e blood. The human heart is divided by a septum
(partition) into two halves, right and left, each half being further
constricted into, two cavities, the upper of the two being termed the
auricle and the lower the ventricle. The heart consists of four chambers
or cavities, two forming the right half, the right auricle and right
ventricle, and two forming the left half, the left auricle and left
ventricle. The right half of the heart contains the venous or impure
blood; the left the arterial or pure blood. From the cavity of the left
ventricle the pure blood is carried into a large artery, the aorta,
through the numerous branches of which it is distributed to all parts of
the body, with the exception of the lungs. In its passage through the
capillaries of the body the blood gives up to the tissues the material
necessary for their growth and nourishment and at the same time receives
from the tissues the waste products resulting from their metabolism, that
is, the building up and tearing down of the tissues, and in so doing
becomes changed from arterial or pure blood into venous or impure blood,
which is collected by the veins and through them returned to the right
auricle of the heart.

From this cavity the impure blood passes into the right ventricle from
which it is conveyed through the pulmonary (lung) arteries to the lungs.
In the capillaries of the lungs it again becomes arterialized by the air
that fills the lungs and is then carried to the left auricle by the
pulmonary veins. From this cavity it passes into that of the left
ventricle, from which the cycle once more begins. The heart, then, is a
hollow muscular organ of a conical form, placed between the lungs and
enclosed in the cavity of the pericardium. It is placed obliquely in the
chest. The broad attached end or base is directed upwards, backwards and
to the right and extends up to the right as high as the second rib and the
center of the base lies near the surface underneath the breast bone. The
apex (point) is directed downwards, forward and to the left and
corresponds to the space between the cartilage of the fifth and sixth
ribs, three-fourths of an inch to the inner side, and one and one-half
inches below the nipple, or about three and one-half inches from the
middle line of the breast bone. The heart is placed behind the lower two-
thirds of the breast bone and extends from the median line three inches to
the left half of the cavity of the chest and one an