151 
ing 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 revo- 
lution of the side and roof bones of the skull upon the basi- 
cranial 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. 
Fie. 30.—Sections of orthognathous (light contour) and prognathous (dark 
contour) skulls, one-third of the natural size. ab, Basicranial axis; be, b' c’, 
plane of the occipital foramen ; dd’, hinder end of the palatine bone ; e e’, front 
end of the upper jaw ; Z’Z7’, insertion of the tentorium. 
