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Page 52
The infectious diseases are those of early life; chronic disease, on
the other hand, is most common in the latter third of life. This is
due to the fact that in consequence of the general wear of the body
this becomes less resistant, less capable of adaptation, and organic
injury, which in the younger individual would be in some way
compensated for, becomes operative. The territory of chronic disease
is so vast that not even a superficial review of the diseases coming
under this category can be attempted in the limits of this book, and
it will be best to give single examples only, for the same general
principles apply to all. One of the best examples is given in chronic
disease of the heart.
The heart is a hollow organ forming a part of the blood vascular
system and serving to give motion to the blood within the vessels by
the contraction of its strong muscular walls. It is essentially a
pump, and, as in a pump, the direction which the fluid takes when
forced out of its cavity by the contraction of the walls diminishing
or closing the cavity space, is determined by valves. The contraction
of the heart, which takes place seventy to eighty times in a minute,
is automatic and is due to the essential quality of the muscle which
composes it. The character, frequency and force of contraction,
however, can be influenced by the nervous system and by the direct
action of substances upon the heart muscle. The heart is divided by a
longitudinal partition into a right and left cavity, and these
cavities are divided by transverse septa, with openings in them
controlled by valves, each into two chambers termed _auricle_ and
_ventricle_. The auricle and ventricle on each side are
completely separated.
The circulation of the blood through the heart is as follows: The
blood, which in the veins of the body is flowing towards the heart,
passes by two channels, which respectively receive the blood from the
upper and lower part of the body, into the right auricle. When this
becomes distended it contracts, forcing the blood into the right
ventricle; the ventricle then contracts and sends the blood into 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 blood
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 kidneys may, by increasing the pressure of the blood within the
arteries, throw more than the usual amount of work upon the heart.
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