head. And this holds alike with all animals possessed of a brain;
and all blooded animals are possessed thereof, and, by the way,
molluscs as well. But, taking size for size of animal, the largest
brain, and the moistest, is that of man. Two membranes enclose it: the
stronger one near the bone of the skull; the inner one, round the
brain itself, is finer. The brain in all cases is bilateral. Behind
this, right at the back, comes what is termed the 'cerebellum',
differing in form from the brain as we may both feel and see.

    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.

    Inside the neck is what is termed the oesophagus (whose other
name is derived oesophagus from its length and narrowness), and the
windpipe. The windpipe is situated in front of the oesophagus in all
animals that have a windpipe, and all animals have one that are
furnished with lungs. The windpipe is made up of gristle, is sparingly
supplied with blood, and is streaked all round with numerous minute
veins; it is situated, in its upper part, near the mouth, below the
aperture formed by the nostrils into the mouth-an aperture through
which, when men, in drinking, inhale any of the liquid, this liquid
finds its way out through the nostrils. In betwixt the two openings
comes the so-called epiglottis, an organ capable of being drawn over
and covering the orifice of the windpipe communicating with the mouth;
the end of the tongue is attached to the epiglottis. In the other
direction the windpipe extends to the interval between the lungs,
and hereupon bifurcates into each of the two divisions of the lung;
for the lung in all animals possessed of the organ has a tendency to
be double. In viviparous animals, however, the duplication is not so
plainly discernible as in other species, and the duplication is
least discernible in man. And in man the organ is not split into
many parts, as is the case with some vivipara, neither is it smooth,
but its surface is uneven.

    In the case of the ovipara, such as birds and oviparous
quadrupeds, the two parts of the organ are separated to a distance
from one another, so that the creatures appear to be furnished with
a pair of lungs; and from the windpipe, itself single, there branch
off two separate parts extending to each of the two divisions of the
lung. It is attached also to the great vein and to what is
designated the 'aorta'. When the windpipe is charged with air, the air
passes on to the hollow parts of the lung. These parts have divisions,
composed of gristle, which meet at an acute angle; from the
divisions run passages through the entire lung, giving off smaller and
smaller ramifications. The heart also is attached to the windpipe,
by connexions of fat, gristle, and sinew; and at the point of juncture
there is a hollow. When the windpipe is charged with air, the entrance
of the air into the heart, though imperceptible in some animals, is
perceptible enough in the larger ones. Such are the properties of
the windpipe, and it takes in and throws out air only, and takes in
nothing else either dry or liquid, or else it causes you pain until
you shall have coughed up whatever may have gone down.

    The oesophagus communicates at the top with the mouth, close to
the windpipe, and is attached to the backbone and the windpipe by
membranous ligaments, and at last finds its way through the midriff
into the belly. It is composed of flesh-like substance, and is elastic
both lengthways and breadthways.

    The stomach of man resembles that of a dog; for it is not much
bigger than the bowel, but is somewhat like a bowel of more than usual
width; then comes the bowel, single, convoluted, moderately wide.
The lower part of the gut is like that of a pig; for it is broad,
and the part from it to the buttocks is thick and short. The caul,
or great omentum, is attached to the middle of the stomach, and
consists of a fatty membrane, as is the case with all other animals
whose stomachs are single and which have teeth in both jaws.

    The mesentery is over the bowels; this also is membranous and
broad, and turns to fat. It is attached to the great vein and the
aorta, and there run through it a number of veins closely packed
together, extending towards the region of the bowels, beginning
above and ending below.

    So much for the properties of the oesophagus, the windpipe, and
the stomach.

                                  17

    The heart has three cavities, and is situated above the lung at
the division of the windpipe, and is provided with a fatty and thick