242 HUMAN PHYSIOLOGY 



of the emergency and the success with which it is usu- 

 ally met make a story which is too fascinating to be 

 slighted. 



The lungs of the embryo are dense and compressed. 

 No air has ever entered them. Their blood-vessels as 

 well as their air passages are nearly closed. But little 

 blood makes its way from the right ventricle to the left 

 auricle through the pulmonary circuit. A temporary 

 provision is made to enable the right ventricle to send 

 out blood in spite of the obstructed state of the vessels 

 in the lungs. The device referred to is the ductus 

 arteriosus, a by-pass from the pulmonary artery to the 

 aorta close by. The blood expelled by the right ventricle 

 slips through to join that pumped by the left and the 

 result is that both ventricles are united to supply the 

 arteries of the child. 



As little blood passes through the lungs there can be 

 little to enter the left auricle through the four pulmonary 

 veins. But there is an opening through the middle 

 partition between the two auricles called the foramen 

 ovale. Through this the left auricle is supplied by an 

 overflow from the right. We have seen that both 

 ventricles deliver to the arteries and now we find that 

 both auricles share the blood returning from the veins. 

 We have next to show how the lack of lungs is supplied. 

 The substitute is the organ called the placenta. 



The placenta is a disc of tissue rich in blood-vessels 

 which is in close contact with the lining of the uterus. 

 A long cord (the umbilical cord) unites the placenta 

 with the navel of the child. The cord serves chiefly to 

 convey blood to and from the placenta. It contains 

 two branches of the arterial system of the embryo which 

 are derived from the permanent vessels in the pelvis. 

 The blood which is diverted through these vessels is sent 

 through minute spaces of the placenta where it is sepa- 

 rated only by the thinnest of walls from the maternal 

 blood in the wall of the uterus. A single, so-called vein 

 comes back in the cord and delivers the blood partly to 



