mon
aorta, consisting of the anterior and posterior aortas, which give off the
large arteries.

The veins take the blood from the capillaries in all parts of the body.
They begin in very small tubes, which unite to become larger in size and
less in number as they approach the heart.

In its course an artery is usually accompanied with a vein and in many
situations with a nerve. The more important arteries are placed deep within
the body; when they are superficial, however, they are generally found
where least exposed to injury, as, for example, on the inner side of the
legs. Arteries are less numerous than veins, and their total capacity is
much less than that of the veins. A great number of veins are in the tissue
immediately beneath the skin and do not generally accompany arteries.

The blood, throughout its course in the heart, arteries, capillaries, and
veins, is inclosed within these vessels. Except where the large lymphatics
empty into the venous blood, there is no opening into the course of the
blood.

All the arteries except the pulmonary and its branches carry bright-red
blood, and all the veins, except the pulmonary veins, carry dark-red blood.
The impure dark-red blood is collected from the capillary vessels and
carried to the right auricle by the veins; it passes down into the right
ventricle, and thence into the pulmonary artery and through its branches to
the capillaries of the lungs, where the carbonic-acid gas and other
impurities are given up to the air in the air cells of the lungs (through
the thin walls between the capillaries and the air cells), and where it
also absorbs from the air the oxygen gas necessary to sustain life. This
gas changes it to the bright-red, pure blood. It passes from the
capillaries to the branches of the pulmonary veins, which convey it to the
left auricle of the heart; it then passes through the auriculo-ventricular
opening into the left ventricle, the contraction of which forces it through
the common aorta into the posterior and anterior aortas, and through all
the arteries of the body into the capillaries, where it parts with its
oxygen and nutritive elements and where it absorbs carbonic-acid gas and
becomes dark colored. (See theoretical diagram of the circulation, Pl.
VII.)

The branches of certain arteries in different parts unite again after
subdividing. This reuniting is called anastomosing, and assures a quota of
blood to a part if one of the anastomosing arter