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 _chordae
tendineae_, connect the edges and apices of these valves with
column-like elevations of the fleshy substance of the walls of the
ventricles, called _columnae carneae_.

[Illustration: Fig. 41.
1. The descending vena cava. 2.
The ascending vena cava. 3. The
right auricle. 4. The opening between
the right auricle and the right
ventricle. 5. The right ventricle. 6.
The tricuspid valves. 7. The pulmonary
artery. 8, 8. The branches
of the pulmonary artery which pass
to the right and the left lung. 9. The
semilunar valves of the pulmonary
artery. 10. The septum between the
two ventricles of the heart. 11, 11.
The pulmonary veins. 12. The left
auricle. 13. The opening between
the left auricle and ventricle. 14.
The left ventricle. 15. The mitral
valves. 16, 16. The aorta. 17. The
semilunar valves of the aorta.]

The valves are so arranged that they present no obstacle to the free
flow of blood from the auricles into the ventricles, but if any is
forced the other way, it gets between the valve and the wall of the
heart, and drives the valve backwards and upwards, thus forming a
transverse partition between the auricle and ventricle, through which no
fluid can pass.

At the base of the heart are given off two large arteries, one on the
right side, which conveys the blood to the lungs, called the _pulmonary
artery_, and one on the left side, which conveys the blood to the system
in general, called the _aorta_. At the junction of each of these great
vessels with its corresponding ventricle, is another valvular apparatus,
consisting of thr