tion: Fig. 72.--Left Cavities of the Heart.

A, B, right pulmonary veins;
with S, openings of the veins;
E, D, C, aortic valves;
R, aorta;
P, pulmonary artery;
O, pulmonic valves;
H, mitral valve;
K, columnae carnoeae;
M, right ventricular cavity;
N, interventricular septum.
]

The flow of blood in the arteries is caused by the muscular force of the
heart, aided by the elastic tissues and muscular fibers of the arterial
walls, and to a certain extent by the muscles themselves. Most of the
great arterial trunks lie deep in the fleshy parts of the body; but their
branches are so numerous and become so minute that, with a few exceptions,
they penetrate all the tissues of the body,--so much so, that the point
of the finest needle cannot be thrust into the flesh anywhere without
wounding one or more little arteries and thus drawing blood.

188. The Veins. The veins are the blood-vessels which carry the
impure blood from the various tissues of the body to the heart. They begin
in the minute capillaries at the extremities of the four limbs, and
everywhere throughout the body, and passing onwards toward the heart,
receive constantly fresh accessions on the way from myriad other veins
bringing blood from other wayside capillaries, till the central veins
gradually unite into larger and larger vessels until at length they form
the two great vessels which open into the right auricle of the heart.

These two great venous trunks are the inferior vena cava, bringing
the blood from the trunk and the lower limbs, and the superior vena
cava, bringing the blood from the head and the upper limbs. These two
large trunks meet as they enter the right auricle. The four pulmonary
veins, as we have learned, carry the arterial blood from the lungs to
the left auricle.

[Illustration: Fig. 73.

A, part of a vein laid open, with two pairs of valves;
B, longitudinal section of a vein, showing the valves closed.
]

A large vein generally accompanies its corresponding artery, but most
veins lie near the surface of the body, just beneath the skin. They may be
easily seen under the skin of the hand and forearm, especially in aged
persons. If the arm of a young person is allowed to hang down a few
moments, and then tightly bandaged above the elbow to retard the return of
the blood, the veins become large and prominent.

The walls of the larger veins, unlike arteries, contain but little of
either elastic or muscul