considering only the following:
The maximum circumference, which is obtained by passing the tape across the ophryon, the occipital points and the supra-auricular points, beginning to apply it at the ophryon. Its measure varies from 520 to 540 mm. in man and from 490 to 510 mm. in woman, if taken from the skull. In the case of a living person 20 mm. should be added.
If we find a circumference greater than normal, we are beginning to enter upon the anomaly which goes by the name of macrocephaly. If, on the other hand, the maximum circumference is notably smaller, we are entering upon the anomaly of microcephaly.
Measurement of Diameters.—Maximum Antero-posterior Diameter.—With the left hand place one branch of the calipers upon the glabella; the other extreme point is to be sought tentatively along a vertical line dividing the occiput in two halves. Partially close the calipers by means of the screw and then make trial by raising and lowering the posterior branch. It ought to move with a slight friction.
This is the classic diameter which measures the maximum length of the cranium and which, as we have seen, it is customary to compare with the width in order to obtain the cephalic index. In the adult man it normally oscillates between 170 and 180 mm.
Fig. 149.—Inspecting cranium (lateral and vertical norms).
Maximum Transverse Diameter.—This measures the width of the cranium. The investigator places himself in front of the subject in order to keep the compass quite horizontal through the guidance of the eyes. The maximum distance is found by experimenting. It normally corresponds very nearly to the supra-auricular points. In children this diameter is frequently situated higher up toward the parietal nodules; in men of tall stature, in whom the cranial vault is generally slightly developed, this diameter may be found, on the contrary, lower down, near the mastoid apophyses. If this diameter occurs similarly low down in children, a notable growth in stature may be prophesied (Manouvrier); and if inquiry is made it will be found that the parents are very tall. This diameter measures, in the adult, from 140 to 150 mm.
Vertical Diameter.—This measures the height of the cranium from the occipital foramen to the bregma. This diameter cannot be measured directly excepting on a skull; in the case of a living person its projection is taken, which, though far from accurate, is given by the distance between the vertex and the external auditory meatus.