ee pouch-like valves, called the _semilunar valves_,
from their resemblance, in shape, to a half-moon. Being placed on a
level and meeting in the middle line, they entirely prevent the passage
of any fluid which may be forced along the artery towards the heart,
but, flapping back, they offer no obstruction to the free flow of blood
from the ventricles into the arteries.

[Illustration: Fig. 42.
A representation of the venous and arterial
circulation of the blood.]

The _Arteries_, being always found empty after death, were supposed by
the ancients, who were ignorant of the circulation of the blood, to be
tubes containing air; hence their name, which is derived from a Greek
word and signifies an _air-tube._ Arteries are the cylindrical tubes
which carry blood to every part of the system. All the arteries, except
the coronary which supply the substance of the heart, arise from the two
main trunks, the pulmonary artery and the aorta. They are of a
yellowish-white color, and their inner surface is smooth. The arteries
have three coats. (1.) The external coat, which is destitute of fat, and
composed chiefly of cellular tissue, is very firm and elastic, and can
readily be dissected from the middle coat. (2.) The middle, or fibrous
coat, is thicker than the external, and composed of yellowish fibers,
its chief property is contractility. (3.) The internal coat consists of
a colorless, thin, transparent membrane, yet so strong that it can, it
is thought, better resist a powerful pressure than either of the others.
Arteries are very elastic as well as extensible, and their chief
extensibility is in length. If an artery of a dead body be divided,
although empty, its cylindrical form will be preserved.

The _Veins_ are the vessels through which the venous blood returns to
the auricles of the heart. They are more numerous than the arteries, and
originate from numerous capillary tubes, while the arteries are given
off from main trunks. In some parts of the body, the veins correspond in
number to the arteries; while in others, there are two veins to every
artery. The veins commence by minute roots in the capillaries, which are
everywhere distributed through the body, and gradually increase in size,
until they unite and become large trunks, conveying the dark blood to
the heart. The veins, like the arteries, have three coats. The external,
or cellular coat, resembles that of the arteries; the middle is fibrous,
but thinner than the