animals, has a double circulation, a fact which explains the double
structure of his heart. The two divisions are known as the _pulmonary_ and
the _systemic_ circulations. By the former the blood passes from the right
ventricle through the lungs, and is then returned to the left auricle; by
the latter it passes from the left ventricle through all parts of the
body, returning to the right auricle.

The general plan of the circulation is indicated in Fig. 23. All the blood
flows continuously through both circulations and passes the various parts
in the following order: right auricle, tricuspid valve, right ventricle,
right semilunar valve, pulmonary artery and its branches, capillaries of
the lungs, pulmonary veins, left auricle, mitral valve, left ventricle,
left semilunar valve, aorta and its branches, systemic capillaries, the
smaller veins, superior and inferior venæ cavæ, and then again into the
right auricle.

In the pulmonary capillaries the blood gives up carbon dioxide and
receives oxygen, changing from a dark red to a bright red color. In the
systemic capillaries it gives up oxygen, receives carbon dioxide and other
impurities, and changes back to a dark red color.

In addition to the two main divisions of the circulation, special circuits
are found in various places. Such a circuit in the liver is called the
_portal_ circulation, and another in the kidneys is termed the _renal_
circulation. To some extent the blood supply to the walls of the heart is
also outside of the general movement; it is called the _coronary_
circulation.

[Fig. 23]