Internally the heart has four cavities: two auricles and two ventricles. The _right ventricle_ is more crescent-shaped than in solipedes, and in a manner envelops the left ventricle in front and to the right, though it does not reach the point of the heart. The _right auricle_ is larger than the left. The _auriculo-ventricular_ valve is not tricuspid as in mammals. This valve instead of being formed as usual by a membranous curtain, with margins retained by cords fixed to the walls of the ventricles, is composed of a wide muscular leaf, which appears to be a portion of the inner wall of the ventricle detached from the _interventricular septum_. This septum is convex; and the auriculo-ventricular orifice is an oblique slit situated between it and the muscular valve in question; so that, when the heart wall contracts at the systole, the valve is applied against this septum and closes the passage. The bicuspid, or auriculo-ventricular valve of the left side usually has two segments, though occasionally there may be three. The _fossa ovalis_ is a depression behind the posterior semi-lunar valve in the septum of the heart. The membranous septum closing the foramen ovale is complete and strong but thin and transparent. The right auricle receives the blood from the two venæ cavæ coming from the anterior extremity, and from the posterior vena cava. These empty into a sinus. The left auricle has two vessels, the pulmonary veins which bring blood to it from the lungs. =Structure of the Heart.=—The heart is lined by a serous membrane, the _endocardium_, which is a continuation of the endothelium of the blood-vessels. There are a few muscular pillars in the inner wall, called the _columnæ carnæ_. To give the heart its pumping power, it is made up of contractile tissue, a specialized kind of muscle called _heart muscle_. It is involuntary-striated and occupies an intermediate position, both morphologically and embryologically, between smooth involuntary muscle and striated voluntary muscle (Fig. 74, No. 4). It, like striated voluntary muscle, is both transversely and longitudinally striated. Heart muscle cells are short, thick cylinders, which are joined end to end to form long fibers. By means of lateral branches the cells of one fiber anastomoses with cells of adjacent fibers. Each cell of heart muscle contains one centrally located nucleus. There is no distinct sarcolemma, but the sarcoplasm is more dense near the surface of the cell, which gives it the appearance of an enveloping cell wall. There is a zone free from fibrillæ around the nucleus. The longitudinal fibrillæ, which make up the cell, are held together by a cement-like substance.