Chap. X.] THE HEART AND CIRCULATION. 211 



In the heart itself the student will make out the general 

 resemblance to that of the rabbit. The main difference is that 

 the tricuspid valve is replaced by a strong muscular fold, nearly 

 as thick as the walls of the right ventricle, which applies itself 

 over the auriculo-ventricular orifice, and converts the whole 

 ventricle, except at the orifice of the pulmonary artery, into a 

 muscular chamber. There are no chordae tendinese nor papillary 

 muscles in this ventricle. 



Notes on Embryonic and Fatal Circulation in Fowl and Rabbit. 



1. The aortic arch is developed from the second branchial 

 arch, the right arch being suppressed in the mammal, the left in 

 the bird. 



2. The carotids are developed from the arch or arches anterior 

 to the second branchial. 



3. The pulmonary arteries are developed from the arch or 

 arches posterior to the second branchial. In the bird the arches 

 of both sides persist ; in the mammal that of the left only. 



4. In both bird and mammal, before lung-respiration sets in, 

 the blood flows in the main from the pulmonary into the aorta 

 by a connecting branch (dudus arteriosus), similar to that shown 

 for the tadpole in Fig. 62, II. 



5. From the aorta the blood is delivered in the main by the 

 vitelline artery to the umbilical vesicle or yolk-sac, and by the 

 allantoic arteries to the allantois. 



6. From the umbilical vesicle and the allantois the blood 

 returns by vitelline and allantoic veins, which unite and receive 

 a small mesenteric branch from the intestines. They then pass 

 into the liver, some of the blood being distributed to that organ, 

 but much of it passing through it by the ductus venosus to the 

 sinus venosus. (See Fig. 66.) 



7. From the anterior part of the body the blood is returned 

 by anterior cardinal veins (a. c.), one on each side; and from 

 the Wolffian bodies and posterior regions by posterior cardinal 

 veins (p. c.), one on each side. The anterior and posterior car- 

 dinals on each side combine and form factors of the ductus 

 Cuvieri (d. c.) which leads the blood to the heart. 



