AMPHIBIA 283 



The four divisions of the heart contract in order, first the sinus 

 venosus, then the two auricles, next the ventricle, and lastly the 

 truncus arteriosus. (Figure 152.) 



Internal Structure. — The heart propels the blood always in one 

 direction and keeps the -pure and impure blood separated. The 

 sino-auricular aperture leads from the sinus venosus into the right 

 auricle. It is a transversely oval opening, guarded by imperfect 

 anterior and posterior valves, in the ventral wall of the sinus venosus 

 near the median plane of the anterior end. The right auricle is the 

 larger of the two. It has thin walls, thickened by muscular strands 

 which form interlacing ridges on its inner surface. In its dorsal 

 wall is the opening from the sinus venosus (sino-auricular aperture). 

 The left auricle is smaller, sometimes much smaller, than the right 

 auricle, which it resembles in the structure of its walls. In its 

 dorsal wall, very close to the sino-auricular aperture, is the opening 

 of the pulmonary vein. The inter-auricular septmn, the partition 

 between the right and left auricles, is much thinner than the walls 

 of the auricles, and is placed obliquely, the left auricle lying more 

 dorsal than the right. The septum ends with a free posterior edge, 

 opposite the auriculo-ventricular aperture. 



The auriculo-ventricular aperture lies at the base of the ven- 

 tricle, and rather to the left side. It is guarded by valves which 

 hang into the ventricle and it is divided by the lower edge of the 

 septum into right and left divisions, admitting blood from the right 

 and left auricles respectively. 



The ventricle is conical in shape with the apex backwards, and 

 has a small central cavity, with thick spongy walls. The spongy 

 character is due to great development of a network of interlacing 

 muscle strands similar to those of the auricles. The pockets 

 formed by muscle ridges serve to keep the pure and impure blood 

 separated. 



Arteries. — The truncus arteriosus., a cylindrical body, arises 

 from the right anterior border of the ventricle and runs forward 

 across the auricles. (See Figure 150.) It divides in front into a 

 right and left branch, each of which again divides into three aortic 

 arches, the carotid arch, the systemic arch and the pulmocutaneous 

 arch. 



(i) The carotid arch is the most anterior of the three arches. 

 It runs around the side of the esophagus and is connected dorsally 

 with the second or systemic arch. 



