The back

of the head is with all animals empty and hollow,
whatever be its size in the different animals. For some creatures have
big heads while the face below is small in proportion, as is the
case with round-faced animals; some have little heads and long jaws,
as is the case, without exception, among animals of the
mane-and-tail species.

The brain in all animals is bloodless, devoid of veins, and
naturally cold to the touch; in the great majority of animals it has a
small hollow in its centre. The brain-caul around it is reticulated
with veins; and this brain-caul is that skin-like membrane which
closely surrounds the brain. Above the brain is the thinnest and
weakest bone of the head, which is termed or 'sinciput'.

From the eye there go three ducts to the brain: the largest and
the medium-sized to the cerebellum, the least to the brain itself; and
the least is the one situated nearest to the nostril. The two
largest ones, then, run side by side and do not meet; the medium-sized
ones meet-and this is particularly visible in fishes,-for they lie
nearer than the large ones to the brain; the smallest pair are the
most widely separate from one another, and do not meet.