176 ELEMENTARY BIOLOGY. [CHAP. 



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-ventricu lar 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 undi- 

 vided 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 semilunar valves separate the pylangium from the 

 synangium, in which are the openings, posteriorly, of the 

 pulmonary arteries, anteriorly of the carotid trunks; while, 

 at the sides, the cavity of the synangium opens into those 

 of the right and left aortic arches. The apparently simple 

 branches into which the truncus arteriosus divides, are, in 

 fact, each made up of three separate trunks, the pulmo- 

 cutancous trunk behind, the aortic arch in the middle and 

 the carotid trunk in front. 



When the heart is in action, the sinus venosus, the atrium, 

 the ventricle and the truncus arteriosus contract in the 



