THE DEVELOPMENT OP THE EMBRYO ITSELF. 129 



isting and gradually, though- at last rapidly, reaching that 

 climax which seems like a vital storm. The law of rhythm 

 aflPects the nervous system as others, and upon this depends 

 the direction and co-ordination of those many activities which 

 make up parturition. We have seen that throughout the whole 

 of foetal life changes in one part are accompanied by correspond- 

 ing changes in others ; and in the final chapter of this history 

 it is not to be expected that this connection should be severed, 

 though it is not at present possible to give the evolution of this 

 process with any more than a general approach to probable 

 correctness. 



CHANGES IN THE CIRCUIiATION AFTER BIRTH. 



When the new-born mammal takes the first breath, effected 

 by the harmonious action of the respiratory muscles, excited to 

 action by stimuli reaching them from the nerve-center (or 

 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 fiowed 

 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 thac 

 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 ruptui'ed and gaping vessels, thus providing against haemor- 

 rhage. 



COITUS. 



In all the higher vertebrates congress of the sexes is essential 

 to bring the male sexual product into contact with the ovum, 

 9 



