of the body, and
a continual return after it has circulated through these parts. Its
presence in every part of the body is one of the essential conditions of
animal life, and is effected by a special set of organs, called the
_circulatory organs_.

* * * * *

CHAPTER VII.

PHYSIOLOGICAL ANATOMY.

CIRCULATORY ORGANS.

Having considered the formation of chyle, traced it through the
digestive process, seen its transmission into the _vena cava_, and,
finally, its conversion into blood, we shall now describe how it is
distributed to every part of the system. This is accomplished through
organs which, from the round of duties they perform, are called
_circulatory_. These are the Heart, Arteries, Veins, and Capillaries,
which constitute the _vascular system_.

Within the thorax or chest of the human body, and enclosed within a
membranous sac, called the _pericardium_, is the great force-pump of the
system, the heart. This organ, to which all the arteries and veins of
the body may be either directly or indirectly traced, is roughly
estimated to be equal in size to the closed fist of the individual to
whom it belongs.

It has a broad end turned upwards, and a little to the right side,
termed its _base_; and a pointed end called its _apex_, turned
downwards, forwards, and to the left side, and lying beneath a point
about an inch to the right of, and below, the left nipple, or just below
the fifth rib. Attached to the rest of the body only by the great
blood-vessels which issue from and enter it at its base, the heart is
the most mobile organ in the economy, being free to move in different
directions.

The heart is divided into two great cavities by a fixed partition, which
extends from the base to the apex of the organ, and which prevents any
direct communication between them. Each of these great cavities is
further subdivided transversely by a movable partition, the cavity above
each transverse partition being called the _auricle_, and the cavity
below, the _ventricle_, right or left, as the case may be.

[Illustration: Fig. 40.
General view of the heart and lungs, _t_. Trachea, or
windpipe, _a_. Aorta, _p_. Pulmonary artery, 1, 2.
Branches of the pulmonary artery, one going to the
right, the other to the left lung. _h._ The heart.]

The walls of the auricles are much thinner than those of the ventricles,
and the wall of the right ventricle is much thinner than that of the