ases there may be temporary
immunity produced by each excursion of the disease, but the immunity
is not permanent nor is the parasite destroyed. There is a further
connection between chronic disease and infection in that the damage to
the organs, which is the great factor underlying chronic disease, is
so often the result of an infection.

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 in