cavity, and of the face, may be said to revolve downwards and forwards
or backwards, according to their position. The arc described by any
one bone or plane, however, is not by any means always in proportion to
the arc described by another.

Now comes the important question, can we discern, between the lowest and
the highest forms of the human cranium anything answering, in however
slight a degree, to this revolution of the side and roof bones of the
skull upon the basicranial axis observed upon so great a scale in the
mammalian series? Numerous observations lead me to believe that we must
answer this question in the affirmative.

The diagrams in Figure 29 are reduced from very carefully made diagrams
of sections of four skulls, two round and orthognathous, two long and
prognathous, taken longitudinally and vertically, through the middle.
The sectional diagrams have then been superimposed, in such a manner,
that the basal axes of the skulls coincide by their anterior ends, and
in their direction. The deviations of the rest of the contours (which
represent the interior of the skulls only) show the differences of the
skulls from one another, when these axes are regarded as relatively
fixed lines.

The dark contours are those of an Australian and of a Negro skull: the
light contours are those of a Tartar skull, in the Museum of the Royal
College of Surgeons; and of a well developed round skull from a
cemetery in Constantinople, of uncertain race, in my own possession.

It appears, at once, from these views, that the prognathous skulls, so
far as their jaws are concerned, do really differ from the
orthognathous in much the same way as, though to a far less degree