THE DEVELOPMENT OF THE UROGENITAL SYSTEM. 191 
centers) which preside over respiration, owing to its being 
roused into action by the lack of its accustomed supply of 
oxygen, the hitherto solid lungs are expanded; the pulmonary 
vessels are rendered permeable, hence the blood now takes the 
path of least resistance along them, as it formerly did through 
the ductus arteriosus. The latter, from lack of use, atrophies 
in most instances. The blood, returning to the left auricle of 
the heart from the lungs in increased volume, so raises the 
pressure in this chamber that the stream that formerly flowed 
through the foramen ovale from the right auricle is opposed 
by a force equal to its own, if not greater, and hence passes by 
an easier route into the right ventricle. The fold that tends to 
close the foramen ovale grows gradually over the latter, so that 
it usually ceases to exist in a few days after birth. 
At birth, ligature of the umbilical cord cuts off the placental 
circulation ; hence the ductus venosus atrophies and becomes a 
mere ligament. 
The placenta, being now a foreign body in the uterus, is ex- 
pelled, and this organ, by the contractions of its walls, closes the 
ruptured and gaping vessels, thus providing against hemor- 
rhage. 
CoITUS BETWEEN THE SEXES, 
In all the higher vertebrates congress of the sexes is essential 
to bring the male sexual product into contact with the ovum. 
Fie. 189.—Section of erectile tissue (Cadiat). a, trabeculee of connective tissue, with elastic 
bers, and bundles of plain muscular tissue (c) ; b, venous spaces (Schifer). 
