262 THE FIFTH DAY. [CHAP. 



The original curvature to the right now forms the 

 apex of the ventricles, and the two auricular appendages 

 are placed at the anterior extremity of the heart. 



The most noticeable difference (in the ventral view) 

 is the still externally undivided condition of the bulbus 

 arteriosus. 



The subsequent changes which the heart undergoes 

 are concerned more with its internal structure than 

 with its external shape. Indeed, during the next three 

 days, viz. the eighth, ninth, and tenth, the external 

 form of the heart remains nearly unaltered. 



In the auricular portion, however, the septum which 

 commenced on the fifth day becomes now more con- 

 spicuous. It is placed vertically, and arises from the 

 ventral wall; commencing at the canalis auricularis 

 and proceeding backwards, it does not as yet reach the 

 opening into the sinus venosus. 



The blood from the sinus, or, as we may call it, the 

 inferior vena cava, enters the heart obliquely from the 

 right, so that it has a tendency to flow towards the lelt 

 auricle of the heart, which is at this time the larger of 

 the two. 



The valves between the ventricles and auricles are 

 now well developed, and it is about this time that the 

 division of the bulbus arteriosus into the aorta and 

 pulmonary artery becomes visible on the exterior. 



By the eleventh or thirteenth day the right auricle 

 has become as large as the left, and the auricular sep- 

 tum much more complete, though there is still a small 

 opening, the foramen ovale, by which the two cavities 

 communicate with each other. Through this foramen 

 the greater part of the blood of the vena cava inferior, 



