The main mass of the heart wall, called _myocardium_, consists of the specialized muscular tissue just described. The myocardium differs in thickness in different parts of the heart wall. It is thickest in the left ventricle and thinnest in the auricles. The left ventricle forces the blood through the systemic circulation and hence must be thicker to give it more power than is needed for the right ventricle, which forces the blood only through the lungs. The auricles are thinnest of all; for they receive the blood and pass it only to the chambers below. The _auricular appendages_ at the base of the heart in fowls are not so well marked as in mammals. The auricular muscles consist of an outer coat common to both auricles, the fibers of which are transverse and of an inner coat, independent for each auricle, the fibers of which are longitudinal. Between the two coats, occur bundles of muscle the fibers of which run in various directions. The disposition of the muscle tissue of the ventricles is much more complicated. It is composed of several layers of fibers intricately interwoven. The _endocardium_, covering the inner surface of the myocardium, forms a serous lining of all the chambers of the heart. At the arterial and venous openings it is continuous with and similar in structure to the intima of the vessels. The endocardium consists of two layers, an external layer closely attached to the myocardium and consisting of mixed fibers, including those of elastic tissue and smooth muscle cells; and an inner, single layer of endothelial cells, spoken of above. The heart is supplied with nutrient blood by the two _coronary arteries_, which are given off from the common aorta just above the semi-lunar valves.