257. When the interior of the heart is laid open there are brought into view four chambers (fig. CXIV. 3, 4, 10, 11), two for each circle. Those belonging to the pulmonic circle are on the right (fig. CXIV. 3, 4), those to the systemic on the left side of the body (fig. CXIV. 10, 11); hence the terms right and left are applied to these respective parts of the heart. 258. The veins which carry the blood to the right or the pulmonic chambers are two, one of which brings it from the upper, and the other from the lower parts of the body: the first is called the superior and the second the inferior vena cava (fig. CXIV. 1, 2). Both pour their blood into the first chamber, termed the right auricle (fig. CXIV. 3); from the right auricle the blood passes into the second chamber, denominated the right ventricle (fig. CXIV. 4): from which springs the artery which carries the blood from the heart to the lung, the pulmonary artery (fig. CXIV. 7). This is the pulmonic circle. From the lung the blood is returned to the heart by four veins, termed the pulmonary veins (fig. CXIV. 9), which pour the blood into the third chamber of the heart, the left auricle (fig. CXIV. 10). From the left auricle it passes into the fourth chamber, the left ventricle (fig. CXIV. 11), from which springs the artery which carries out the blood to the system, termed the aorta (figs. CXIV. 12, and CXVII. 11). This is the systemic circle. In the system the minute branches of the aorta unite with the minute branches that form the venæ cavæ, which return the blood to the right auricle of the heart, and thus the double circle is completed. 259. The two chambers called the auricles occupy the basis of the heart (fig. CXIV. 3, 10). The right auricle is situated at the basis of the right ventricle (figs. CXIV. 3, and CXVI. 4). It is partly membranous and partly muscular. At its upper and back part is the opening of the vena cava superior (fig. CXVI. 1), which returns the blood to the heart from the head, neck, and all the upper parts of the body. At its lower part is the opening of the vena cava inferior (fig. CXVI. 2), which returns the blood from all the lower parts of the body. [Illustration: Fig. CXVI.