ricular appendices) are the ventral extensions of the right and left atria, respectively, which extend ventrally on each side of the pulmonary artery. Notice on the surface of the heart the injected coronary artery, which carries blood to the tissues of the heart, and the coronary veins, which drain the venous blood back into the general circulation.
The heart is a muscular pump which propels the blood throughout the body. Its complete failure to function means almost instant death. Venous blood returning from the capillaries of the muscles, brain, digestive organs, kidneys, etc. of the body enters the right atrium through the superior and inferior venae cavae. Contraction of the right atrium forces the blood into the right ventricle. The two atria contract simultaneously, but their contraction alternates with the simultaneous contraction (systole) of the ventricles. When the right ventricle is full it drives the blood through the pulmonary artery to the lungs, then relaxes (diastole) to receive more blood from the atrium. The excess of carbon dioxide carried by the blood is eliminated in the lung capillaries into the air contained in the minute air sacs (alveoli) of the lung. At the same time oxygen from the alveolar air penetrates the walls of the air sacs and capillaries and unites chemically with the hemoglobin, or red coloring matter of the blood. The blood then traverses the pulmonary veins to the left atrium of the heart, thence to the left ventricle, and finally out through the aorta to the tissues of the body. The internal mechanism of the heart may now be elucidated by an examination of the dissected organ.
Insert one point of the scissors into the severed end of the pulmonary artery, carefully slit it lengthwise, continuing the cut along the lateral wall of the right ventricle. Bend back the cut walls and carefully wash out the coagu-
