194 THE FIFTH DAY. [CHAP. 



bulbus is much less marked on the seventh day than it was 

 before. 



At the end of the sixth day, and even on the fifth day 

 (Figs. 60, CI), the appearance of the heart itself, without 

 reference to the vessels which come from it, is not very 

 dissimilar from that which it presents when adult. 



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 arte- 

 riosus. 



About the sixth or, perhaps, even on the fifth day, the 

 pericardium, according to Von Baer, makes its appearance. 

 Its mode of formation is not exactly known, but it probably 

 takes origin from folds of the lining of the thoracic cavity 

 which meet and coalesce. 



9. 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 re- 

 mains nearly unaltered. 



In the auricular portion however, the septum which com- 

 menced on the fifth day becomes now more conspicuous. It 

 is placed vertically, and arises from the ventral wall ; com- 

 mencing 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 left 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 septum much 

 more complete, though there is still a small opening, the 

 foramen ovale, by which the two cavities communicate 

 with each other. Througli this foramen the greater part of 

 the blood of the vena cava inferior, which is now joined just 



