DEVELOPMENT OF THE FCETUS. 



45 



collected by the vena cava 

 descendens, and emptied also 

 into the right auricle, from 

 whence it passes into the right 

 ventricle^ from thence mainly- 

 through the ductus arteriosus 

 into the aorta, a small portion 

 only passing into the pulmo- 

 nary artery. From this it 

 will be seen that the liver is 

 the only organ that receives 

 the blood from the placenta 

 unmixed. Every other organ 

 receiving mixed arterial and 

 venous. 



After Urth, the current is 

 entirely changed by the esta- 

 blishment of respiration, and 

 its diversion into the lungs. 

 The ductus venosus, and the 

 ductus arteriosus shrivel up 

 into ligamentous cords ; the 

 foramen ovale shortly closes 

 by the apposition of its valve ; 

 the pulmonary artery and 

 vein become dilated to receive 

 the increased current, and the 

 circulation, which before re- 

 sembled that of the higher 

 reptiles, becomes now that of 

 the perfect mammal. It not 

 unfrequently happens, how- 

 ever, that some arrest of de- 

 velopment takes place, and prevents the completion of these changes ; 



* The fcelal 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 distributed 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 in- 

 testines, and uniting with the right hepatic branch ; 8, the right auricle ; the course of the 

 blood is denoted by the arrow, proceeding from 8 to 9, the left auricle ; JO, 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 auricb- (8), and in the 

 course of the arrow through the right ventricle (J5), to the pulmonary artery (16); 17, the 

 ductus arteriosus, which appears to be a proper continuation of the pulmonary artery— ihe 

 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 descend- 

 ing aorta (18, T8), 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; while the other divisions, the external iliacs (20), 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. 



