to the
arteries of the lungs, the passage of blood into the auricle being
prevented by valves which close the opening between auricle and
ventricle when the latter contracts upon its contents. When the
ventricle empties by its contraction the wall relaxes and the back
flow from the artery is prevented by crescentic-shaped valves placed
where the artery joins the ventricle. A similar arrangement of valves
is on the left side of the heart. The pressure given the blood by the
contraction of the right ventricle sends it through the lungs; from
these, after it has been oxygenated, it passes into the left auricle,
then into the left ventricle and from this into the great artery of
the body, the aorta, which gives off branches supplying the
capillaries of all parts of the body. Both of the auricles and both of
the ventricles contract at the same, time, the ventricular contraction
following closely upon the contraction of the auricles. Contraction or
systole is followed by a pause or diastole during which the blood
flows from the veins into the auricles. The work which the right
ventricle accomplishes is very much less than that of the left, and
the right ventricle has a correspondingly thinner wall. The size of
the heart is influenced by the size and the occupation of the
individual being larger in the large individual than in the small, and
larger in the active and vigorous than in the inactive. Generally
speaking, the heart is about as large as the closed fist of its
possessor.

Imperfections of the heart which interfere with its action may be the
result of failure of development or disease. An imperfect heart which
can, however, fully meet the limited demands made upon it in
intra-uterine life, may be incapable of the work placed upon it in
extra-uterine life. Children with imperfectly formed hearts may be
otherwise perfect at birth, but they have a bluish color due to the
imperfect supply of the blood with oxygen, and are known as blue
babies. The condition becomes progressively worse due to the
progressive demands made upon the heart, and death takes place after
some days or months or years, the time depending upon the degree of
the imperfection.

Much of the damage of the heart in later life is due to infection. The
valves of the heart are a favorite place for attack by certain sorts
of bacteria which get into the blood. This is due to the prominent
position of the valves which brings them in contact with all the bloo