The blood, the characters of which have just been considered, circulates through the body by the agency of the heart and blood-vessels. The heart is a portion of the vascular system, consisting of a compact but hollow mass of muscle that acts rhythmically as a pump, and, owing to the presence of valves suitably placed, drives the blood in one constant direction through the body. It is placed in the thorax or chest, and is protected from injury by the breast-bone and the firm but elastic ribs and spine, and also owing to its being enveloped to a large extent by the spongy tissue of the lungs. It is enclosed in a tough membranous bag named the pericardium, lined internally by a serous membrane, the smooth and polished surfaces of which, moistened with the fluid they secrete, reduce friction to a minimum. The general course pursued by the blood is from the heart, through the arteries and their ramifications, named capillaries, onward to the veins, by which it is again conducted to the heart. Although in appearance single, the heart is really a double organ, the two parts being united for the sake of economy of space and material, and also to enable them to work equably and simultaneously.

One of these hearts, the right, forces the blood through the pulmonary artery to