5. Attached to the right and lower margins of this opening are the remains of the Eustachian valve (fig. 2, h), which in the foetus directs the blood from the inferior vena cava, through the foramen ovale, into the left auricle.
6. Below and to the left of this is the opening of the coronary sinus (fig. 2, k), which collects most of the veins returning blood from the substance of the heart.
7. Guarding this opening is the coronary valve or valve of Thebesius.
8. On the posterior or septal wall, between the two auricles, is an oval depression, called the fossa ovalis (fig. 2, g), the remains of the original communication between the two auricles. In about a quarter of all normal hearts there is a small valvular communication between the two auricles in the left margin of this depression (see “7th Report of the Committee of Collective Investigation,” J. Anat. and Phys. vol. xxxii. p. 164).
9. The annulus ovalis is the raised margin surrounding this depression.
10. On the left side, opening into the right ventricle, is the right auriculo-ventricular opening.
11. On the right wall, between the two caval openings, may occasionally be seen a slight eminence, the tubercle of Lower, which is supposed to separate the two streams of blood in the embryo.
12. Scattered all over the auricular wall are minute depressions, the foramina Thebesii, some of which receive small veins from the substance of the heart.
The right ventricle is a triangular cavity (see fig. 2) the base of which is largely formed by the auriculo-ventricular orifice. To the left of this it is continued up into the root of the pulmonary artery, and this part is known as the infundibulum. Its anterior wall forms part of the anterior surface of the heart, while its posterior wall is chiefly formed by the septum ventriculorum, between it and the left ventricle. Its lower border is the margo acutus already mentioned. In transverse section it is crescentic, since the septal wall bulges into its cavity. In its interior the following structures are seen:
1. The tricuspid valve (fig. 2, l, m, n) guarding against reflux of blood into the right auricle. This consists of a short cylindrical curtain of fibrous tissue, which projects into the ventricle from the margin of the auriculo-ventricular aperture, while from its free edge three triangular flaps hang down, the bases of which touch one another. These cusps are spoken of as septal, marginal and infundibular, from their position.