SANGUIFEROUS SYSTEM. 519 



higher vertebrata. The development of a second auricle by a 

 septum rising through the first, the partial division of the 

 ventricle by a muscular septum, the imperfect separation of 

 the venous from the arterialized blood, the entire metamor- 

 phosis of the branchial arches, and the development of the 

 pulmonary arteries, change, at length, this system of the 

 human embryo to that of an air-breathing reptile. The 

 ascending septum of the yet single ventricle, draws up and 

 cleaves the apex of the embryo's heart, and makes it double, 

 like that preserved through life in the lamantin and the 

 dugong ; and this septum, on reaching to the origin of the still 

 single systemic artery, divides it also to the extent of the 

 ductus arteriosus, arid entirely severs that portion, con- 

 taining the pulmonary arteries, from the primitive bulb of the 

 aorta. The septum of the auricles is developed in man, as 

 in the reptile, before that of the ventricles; but the sep- 

 tum of his ventricles is necessarily completed, before the 

 entire separation of the auricles, at birth, by the closing 

 of the foramen ovale; and the reptile circulation of man in 

 the amniotic fluid, must be continued for a time, by a diffe- 

 rent route from that followed in the air-breathing animal. 



The arterial blood of the human foetus (142. B. C.), as of 

 other mammalia, aerated and replenished by traversing the 

 placenta, is returned by the umbilical vein (B. 4.), to be sent, 

 along with the visceral blood poured into it from the vena 

 portce (B. 1.2. 3.), through every part of the liver (B. 5. 6). 

 A small portion only of the placental fluid now follows its 

 primitive course, from the umbilical vein (B. 4.) directly 

 through the ductus venosus (B. 7-)> into the inferior vena cava 

 (B. 11.) The portal blood (B. 1. 2. 3.) entering the dilated 

 sinus (B. 5.) of the umbilical vein (B. 4.), is distributed with 

 that of the minute hepatic artery, chiefly through the right 

 lobe (B. 6.) of the liver, which here is less than the left. 

 The arterialized fluid received from the hepatic veins, (142. 

 C. c.) and the ductus venosus (C. b.), is conveyed, along with 

 the blood of the abdominal cava (C. .), into the great vena 

 cava inferior (C. d.) and the right auricle (C. e.) of the heart. 

 By the aid of the Eustachian valve, it is directed from the 

 right auricle (C. e.), through the foramen ovale (C. p.), into 

 the left auricle (C. /.), where it mingles with the blood from 

 the yet small pulmonary veins (C. v.) From the left auricle 

 (C./.), this aerated placental fluid is conveyed through the 



