CHAP. VII.] TUB CAT-S ORGANS OF CIRCULATION. 205 



much thicker and stronger than the tri-cuspid valve, but its essential 

 arrangement is the same. 



The action of the tri- and bi- cuspid valves is similar, and is 

 inverse to that of the semi-lunar valves. When the auricles 

 contract and the blood tends to flow into the ventricles, these 

 valvular flaps readily bend backwards and allow it free passage ; 

 but when, by the contraction of the ventricles, the blood tends to be 

 driven into the auricles as well as into the great arteries, then these 

 valves immediately close, and are pressed towards the auricles, while 

 they are prevented from going too far (and so being driven into the 

 auricles) by the numerous chorda) tendiueaj which hold them firmly 

 attached, as the cords of a tent hold and sustain it. The exactness 

 with which they meet together is rendered complete by the cohonme 

 carnece themselves, which participate in the contraction of the cavity 

 from the walls of which they proceed. 



§ 13. The KiGHT AURICLE (Fig. 102, B, RA) is situated at the 

 ventral side of the base of the heart, and has a smooth surface. 

 The superior vena cava (which is very short) opens into the anterior 

 portion of this chamber, and the inferior vena cava opens by a larger 

 opening into its posterior part — above the auriculo-ventricular 

 aperture — while between them is the orifice of a small vein called 

 coronary, which brings back the blood from the walls of the heart 

 itself. This part of the cavity (as distinguished from the cavity of 

 the auricular appendage) is called the mina venosus. At the hinder 

 part of the septum, which divides this auricle from the left one, is 

 an oval depression called the fossa ova I is (FO), which is the indica- 

 tion of a foetal condition, for, as we shall see, in the embryo an 

 opening leads directly from one auricle to the other. Immediately 

 in front of this is a marked transverse prominence called the tubercle 

 of Lower (TL). 



The LEFT AURICLE, wliicli, wlieii distended, has a somewhat 

 quadrangular appearance, lies to the left on the dorsal aspect of the 

 base of the heart. The openings into it of the pulmonary veins are 

 dorsally placed, and there are usually two on each side. Those of 

 the right side, however, meet together close to the auricle, so that 

 there seems to be but one pulmonary vein opening into it on that 

 side. The wall of the septum shows a slight depression, correspond- 

 ing with the fossa ovalis of the right auricle. 



The RIGHT VENTRICLE forms the right side of the ventral aspect 

 of the heart from the transverse furrow onwards towards its apex. 

 Internally it presents the valvular structure (the tri-cuspid valve) 

 already described, but its upper and anterior part is smooth and 

 free fi'om columns carnete. It is also prolonged forwards in a 

 conical manner, whence this part has been named the conns 

 arteriosus (CA). The left wall of the ventricle is convex, the septum 

 between the ventricles so bulging into its cavity as to make that 

 cavity appear crescentic when the right ventricle is transversely 

 bisected. The muscular wall of this chamber is much thicker than 

 that of either of the auricles. The opening (with its semi-lunar 



