At the base of the heart are given off two large arteries, one on the
right side, which conveys the blood to the lungs, called the _pulmonary
artery_, and one on the left side, which conveys the blood to the system
in general, called the _aorta_. At the junction of each of these great
vessels with its corresponding ventricle, is another valvular apparatus,
consisting of three 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