Chap. X.] 



THE HEART AND CIRCULATION. 



213 



passes in Fig. 67, A. -An Eustachian valve or fold of membrane, 

 running from the opening of the postcaval (pi c.) to the foramen 

 ovale, directs most of the blood from the posterior regions of the 

 body through that orifice into the left auricle, while it pravents 

 the blood passing in by the precavals (pr. c.) from leaving the 

 right auricle. The blood from the posterior regions has been 

 arterialisecl in the allantois. 



/P r.c. 



l.au. 



pal. a. 



FlG. 67. FCETAL AND xVDULT CIRCULATION IN HEART. 



A. Foetal. B. Adult. 



ao. Aorta. b. a. Bulbus arteriosus. ca. Carotid artery, d. a. Ductus 

 arteriosus. 1. aw. Left auricle. I. v. Left ventricle, pr. c. Precavalvein. pt. c. 

 Postcaval vein. pul. a., pul. v. Pulmonary artery and vein. r. au. Right auricle. 

 r. v. Right ventricle. 



12. The blood leaves the ventricles by a bulbus arteriosus 

 (b. a.), which is divided by a septum into two chambers, which 

 will be converted into the root of the aorta and that of the 

 pulmonary artery. 



13. The blood from the left ventricle (arterialised in the 

 allantois) flows along the aortic chamber of the bulbus arteri- 

 osus, and so into the carotid (ca.) and aorta (ao.). The blood 

 from the right ventricle which has returned from the head 

 passes into the pulmonary chamber, which thus delivers blood 

 along the ductus arteriosus (d. a.) to the aortic arch, and in 

 small quantities to the as yet functionless lungs (by pul. a.). 



