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.
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).
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).