The Circulatory Apparatus. The organs of circulation, or the circulatory apparatus, constitute the means by which the blood, the nutritive fluid of the body, is circulated through all its different parts, carrying new material to parts requiring it for repairs, and carrying away to be expelled from the body worn-out and useless or clogging elements wherever found. The circulatory apparatus consists of the heart, the blood-vessels, and the lymphatics, the structure and functions of which we will now briefly examine. The Heart.-Fig. 88. The heart is the central organ of the circulation, and hence is very properly placed near the center of the body, in the thorax, its exact position being a little to the left of the median line in the central part of the chest, between the two lungs. The heart is a muscular organ. It is, in fact, a hollow muscle. It is conical in shape, and is suspended in the chest, with the base upward and the apex downward. The apex is free, and when the heart is beating may be felt to strike the chest just below the fifth rib