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