nsidered as a continuation of the veins and the
arteries, forming their internal lining. The walls of the ventricles are
thicker than those of the auricles, and the walls of the left ventricle are
much thicker than those of the right.

The heart is enveloped by a fibrous sac (or bag), called the pericardium,
which assumes much of the general shape of the outer surface of the heart.

The action of the heart is similar to that of a pump and its function is to
keep the blood in circulation. The auricles may be considered as the
reservoirs or receivers of the blood and the ventricles as the pump
chambers. During the interval between contractions, the heart being in
momentary repose, the blood pours into the auricles from the veins; the
auriculo-ventricular orifices being widely open, the ventricles also
receive blood; the auricles contract and the ventricles are filled;
contraction of the ventricles follows; the auriculo-ventricular valves are
forced up by the pressure of the blood and close the auriculo-ventricular
openings and prevent the return of blood into the auricles; the contraction
of the ventricles forces the blood from the right ventricle into the lungs
through the pulmonary artery and its branches, and from the left ventricle
into the aorta, thence through the arteries to all parts of the body. After
the contraction of the ventricles the heart is again in momentary repose
and is being filled with blood, while the valves in the aorta and pulmonary
artery close to prevent the return of blood into the ventricles. (See Pl.
VII.)

The average weight of the heart of an ox is said to be from 3-1/2 to 5
pounds; but, of course, owing to the many breeds and sizes of cattle, it
must vary in different animals.

The vessels that convey the blood from the heart to all parts of the body
are called arteries; those which return the blood to the heart are called
veins. The arteries divide and subdivide (like the branches of a tree),
become smaller and smaller, and ultimately ramify into every part of the
body. Between the ultimate ramifications of the arteries and the beginning
of the veins there is an intermediate system of very minute vessels called
capillaries, which connect the arterial with the venous system of the
circulation. The walls of the arteries are possessed of a certain amount of
rigidity, sufficient to keep the tubes open when they are empty.

The blood leaves the left ventricle through a single vessel, the com