This is a transparent membrane, much more delicate than the serous membranes, which, however, it strongly resembles. Its free surface is highly polished and glistening ; its attached surface is united to the subjacent tendinous and muscular structures by very fine areolar tissue, which is often found thickened and altered by disease, particularly at the left side. The endocardium is thicker in the left cavities of the heart than in the right, and thickest opposite the auriculo-ventricular and arterial orifices, in which situations it is often found morbidly thick and rough, in consequence of chronic inflammation. It consists of a layer of epithelium placed on a stratum of fine fibres, which exhibit minute wavings. The epithelium appears to be extremely delicate, but the same in all its characters as that of the blood-vessels. It is so delicate, that to be seen satisfactorily it must be examined in animals just killed. We observe two forms of epithelial particles: one soft, rounded, and globular; the other somewhat compressed and drawn out at opposite poles into pointed or fibre-like processes. It is difficult to determine the precise relative position of these two forms of epithelium; but it seems probable that the pointed processes are the more deeply seated, and are in immediate contact with the subjacent fibrous layer, which here corresponds to the basement membrane beneath the epithelium of serous and mucous membranes.*

* Todd's Cyclopaedia, p. 619.

The Arteries of the heart are two in number, viz., the posterior and anterior coronary.

The posterior, or right coronary artery, arises from the aorta, above the margin of one of the semilunar valves; and after communicating with the left coronary behind the pulmonary artery, proceeds outwards in the groove between the right auricle and right ventricle. Having reached the inferior surface of the heart, it divides into two branches; one of which continues in the same groove, and winding around the base of the heart, anastomoses with the left coronary artery; it supplies the right auricle and ventricle: the second, from its size, appears the continued trunk : it descends in the groove on the posterior inferior surface of the heart, accompanied by the posterior coronary vein, along the septum ventriculorum, supplies both ventricles, and near the apex of the heart anastomoses with the left coronary. The branches of the right coronary, before its division, are the following: first, auricular branches, five or six in number, which supply the right auricle, the septum auricularum, and the parietes of the venae cavae; secondly, ventricular branches, much larger, which are distributed to the right ventricle ; some of these descend on the superior surface of the heart, others on the inferior, and one along its right or thin margin.

* Todd and Bowman's Physiological Anatomy, vol. ii. p. 335.

The anterior, or left coronary artery, smaller than the right, arises from the aorta, above the margin of one of the semilunar valves; it then proceeds to the left, till it escapes from beneath the pulmonary artery and divides into a superior and inferior branch. The superior winds round the base of the heart in the groove between the left auricle and left ventricle, concealed by the coronary vein, and anastomoses with the right coronary artery: in this course its branches are distributed principally to the left ventricle; others go to the left auricle and the pulmonary veins. The inferior branch is the larger; it descends on the anterior superior surface of the heart, accompanied by the anterior coronary vein, in the groove between the two ventricles. Its first branches ramify on the commencement of the aorta and pulmonary artery; the rest are distributed to the ventricles, principally to the left.