THE MECHANISM OF TRANSPORT 117 



and a deeper layer separate for each chamber. The auric ulo- ventri- 

 cular ring consists of connective tissue separating the muscle of the 

 auricles from that of ventricles except at one spot on the septum (see 

 p. 121, the A.-V. bundle), and possibly at the right lateral external 

 margin. 



The right auricle is more or less quadrilateral in shape, being 

 prolonged in the upper corner to an ear -like process the right auricular 

 appendix. Into it the superior and inferior venae cavae open. At 

 the junction of the superior vena cava and auricle is situated a small 

 mass of tissue known as the " sinu-auricular node." 



The right ventricle forms the chief part of the anterior surface 

 of the heart. It communicates with the right auricle and with the 

 pulmonary artery. At the entrance from auricle to ventricle are 



*st arch 

 ventral aorta 



conus arteriosus 



prim. ventricle\ . / .. \prim.auricle 



'inus uenosus 



vit. vein- > '-^ ^^.vit. vein 



FIG. 26. THE PRIMITIVE DIVISIONS OF THE EMBRYONIC HEART. (Keith.) 



situated the tricuspid valves, while the entrance to the pulmonary 

 artery is guarded by thin watch-pocket-like valves the semilunar 

 valves. 



The left auricle is situated posteriorly. It likewise possesses an 

 appendix. Into it the four pulmonary veins open. The left auricle 

 communicates with the left ventricle, the orifice being guarded by the 

 two flapped bicuspid or mitral valves. 



The left ventricle forms the chief part of the posterior surface, 

 and also the apex of the heart. It forms the chief muscular mass of 

 the heart, the wall being in places inch in thickness. It com- 

 municates with the left auricle and with the aorta. At the orifice of 

 the aorta are situated delicate semilunar or watch-pocket valves. 

 Opposite the cusps are bulgings of the aortic wall the sinuses of 

 Valsalva. From the anterior one arises the right coronary artery, 



