Page:Popular Science Monthly Volume 59.djvu/404

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394
THE POPULAR SCIENCE MONTHLY.

roof of the orbit is low like that of the long skull, but the lower border does not extend so far downward and the direction of the transverse diameter is more oblique.

Measurements of the direction of the axis of the orbit in these three classes show that in the long skull the direction is usually quite

PSM V59 D404 Cephalic indexes of skull shapes.png
Fig. 8.
Front view of Long Skull:
Cephalic Index 71.4:100.
Fig. 9.
Front view of Tall Skull:
Cephalic Index 81:100.
Fig. 10.
Front view of Broad Skull:
Cephalic Index 85:100.

low, that in the tall skull it is much higher and that, while the axis of the broad skull is lower than that of the tall one, it is scarcely as low on the average as in the case of the long skull; and these comparative positions of the axes of the orbits in the prepared skulls correspond remarkably with the positions of the visual plane in the case of living subjects with heads of corresponding types. That is, the visual plane of the long head is low, of the broad head also low and that of the tall head is high.

Notwithstanding the apparent simplicity of these relations of the form of the orbit with the type of the skull and of the direction of the visual plane to the type of cranium, there are, in practice, certain modifying features.

The most important of these from the anatomical standpoint is found in the angle of the face. In forming a judgment, therefore, of the probable direction of the normal visual plane in the living subject without resorting to measurements by the tropometer, it is necessary to measure or to estimate this angle. None of the measurements employed for the prepared skull will serve the purpose here, and it has been found most practical to use for fixed points for the measurement of this angle the following: the glabella, which is the elevation above the root of the nose and just between the ridges of bone above the orbits; the depression just below the nose, and the tip of the chin.

If these three fixed points are selected for the measurement of the angle of the face, it will readily be seen that this angle varies greatly in different individuals. It may, indeed, vary considerably in heads belonging to the same type. Yet, on the whole, there is a pretty