678 DEVELOPMENT OF THE CIRCULATORY APPARATUS. 



from right to left, to be distributed to the capillaries of the left 



lobe. 



According to Dr. Guy, the umbilical vein is completely closed at 

 the end of the fifth day after birth. 



Development of the Heart, and the Ductus Arteriosus. When the 

 embryonic circulation is first established, the heart is a simple tubu- 

 lar sac (Fig. 266), receiving the veins at its lower extremity, and 

 giving off the arterial trunks at its upper extremity. By the pro- 

 gress of its growth, it soon becomes twisted upon itself; so that the 

 entrance of the veins, and the exit of the arteries, come to be placed 

 more nearly upon the same horizontal level (Fig. 267); but the 

 entrance of the veins (i) is behind and a little below, while the exit 

 of the arteries (2) is in front and a little above. The heart is, at 

 this time, a simple twisted tube ; and the blood passes through it 

 in a single continuous stream, turning upon itself at the point of 

 curvature, and passing directly out by the arterial orifice. 



Fig. 266. 



Fig. 267. 



Fig. 268. 



\ 



Earliest form of F<ETAL 

 HEART. 1. Venous ex- 

 tremity. 2. Arterial ex- 

 tremity. 



FCETAL HF.ART, twisted 

 upou itself. 1. Venous ex- 

 tremity. 2. Arterial extre- 

 mity. 



FCETAL HEART, divided 

 into right and left cavities. 

 1. Venous extremity. 2. 

 Arterial extremity. 3, 3. 

 Pulmonary branches. 



Soon afterward, this single cardiac tube is divided into two paral- 

 lel tubes, right and left, by a longitudinal partition, which grows 

 from the inner surface of its walls and follows the twisted course 

 of the organ itself. (Fig. 268.) This partition, which is indicated 

 in the figure by a dotted line, extends a short distance into the 

 commencement of the primitive arterial trunk, dividing it into two 

 lateral halves, one of which is in communication with the right side 

 of the heart, the other with the left. 



About the same time, the pulmonary branches (3, 3) are given 

 off from each side of the arterial trunk near its origin ; and the 

 longitudinal partition, above spoken of, is so placed that both these 

 branches fall upon one side of it, and are both, consequently, given 

 off from that division of the artery which is connected with the right 

 side of the heart. 



