328 Veterinary Obstetrics 



The Fetal Circulation. 



In the earlier stages of the fetal circulation, all the blood re- 

 turned to the heart, whether from the embryo, the placenta or 

 the vitelline sac, is emptied into the sinus venosus, from which 

 it passes into the cavity of the common auricle. In this way a 

 complete mixture of the blood from all sources necessarily 

 occurs. This mixed blood is then forced out by the auricle, 

 through the truncus arteriosus, toward the head of the fetus. 



The sinus venosus later becomes a part of the auricle and into 

 it empty the Cuvierian veins and the posterior vena cava. The 

 septum between the two auricles is gradually formed, but there 

 remains the foramen ovale, through which the blood can pass 

 with more or less freedom. The po.sterior vena cava opens very 

 near to the foramen ovale and a small valve at this point tends 

 to cause the blood to flow from the posterior vena cava to the 

 left auricle, by way of the foramen. In this way, considerable 

 admixture of blood from the three different vessels still ensues. 

 L,ater, as the left Cuvierian vein disappears, the blood from the 

 head and anterior limbs is all returned through the right vein, 

 or anterior vena cava, so that there are now only two vessels, 

 both of which empty into the right auricle. At this period, the 

 blood which enters the auricle from the anterior vena cava, com- 

 ing from the head and both fore-limbs, is entirely venous in 

 character, while that from the posterior vena cava is chiefly 

 arterial, or red, blood coming irom the placenta, with a small 

 portion of venous blood added from the posterior limbs and 

 other parts of the body. 



By an elaboration of the valve in the foramen ovale and a 

 valvular fold at the opening of the posterior vena cava, the 

 blood from this vein is finally all carried directly across the 

 right auricle into the left auricular chamber. 



The right auricle now receives blood from the anterior and 

 posterior vena cavse and a small amount from the coronary 

 sinus. The blood comes respectively from the head and fore- 

 limbs, from the hinder part of the body, the placenta, the intes- 

 tine and the liver and from the walls of the heart itself. The 

 blood entering the heart through the posterior vena cava de- 

 serves special consideration. The right allantoic vein disappears 

 early in fetal life, while the left persists and enters the body 



