Since better understood, the subject of facial prognathism has lost
much of its allure in anthropology; yet the matter is not wholly
without interest.
Facial protrusion is as a rule secondary to and largely caused by
alveolar protrusion, which in turn is caused by the size and shape of
the dental arch; and the dental arch is generally proportional to the
size of the teeth. The form of the arch is, however, quite influential.
With the teeth identical in size a narrow arch will be more, a broad
arch less protruding, and a narrow arch with small teeth may protrude
more than a broad one with larger teeth. Another influence is that of
the height of the upper face, the same arch protruding more in a low
face than in a high one. And still another factor is the incline of the
front teeth, though this affects merely the appearance of prognathism
and not its measurements.
There are different ways of measuring facial prognathism, and with
sufficient care all may be effective; I prefer, for practical reasons,
linear measurements from the basion, which, together with the facial
and subnasal heights, give triangles that can readily be reconstructed
on paper and allow a direct measurement of both the facial and the
alveolar angle. The three needed diameters from basion are taken, the
first to the "prealveolar point," or the _most anterior_ point on the
upper dental arch above the incisors; the second to the "subnasal
point," or the point on the left (for convenience) of the nasal
aperture, where the outer part of its border passes into that which
belongs to the subnasal portion of the maxilla (the point where the
subnasal slant begins); and the third to nasion. The facial height is
that from the alveolar point (_lowest_ point of the upper alveolar
border in the median line) to nasion; while for the subnasal height,
which can not be measured directly, I utilize the difference between
the facial and nasal heights, which is very close to the needed
dimension.
The important basion-nasion diameter has already been considered. That
to the subnasal point needs no comment. That to the prealveolar point
shows in the western and other Eskimo as follows: