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
left, from the fact that the ventricles have more work to perform than
the auricles, and the left ventricle more than the right.

In structure, the heart is composed almost entirely of muscular fibers,
which are arranged in a very complex and wonderful manner. The outer
surface of the heart is covered with the pericardium, which closely
adheres to the muscular substance. Inside, the cavities are lined with a
thin membrane, called the _endocardium_. At the junction between the
auricles and ventricles, the apertures of communication between their
cavities are strengthened by _fibrous rings_. Attached to these fibrous
rings are the movable partitions or valves, between the auricles and the
ventricles, the one on the right side of the heart being called the
_tricuspid valve_, and the one on the left side the _mitral valve._ A
number of fine, but strong, tendinous chords, called _chordæ