View of the heart with the great vessels in connection with it, on the right side, its different chambers being laid open and its structure shown. 1. The vena cava superior; 2. the vena cava inferior; 3. cut edge of the right auricle turned aside to show, 4. the cavity of the right auricle into which the two venæ cavæ pour the blood returned from all parts of the body; 5. hook suspending the reflected portion of the wall of the auricle; 6. the right ventricle; 7. cut edge of the wall of the ventricle, a portion of which has been removed to show 8. the cavity of the ventricle; 9. situation of the opening between the auricle and ventricle, called the auricular orifice of the ventricle; 10. valve placed between the auricle and ventricle, one margin being firmly attached to the auriculo-ventricular opening in its entire extent, the other lying loose in the cavity of the ventricle; 11. probe passed from the auricle into the ventricle underneath the valve, showing the course of the blood from the former chamber to the latter; 12. the columnæ carneæ attached by one extremity to the walls of the ventricle, the other extremity ending in tendinous threads attached to the loose margin of the valve; 13. passage to the pulmonary artery; 14. the three semilunar valves placed at the commencement of 15. the pulmonary artery; 16. the two great branches into which the trunk of the pulmonary artery divides, one branch going to each lung.] 260. The auricle communicates with its corresponding ventricle by a large opening, termed the auricular orifice of the ventricle (figs. CXIV. 5, and CXVI. 9). All around the opening is placed a thin but strong membrane (fig. CXVI. 10), one margin of which is firmly attached to the wall of the ventricle (figs. CXIV. 5, and CXVI. 9), while the other is free (fig. CXVI. 10). This membrane receives the name, and, as will be seen immediately, performs the office of a valve. 261. The ventricle is much thicker and proportionally stronger than the auricle (fig. CXVI. 3, 6). It is composed almost entirely of muscular fibre. Over nearly the whole extent of its internal surface are placed irregular masses of muscular fibres, many of which stand out from the wall of the ventricle like columns or pillars (fig. CXVI. 12); hence they are called fleshy columns (columnæ carneæ). Some of these fleshy columns are adherent by one extremity to the wall of the ventricle, while the other extremity terminates in tendinous threads which are attached to the membrane that forms the valve (fig. CXVI. 12). 262. From the upper and right side of this chamber springs the pulmonary artery (fig. CXVI. 15); at the entrance of which are placed three membranes of a crescent or semilunar shape, termed the semilunar valves (fig. CXVI. 14).