Left Ventricle 167 



from the inferior cava passed into the left auricle ; the ridge around 

 the fossa is the annulus ovalis. 



The coronary sinus collects the blood from the two coronary veins, 

 and returns it into the back of the right auricle ; its orifice, which is 

 guarded by an imperfect valve (Thebesian), is between the inferior 

 caval and the ventricular orifices. Some small cardiac veins open 

 independently by foramina Thebesii into the right auricle. 



The auricular appendix has its wall strengthened by muscular 

 bands which are arrayed like the teeth of a comb the musculi pec- 

 tinati. 



The right ventricle has its flat side resting upon the diaphragm ; 

 its convex surface forms the chief part of the front of the heart, but it 

 does not quite reach to the apex. The auriculo-ventricular opening 

 is to the right side of its base, and is guarded by the tricuspid valve, 

 which lies behind the sternum between the third intercostal spaces. 

 The most important flap of the tricuspid valve is, of course, towards the 

 left, so that as blood is being driven into the pulmonary artery there 

 may be no risk of it flowing back into the auricle. Of the other seg- 

 ments one is anterior, the other posterior. 



The bases of the flaps are attached to a fibrous ring around the 

 orifice, and blend with each other laterally. To the free borders of the 

 valve segments, and also to their ventricular surfaces, chorda tendinea 

 are attached, so that they may not be swept up into the auricle with 

 the stream of blood when the ventricle contracts. If the tendinous 

 cords were connected by their other end with the ventricular wall they 

 would become slack during systole as the walls closed in upon their 

 contents and the tension of the valves would cease. They, therefore, 

 lose themselves below on fleshy columns (musculi papillares), which 

 contract simultaneously with the ventricular wall, and thus they hold the 

 valves taut. 



The pulmonary artery leads up from the conus arteriosus, and is 

 near to the interventricular septum that is, on the left side of the roof 

 of the ventricle. Its mouth is guarded by semilunar valves, which are 

 placed behind the third left chondro-sternal joint. 



The left auricle has an appendix like that of the right ; and 

 it overlaps the root of the pulmonary artery on the left side. In front 

 of the auricle are the aorta and the pulmonary artery. The four pul- 

 monary veins enter the back of the sinus, two to the right and two 

 to the left ; they have no valve. Behind the auricle passes the left 

 bronchus. On the inner wall is a depression marking the situation of 

 the foetal foramen ovale. 



The left ventricle makes the chief part of the back of the heart, 

 only a small part of it being seen near to the apex on the anterior view, 

 but it reaches beyond the right ventricle and forms the apex. Its 

 opening into the left auricle is guarded by a valve of two flaps like 

 a bishop's mitre which is behind the sternum at the level of the third 



