in the body and attacking when the defences of the body are in some way weakened.  In such cases 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 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