and vomer--the "basifacial axis" ('f
e'.) forms an exceedingly obtuse angle, where, when produced, it cuts
the 'basicranial axis.'

If the angle made by the line 'b c'. with 'a b'., be called the
'occipital angle,' and the angle made by the line 'a d'. with 'a b'. be
termed the 'olfactory angle,' and that made by 'i T'. with 'a b'. the
'tentorial angle,' then all these, in the mammal in question, are nearly
right angles, varying between 80 degrees and 110 degrees. The angle 'e f
b'., or that made by the cranial with the facial axis, and which may be
termed the 'cranio-facial angle,' is extremely obtuse, amounting, in the
case of the Beaver, to at least 150 degrees.

But if a series of sections of mammalian skulls, intermediate between a
Rodent and a Man (Figure 28), be examined, it will be found that in the
higher crania the basicranial axis becomes shorter relatively to the
cerebral length; that the 'olfactory angle' and 'occipital angle' become
more obtuse; and that the 'cranio-facial angle' becomes more acute by
the bending down, as it were, of the facial axis upon the cranial axis.
At the same time, the roof of the cranium becomes more and more arched,
to allow of the increasing height of the cerebral hemispheres, which is
eminently characteristic of man, as well as of that backward extension,
beyond the cerebellum, which reaches its maximum in the South America
Monkeys. So that, at last, in the human skull (Figure 29), the cerebral
length is between twice and thrice as great as the length of the
basicranial axis; the olfactory plane is 20 degrees or 30 degrees on the
'under' side of that axis; the occipital angle, instead of being
less than 90 degrees, is as much as 150 degrees or 160 degrees; the
cranio-facial angle may be 90 degrees or less, and the vertical height
of the skull may have a large proportion to its length.

It will be obvious, from an inspection of the diagrams, that the
basicranial axis is, in the ascending series of Mammalia, a relatively
fixed line, on which the bones of the sides and roof of the cranial
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