DEVELOPMENT 633 



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 ductus 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 animal is born, the flow of blood between it and 

 the 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 are thus expanded and the blood 

 flows through them. In the ductus venosus a clot forms 

 and the vessel becomes obliterated. The ductus arteriosus also 

 closes up, and the foramen ovale is occluded. The circulation 

 now takes the normal course in post-natal life. 



VIII. Gestation and Delivery. 



The length of gestation is different in different animals — 



At the end of this period labour occurs, and the foetus and 

 its membranes are expelled. 



The mechanics of labour must be studied with obstetrics. 



Nervous Control. — The uterus is supplied by fibres leaving 

 the spinal cord by true sympathetic fibres from the splanchnics. 

 These fibres pass to the inferior mesenteric ganglion and on in 

 the hypogastric nerves to end in two large plexuses or ganglia, 

 containing numerous nerve cells, one on each side of the cervix. 

 From these, fibres pass to the uterus and to the vagina. 



There is evidence that, in lower animals at least, the 

 contents of the uterus may be expelled after complete separa- 



