of the capillaries of the
pulmonary artery. They form larger and larger vessels until they become
two large veins in each lung, and pour their contents into the left
auricle. Thus the pulmonary artery carries venous blood from the right
ventricle _to_ the lungs, as the pulmonary veins carry arterial blood
_from_ the lungs to the left auricle.

From the left ventricle springs the largest arterial trunk in the body,
over one-half of an inch in diameter, called the aorta. From the
aorta other arteries branch off to carry the blood to all parts of the
body, only to be again brought back by the veins to the right side,
through the cavities of the ventricles. We shall learn in Chapter VIII.
that the main object of pumping the blood into the lungs is to have it
purified from certain waste matters which it has taken up in its course
through the body, before it is again sent on its journey from the left
ventricle.

187. The Arteries. The blood-vessels are flexible tubes through which
the blood is borne through the body. There are three kinds,--the
arteries, the veins, and the capillaries, and these differ
from one another in various ways.

The arteries are the highly elastic and extensible tubes which carry
the pure, fresh blood outwards from the heart to all parts of the body.
They may all be regarded as branches of the aorta. After the aorta leaves
the left ventricle it rises towards the neck, but soon turns downwards,
making a curve known as the arch of the aorta.

From the arch are given off the arteries which supply the head and arms
with blood. These are the two carotid arteries, which run up on each
side of the neck to the head, and the two subclavian arteries, which
pass beneath the collar bone to the arms. This great arterial trunk now
passes down in front of the spine to the pelvis, where it divides into two
main branches, which supply the pelvis and the lower limbs.

The descending aorta, while passing downwards, gives off arteries to the
different tissues and organs. Of these branches the chief are the
coeliac artery, which subdivides into three great branches,--one
each to supply the stomach, the liver, and the spleen; then the renal
arteries, one to each kidney; and next two others, the mesenteric
arteries, to the intestines. The aorta at last divides into two main
branches, the common iliac arteries, which, by their subdivisions,
furnish the arterial vessels for the pelvis and the lower limbs.

[Illustra