MAMMALIA 285 



auricle by the posterior vena cava is directed through the 

 foramen ovale into the left auricle. While these changes have 

 been taking place the conus has become divided by a septum 

 which separates the vessel into the pulmonary artery related 

 to the right ventricle, and into an aorta opening from the 

 left ventricle. During embryonic life the former maintains 

 its connexion with the dorsal aorta. 



The blood, therefore, from the placenta is directed through 

 the foramen ovale into the left auricle, and so to the left 

 ventricle, from which it is despatched to the head. The venous 

 blood of the embryo, brought back mainly by the anterior 

 venae cavae to the right auricle, is sent by the right ventricle 

 to the dorsal aorta by the ductus arteriosus that is to say, the 

 original remaining part of the pulmonary arch communicating 

 with the dorsal aorta. 



At birth, all this is changed ; for with the distension of the 

 lungs the blood from the right ventricle is sent to the pul- 

 monary vessels and the ductus arteriosus closes. The foramen 

 ovale is closed, and thereafter the posterior vena cava is 

 concerned entirely with venous blood. Moreover, with the 

 stoppage of the placental circulation, the allantoic vessels 

 atrophy. A circulation is thus established which is that of 

 the adult. The venous blood is received by the right auricle 

 and sent to the right ventricle, and thence to the lungs by the 

 two branches of the pulmonary artery. The arterial blood is 

 brought to the left auricle by the pulmonary veins and sent 

 to the left ventricle, from which it is carried by the aorta to all 

 parts of the body. 



The heart may be more conveniently studied by procuring 

 the heart of a sheep. Externally, it may be at once correctly 

 orientated by noting that the apex of the heart is that of the 

 left ventricle, and that the ventral side of the heart is rounded, 

 while the dorsal side is flatter. The outward evidence of the 

 septum separating the ventricles is the sulcus containing the 

 coronary vein, and the ventral one is more conspicuous than 

 the dorsal. The anterior broad end of the heart is occupied 

 by the large blood-vessels and the auricular appendages. If 

 the heart be held, therefore, with the apex downwards and the 



