DEVELOPMENT OF THE E-MBRYO. 



997 



rating organs, the Placenta and the foetal Liver, it is in the condition of arterial 

 blood; but, being mixed in the vessels with that which has been returned from 

 the trunk and lower extremities, it loses this character in some degree by the 

 time that it arrives at the Heart. In the right auricle, which it then enters, it 



The Foetal Circulation : 1. The umbilical cord consisting of the umbilical vein and two umbilical arteries, 

 proceeding from the placenta (2). 3. The umbilical vein dividing into three branches ; two (4, 4) to be dis- 

 tributed to the liver; and one (5), the ductus venosus, which enters the inferior vena cava (6). 7. The portal 

 vein, returning the blood from the intestines, and uniting with the right hepatic branch. 8. The right auri- 

 cle ; the course of the blood is denoted by the arrow proceeding from 8 to 9, the left auricle. 10. The left 

 ventricle; the blood following the arrow to the arch of the aorta (11), to be distributed through the branches 

 given off by the arch to the head and upper extremities. The arrows 12 and 13 represent the return of the 

 blood from the head and upper extremities through the jugular and subclavian veins, to the superior vena 

 cava (14), to the right auricle (8), and in the course of the arrow through the right ventricle (15) to the pul- 

 monary artery (16). 17. The ductus arteriosus, which appears to be a proper continuation of the pulmonary 

 artery; the offsets at each side are the right and left pulmonary artery cut off; these are of extremely small 

 size as compared with the ductus arteriosus. The ductus arteriosus joins the descending aorta (18, 18), which 

 divides into the common iliacs, and these into the internal iliacs, which become the umbilical arteries (19) 

 and return the blood along the umbilical cord to the placenta and external iliacs (20), which are continued 

 into the lower extremities. The arrows at the termination of these vessels mark the return of the venous 

 blood by the veins to the inferior cava. 



would be also mixed with the venous blood brought thither by the descending 

 Cava, were it not that a very curious provision t exists, to prevent (in great de- 

 gree, if not entirely) any such further dilution. The Eustachian valve has 



