263. The structure of the left side of the heart is perfectly analogous to that of the right. Its auricle, like that on the left side, is placed at the base of the ventricle (figs. CXIV. 10, and CXVII. 2), and like it also is thin, being composed chiefly of membrane. At its upper and back part (figs. CXIV. 9, and CXVII. 1) are the openings of the four pulmonary veins, two from the right, and two from the left lung. 264. At the passage of communication between the left auricle and ventricle is placed a valve analogous to that on the right side (fig. CXVII. 7). [Illustration: Fig. CXVII. View of the heart with the great vessels in connection with it, on the left side, its chambers being laid open as in the preceding figure. 1. The four pulmonary veins opening into, 2. the cavity of the left auricle; 3. the cut edge of the wall of the auricle; 4. the appendix of the auricle; 5. the cavity of the left ventricle; 6. the cut edge of the wall of the ventricle, the greater portion of the wall having been removed to show the interior of the chamber; 7. valve placed between the auricle and ventricle; 8. columnæ carneæ terminating in tendinous threads attached to the loose margin of the valve; 9. probe passed underneath the valve and its tendinous threads, raising them from the wall of the ventricle similar to a refluent current of blood; 10. passage to 11. the aorta; 12. two of the semilunar valves placed at the mouth of the aorta, the third having been cut away; 13. arch of the aorta; 14. the three semilunar valves at the commencement of the pulmonary artery seen in action, completely closing the mouth of the vessel.] 265. The walls of the left ventricle are nearly as thick again as those of the right, and its fleshy columns are much larger and stronger. From the upper and back part of this fourth chamber (fig. CXVII. 11) springs the great systemic artery, the aorta, around the mouth of which are placed three semilunar valves (fig. CXVII. 12), similar to those at the mouth of the pulmonary artery. 266. The partition which divides the two sets of chambers from each other (fig. CXIV. 6) is wholly composed of muscular fibres, and is called the septum of the heart.