

618 POSTAL CIRCULATION. 



lobe. 2dly, A single branch, which communicates with the portal vein 

 in the transverse fissure, and supplies the right lobe. 3dly, A large 

 branch, the ductus venosus, which passes directly backwards, and joins 

 the inferior cava. In the inferior cava the pure blood becomes mixed 

 with that which is returning from the lower extremities, and is carried 

 through the right auricle, guided by the Eustachian valve, and through 

 the foramen ovate into the left auricle. From the left auricle it passes 

 into the left ventricle, and from the left ventricle into the aorta, 

 whence it is distributed, by means of the carotid and subclavian 

 arteries, principally to the head and upper extremities. From the head 

 and upper extremities, the impure blood is returned by the superior 

 vena cava to the right auricle ; from the right auricle, it is propelled 

 into the right ventricle ; and from the right ventricle into the pul- 

 monary artery. In the adult, the blood would now be circulated 

 through the lungs, and oxygenated ; but in the foetus the lungs are 

 solid, and almost impervious. Only a small quantity of the blood 

 passes therefore into the lungs ; the greater part rushes through the 

 ductus arteriosus, into the commencement of the descending aorta, 

 where it becomes mingled with that portion of the pure blood which 

 is not sent through the carotid and subclavian arteries. 



Passing along the aorta, a small quantity of this mixed blood is dis- 

 tributed by the external iliac arteries to the lower extremities ; the 

 greater portion is conveyed by the internal iliac, hypogastric, and um- 

 bilical arteries to the placenta; the hypogastric arteries proceeding 

 from the internal iliacs, and passing by the side of the fundus of the 

 bladder, and upwards along the anterior wall of the abdomen to the 

 umbilicus, where they become the umbilical arteries. 



From a careful consideration of this circulation, we shall perceive 

 1st. That the pure blood from the placenta is distributed in consider- 

 able quantity to the liver, before entering the general circulation. 

 Hence arises the abundant nutrition of that organ, and its enormous 

 size in comparison with the other viscera. 



2dly. That the right auricle is the scene of meeting of a double 

 current ; the one coming from the inferior cava, the other from the 

 superior, and that they must cross each other in their respective course. 

 How this crossing is effected the theorist will wonder ; not so the 

 practical anatomist ; for a cursory examination of the foetal heart will 

 show, 1. That the direction of entrance of the two vessels is so oppo- 



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 pulmonary 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 hypo- 

 gastric arteries (19), and return the blood along the umbilical cord to the pla- 

 centa ; while the other divisions, the external macs (20), are continued into the 

 lower extremities. The arrows at the terminations of these vessels mark the 

 return of the venous blood by the veins to the inferior cava. 



