900 ANATOMY VASCULAR the 4th to the 8th dorsal vertebra}. The apex is directed forwards, downwards, and to the left, opposite to the inter val between the 5th and Gth left ribs. The heart has on its surface grooves which indicate its division internally into four chambers, two in its right half, two in its left half. The right chambers are the right auricle and right ventricle. The left chambers are the left auricle and left ventricle. All these chambers are lined by a smooth membrane, the endocardium, which is continuous on the one hand with the lining membrane of the veins, on the other with the lining membrane of the arteries. The Right Auricle occupies the right part of the base of the heart. It consists of a large dilated; portion, the sinus vcnosus, and of a small ear-shaped appendage, the auricula. Its muscular wall is smooth internally, except in the auri cula and adjacent anterior wall of the sinus venosus, where it is thrown into parallel ridges like the teeth of a comb, and named musculi pectinati. Into the sinus venosus open the great systemic veins or vence cavoe. The superior vena cava conveys to the auricle the systemic blood that has been circulating in the body above the diaphragm ; it opens by a patent mouth into the upper and back part of the sinus venosus. The inferior vena cava conveys to the auricle the blood that has been circulating in the parts of the body below the diaphragm ; it opens into the lower and back part of the auricle, and at its mouth is a rudimentary valve, the Eustachian valve. Close to its orifice is the mouth of another large vein, the coronary venous sinus, which also possesses a small valve. Several minute open ings, the foramina Thebesii, scattered over the inner wall of the auricle, are the mouths of small veins ramifying in the wall itself. Through these various orifices the venous blood pours into the auricle, and then flows into the right ventricle through a large orifice of communication between them. The right auricle is separated by a partition, the auricular septum, from the left auricle. On the surface of this septum is a depression, the fossa ovalis, surrounded by a raised border, the annulus ovalis, with which border the inner end of the Eustachian valve is continuous. Before the birth of the child the septum is perforated by a hole, called foramen ovale, through which the blood flows directly into the left auricle, but this foramen is obliterated after the birth of the child. The Rigid Ventricle forms the right border, a large part of the anterior surface, but only a small part of the posterior surface of the heart. It is shaped somewhat like a flattened cone, its apex being directed downwards towards the apex of the heart, its base to the corresponding auricle. The inner surface of its wall is very irregular, owing to the muscular bundles being elevated into strong ridges, called columnas carneoz. Two, or it may be three, of these fleshy columns project like nipples or big papilla? into the cavity of the ventricle, and are called musculi papillares. Attached to the free apex of each papillary muscle are several fibrous threads, the chordae tendinece, which, by their opposite extre mities, are connected to the segments of a large valve situ ated around the opening between the right auricle and ventricle. The right auriculo-ventricular opening, situated at the base of the ventricle, is sufficiently large to admit three fingers, and possesses a valve which consists of three large pointed segments or cusps (hence the name tricuspid given to it), between which three smaH intermediate cusps lie. One of the large cusps lies opposite the anterior wall of the ven tricle, another opposite the posterior, whilst the third is between the auriculo-ventricular and pulmonary openings. The cusps are flattened triangular folds of membrane con nected by their bases around the opening ; when the valve is not in action the apex of each cusp hangs pendulous in the ventricle : one surface is smooth, and looks to the cavity of the ventricle, the other surface is rough and directed to its wall; to this rough surface, to the apex, and to the edges of the cusp, the chordae tendineaB are attached. As the- musculi papillares, from which the chordae tendineee spring, lie opposite the intervals between the cusps, the chordas tendinea? from any given papillary muscle divide them selves into two groups, one for each of the two cusps between which it is situated. Attention has recently been FIG. 89. Cavities of the right side of the Heart. a, superior, and 6, inferior vena cava; c, arch of aorta; d, pulmonary artery; e. right, and /, left auricular appendage ; g, fossa ovalis ; h, Eustachian valve; k, mouth of coronary vein; /, m, n, cnsps of the tricuspid valve; 0, o, rapil- lary muscles; p, semilunar valve; q, corpus Arantii ; r, lunula. drawn by Kolleston to a band which passes from the base of the anterior papillary muscle to the septal wall of the ventricle. As it prevents over-distension of the ventricle, he has named it the moderator band. The base of this ventricle forms to the left and in front of the auriculo- ventricular opening, a funnel-shaped prolongation, the conns arteriosus, from which the pidmonary artery arises, through the intermediation of a strong fibrous ring. Surrounding the mouth of this artery is a valve called semilunar, which consists of three semilunar segments. Each segment is attached by its convex border to the artery where it springs from the ventricle. The opposite border is free, and pos sesses at its centre a minute nodule, the corpus Arantii, from which slender threads curve outwards at the free border and in the substance of the valve to strengthen it. A thin lunated portion lies immediately within the free border. One surface of the valve is convex, and directed to the lumen (i.e., the space contained by the walls) of the artery; the other is concave, and directed to the wall of the artery, and between it and the wall is a pouch named sinus of Valsalva. The pulmonary artery extends upwards and to the left for about If inch, and then divides into two branches, one for each lung. The right ventricle is com pletely separated from the left by the ventricular septum, which passes obliquely from left to right, and from before backwards, so that it forms the posterior wall of the right ventricle and the anterior wall of the left. The Left Auricle occupies the left part of the base of the heart, and, like the right auricle, consists of a dilated sinus venosus and an ear-shaped auricula. Its muscular wall forms a smooth surface internally, except in the auricula, where the ridge-shaped musculi pectinati occur. Opening into the sinus are the orifices of, the four pulmonary
veins, two from the right, two from the left lung : theso
