4i8 



HUMAN PHYSIOLOGY 



JV.S.P 



FlQ. 161. Longitudinal Section 

 through the tip of a villus of 

 the human placenta, covered by 

 its trophoblast layer, and con- 

 taining a loop of blood vessels, 

 and projecting into a large blood 

 sinus, I.V.S., in the maternal 

 mucosa. 



It is the foetal alimentary canal supplying the necessary 

 material for growth and development; and it is the foetal 

 kidney through which the waste nitrogenous constituents are 



thrown otf. In the mesoblast, 

 through which the allantoic 

 arteries pass out, a large vesicle 

 filled with fluid, and at first 

 communicating with the pos- 

 terior gut, is developed. This 

 is the allantois (Fig. 159, ////.). 



3. Foetal Circulation. 



P er f rmance f these 

 functions by the placenta is 

 associated with a course of 

 circulation of the blood some- 

 what different to that in the 

 post-natal state (Fig. 162). 



The blood coming from the 

 placenta to the foetus is collected 

 into a single umbilical vein which passes to the liver. This 

 divides into the ductus venosus, passing straight through 

 the organ, and into a series of capillaries among the cells. 

 From these the blood flows away in the hepatic vein to the 

 inferior vena cava, which carries it to the right auricle. In 

 this it is directed by a fold of endocardium, through the 

 foramen ovale, a hole in the septum between the auricles, 

 and it thus passes to the left auricle, and thence to the left 

 ventricle, which drives it into the aorta, and chiefly up to 

 the head. From the head the blood returns to the superior 

 vena cava, and, passing through the right auricle, enters the 

 right ventricle, which drives it into the pulmonary artery. 

 Before birth this artery opens into the aorta by the ductus 

 arteriosus, while the branches to the lungs are still very 

 small and unexpanded. In the aorta, this impure blood 

 from the head mixes with the purer blood from the left ven- 

 tricle, and the mixture is sent to the lower part of the body 

 through the descending aorta. From each iliac artery an 

 umbilical artery passes off, and these two vessels carry the 

 blood to the placenta. 



