DEVELOPMENT OF THE VASCULAR SYSTEM. 697 



the ductus venosus is also obliterated. Thirdly, the blood entering the 

 liver by the portal vein ( x ) passes by its right branch, as before, to the 

 right lobe. But in its left branch ( 4 ) the course of the blood is reversed. 

 This was formerly the right branch of the umbilical vein, its blood 

 passing from left to right. It now becomes the left branch of the 

 portal vein ; and its blood passes from right to left, for distribution to 

 the capillary vessels of the left lobe. 



According to Guy, the umbilical vein, in man, is completely closed 

 at the end of the fifth day after birth. 



The Heart, and Ductus Arteriosus. When the embryonic circulation 

 is first established, the heart is a straight tubular canal (Fig. 246), 

 receiving the veins at its lower extremity and giving off an arterial 



FIG. 246. 



FIG. 247. 



Earliest form of the 

 FCETAL HEART. 1. 

 Venous extremity. 2. 

 Arterial extremity. 



FCETAL HEART, bent 

 upon itself. 1. Ven- 

 ous extremity. 2. Ar- 

 terial extremity. 



F<ETAL HEART still farther de- 

 veloped. 1. Aorta. 2. Pul- 

 monary artery. 3, 3. Pul- 

 monary branches. 4. Ductus 

 arteriosus. 



trunk at its upper extremity. It soon afterward becomes bent in a 

 sharp curve (Fig. 247), which brings its venous and arterial extremities 

 nearer the same level ; but in such a way that its venous portion is 

 situated behind, and its arterial portion in front. It has still a single, 

 undivided cavity ; and the blood passes through it in a continuous 

 stream, turning upon itself at the point of curvature and emerging by 

 the arterial orifice. 



Subsequently the venous extremity of the heart shows a longitudinal 

 furrow which divides its originally single cavity into two secondary 

 compartments, placed side by side. These compartments become the 

 right and left auricles ; and they are furthermore separated, by trans- 

 verse constrictions, from the curved portion of the heart, which is to 

 form the ventricles. The cavities of the two ventricles become sepa- 

 rated from each other by the growth of a septum, which begins at the 

 most prominent part of the curvature or apex of the organ, and gradu- 

 ually extends toward its base. When the interventricular septum is 

 completely formed, its situation is indicated by a corresponding furrow 

 on the external surface of the organ. 



The primitive arterial trunk, springing from the upper extremity of 

 the heart, has already been divided, by a longitudinal furrow, into two 

 secondary trunks, lying side by side and nearly parallel with each other 



