168 ELEMENTARY BIOLOGY. [XIII. 



aperture into the atrium. The latter shews no signs of 

 division externally, but, internally, it is divided by a delicate 

 partition, the septum of the auricles, into a smaller left 

 auricle and a larger right auricle. The sinus venosus opens 

 into the atrium, to the right of the septum and therefore 

 into the right auricle. Into the left auricle, the common 

 pulmonary vein, a small trunk formed by the junction of the 

 veins from the right and left lungs, opens. 



At its posterior end the atrium opens by the auriculo- 

 ventricular aperture into the ventricle. 



A small valve, prevented from flapping back by fine ten- 

 dinous cords, exists on each side of this aperture, and the 

 septum of the auricles is continued back upon the faces of 

 these valves and ends by a free edge between them, thus 

 dividing the auriculo-ventricular aperture itself into two 

 openings. 



The walls of the sinus and of the atrium are very thin. 

 Those of the ventricle, on the other hand, are thick and 

 spongy, only a comparatively small, transversely elongated, 

 clear cavity being left at the anterior end or base of the 

 ventricle. At the right extremity of this is the aperture 

 which leads into the truncus arteriosus. Three semilunar 

 valves, which open from the ventricle into the truncus, sur- 

 round this opening. 



The walls of the truncus arteriosus are thick and mus- 

 cular, though not nearly so thick as those of the ventricle. 

 At its anterior end it appears to divide into two trunks, 

 which diverge and immediately leave the pericardium to 

 pass on to the sides of the gullet. The elongated undivided 

 part is the pylangium, the terminal part common to the 

 divergent trunks is the synangium. The former is divided 

 throughout its length by a sort of fold which is attached to 

 the dorsal wall while its opposite edge is free. Three semi- 

 lunar valves separate the pylangium from the synangium, in 



