KEPKODUCTION 



439 



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



vena eava, p.v.c., which carries it to the right auricle. In this 



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



ovale,/o., a hole in the septum between the auricles, and it thus 



passes to the left auricle, and thence to the left ventricle, l.v., 



which drives it into the aorta, a.a., and chiefly up to the head, 



ant.a. From the head the blood returns to the superior vena 



eava, a.v.c., and, passing through 



the right auricle, enters the right 



ventricle, r.v., which drives it 



into the pulmonary artery, p.a. 



Before birth this artery opens into 



the aorta by the duetus arteriosus, 



d.a., 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 ventricle, and the mixture is 



sent to the lower part of the body 



through the descending aorta, 



po.a. From each iliac artery, i.a., 



an umbilical artery, u.a., passes 



off, and these two vessels carry 



the blood in the umbilical cord, 



u.c., to the placenta. 



When the foetus is born in the deciduata, the placenta is 

 pressed upon by the contraction of the uterus, and the flow of 

 blood between child and mother is arrested. As a result of this 

 the respiratory centre is no longer supplied with pure blood 

 and is stimulated to action. The lungs expand and the blood 

 flows through them. In the non-deciduata the same result is 

 brought about by the separation of the foetal from the maternal 

 placenta. In the duetus venosus a clot forms and the vessel 

 becomes obliterated. The duetus arteriosus also closes up, and 

 the foramen ovale is occluded. The circulation now takes the 

 normal course in post-natal life. 



Our knowledge of the differences between the physiological 

 processes in intra-uterine and in extra-uterine life is still very 

 imperfect, and the subject cannot be further discussed here. 



FIG. 176. Longitudinal Section 

 through the tip of a villus of 

 the human placenta, covered by 

 its trophoblast layer, and contain- 

 ing a loop of blood vessels, and 

 projecting into a large blood 

 sinus, /. V.S. in the maternal 



