Placental Cii'cnlation. 473 



The foetal eirenlatioii. 



Daring embryonic life the apparatus concerned in the circulation of the blood under- 

 goes various transfoiinations. 



The arrangement which exists in the second half of pregnancy up to birth is designated thi; 

 placental circulation. This presents the following peculiarities (see Figs. 510 and 511): 



The T. iimbiliealis (u7nbilical vein), devoid of valves, leads the blood, which has become 

 arterial in the placenta, through the umbilical cord (funiculus umbilicalis) to the navel and 

 thence in the free margin of the lig. falciforme hepatis to the fossa' venae umbilicalis of the 

 liver. There a part of the blood goes through direct branches to the capillary system of the 

 liver ; another part becomes mixed, in that the v. umbilicalis connects with the left branch of 

 the V. portae, with the venous contents of the portal vein (from the stomach, intestine, pancreas 

 and spleen) and also anives, along with this, in the capillary system of the liver; finally, 

 the third, smaller part runs directly to the left v. hepatica and thus to the v. cava inferior 

 through the ductus venosus [Arantii], which is situated in the fossa ductus venosi of the liver 

 (see also Fig. 5S1) and connects the v. umbihcaUs with the left v. hepatica. The v. cava 

 inferior carries mixed blood therefore on its passage through the foramen venae cavae of the 

 diaphragm. This is composed of 1st the pure arterial contents of the ductus venosus (Arantii], 

 2nd the contents of the v\. hepaticae, which cany the blood away from the capillary system 

 of the liver, and 3rd the venous contents of the v. cava inferior (especially from the renal veins, 

 pelvic veins and those of the lower extremities). Passing through the v. cava inferior, this 

 blood reaches the right atrium , which, in the foetus, is freely connected with the left atriimi 

 through the foramen ovale (see Fig. 423). Since the foramen ovale is situated in the direction 

 of the cun-ent of bL lod arriving through the v. cava inferior, and since, in addition, the valvula 

 venae cavae [inferioris, Eustachii] to a certain extent hinders an alteration of this direction 

 of the cm-rent, the main mass of blood streams into the leit atrium and in so doing receives 

 and mixes with only part of the venous blood which arrives in the heart through the v. cava 

 superior. Into the left atrium open the vv. pulmonales and their l)lood, which in the foetus 

 is venous, mixes with the other contents. The so-called mixed arterial blood thus arising 

 goes especially to the upper half of the body through the loft ventricle, the aorta ascendens 

 and the arcus aortae and its large branches, the aa. anonjTua , carotis sinistra and subclavia 

 sinistra ; only a part comes into the aorta thoracalis. 



The V. cava superior carries the venous lilood from the upper half of the body and 

 (through the v. azygos which is markedly developed in the foetus) fi-om the wall of the trunk, 

 to the right atrium. Thence a small part passes through the foramen ovale into the left 

 atrium, the larger part, however, mixed with a part of the blood from the v. cava infei-ior 

 and hence called mixed venous blood, enters the right ventricle and passes into the a. pid- 

 monaLis. The a. pulmonalis divides, in the foetus, into three branches: tlie rami dexter et 

 sinister, which go to the lungs (as yet not functioning) and the ductus arteriosus [Botalli], 

 which runs ia the direction of the main trunk and enters, at the end of the arcus aortae, 

 into the concave anterior wall of the latter. The greater part of the mixed venous blood now 

 rims through this ductus arteriosus as far as the beginning of the aorta thoracalis; there it 

 mixes with the residue of the mixed arterial blood coming from the end of the arcus aortae 

 and passes through the 1)ranches of the aorta thoracalis and aorta abdominahs to the intestmal 

 canal, to the inferior portion of the wall of the trunk and to the lower extremities, chiefly, 

 however, through the two aa. umbilicales (see p. 428) into the umbilical cord and to the 

 placenta, where it again becomes arterialized. 



