REPRODUCTION 



419 



When the child is born, the flow of blood between it and 

 the mother is arrested, the umbilical cord being tied. 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 ductus venosus a a - KC - 



clot forms and the vessel 

 becomes obliterated. The 

 ductus arteriosus also 

 closes up, and the fora- 

 men ovale is occluded. 

 The circulation now takes 

 the normal course in 

 post-natal life. 



Our knowledge of the 

 differences between the 

 physiological processes 

 in embryonic and in 

 extra- uterine life is still 

 very imperfect, and the 

 subject cannot be further 

 discussed here. 



$. Gestation and 

 Delivery. 



The child remains in 

 the uterus for nine 

 months, and at the end 

 of that period it is 

 expelled during labour. 

 Labour may be 



FIG. 162. Scheme of Circulation in the 

 Foetus, u.v., umbilical vein ; d.v., ductus 

 venosus ; p.v.c., inferior vena cava pour- 

 ing blood through the right auricle and 

 through the foramen ovale, jjf.o. , into the 

 left heart; a.v.c., superior venfa cava bring- 

 ing blood from head to pass through the 

 right side of the heart, and through the 

 ductus arteriosus, d.a. 



divided 

 into three stages. In the 



first stage the uterus passes into contractions at intervals, 

 and the lower part or cervix is dilated. In the second stage 

 the contractions become stronger, and with the help of the 

 contractions of the abdominal muscles, the child is expelled 

 through the vagina. After this the uterus is usually 



