d
in the body, the large extent and unevenness of the surface and to the
rubbing together and contact of their edges when closed. At the site
of infection there is a slight destruction of tissue and on this the
blood clots producing rough wart-like projections. The valves in some
cases are to a greater or less extent destroyed, they may become
greatly thickened and by the deposit of lime salts converted into
hard, stony masses. Essentially two conditions are produced. In one
the thickened, unyielding valves project across the openings they
should guard, and thus by constricting the opening interfere with the
passage of blood either through the heart or from it. In the other the
valves are so damaged that they cannot properly close the orifices
they guard, and on or after the contraction of the cavities there is
back flow or regurgitation of the blood. If, for instance, the orifice
of the heart into the aorta is narrowed, then the left ventricle can
only accomplish its work of projecting into the aorta a given amount
of blood in a given time by contracting with greater force and giving
a greater rapidity to the stream passing through the narrow orifice.
This the heart can do because, like all other organs of the body, it
has a large reserve force which enables it, even suddenly, to meet
demands double the usual, and like all other muscles of the body it
becomes larger and stronger by increased work. The condition here is
much simpler than when the same valve is incapable of perfect closure,
or when both obstruction and imperfect closure, are combined as they
not infrequently are. In such cases the ventricle must do more than in
the first case. It must force through the orifice, which may be
narrowed, the amount of blood which is necessary to keep up the
pressure within the aorta and give to the circulation the necessary
rapidity of flow, and also the amount which flows back into the heart
through the imperfectly acting valve. This it can do by contracting
with greater force upon a larger amount of blood, the cavity becoming
enlarged to receive this. Not only may such damage to the valves be
produced, but the muscular tissue of the heart may suffer from
defective nutrition or from the effect of poisons, whether these are
formed in the body as the effect of disease or introduced from
without; or in consequence of disease in the lungs the flow of blood
through them may be impeded, or disease elsewhere in the body, as in
the