364 REPRODUCTION 



centre is to co-ordinate the contractions of the abdominal 

 muscles with those of the uterus. 



THE FCETAL CIRCULATION 



The oxygenated blood from the placenta travels by the 

 umbilical veins and reaches the inferior vena cava either 

 through the hver or directly through the ductus venosus. 

 In the inferior vena cava it becomes mixed with the venous 

 blood returning from the lower hmbs. Entering the right 

 auricle, it is directed across that chamber through the 

 foramen ovale into the left auricle — ^from there into the 

 left ventricle and aorta. This blood supphes chiefly the 

 head, neck, and upper hmbs. The venous blood from 

 these parts is collected in the superior vena cava. It 

 passes through the right auricle into the right ventricle. 

 From this chamber it passes by the pulmonary artery and 

 ductus arteriosus into the descending aorta, by which it 

 reaches the abdomen, lower hmbs and placenta (by the 

 umbihcal arteries). The special features to note are : — 

 (1) Very httle blood traverses the Imigs. (2) In the right 

 auricle there are two independent currents of blood, one 

 from the inferior vena cava to the left auricle, the other 

 from the superior vena cava to the right ventricle. The 

 two streams are kept apart by the Eustachian valve. 

 (3) Blood leaves the heart in two degrees of purity — the 

 more oxygenated blood from the left ventricle which 

 supphes the upper part of the body, and the venous blood 

 from the right ventricle which supplies the abdomen and 

 lower hmbs. (4) No part of the foetus receives fully- 

 oxygenated blood, since the blood which leaves the placenta 

 is mixed with that which is returning from the lower part 

 of the body of the foetus. 



The expansion of the lungs which occurs at the first in- 

 spiration determines a flow of blood from the right ventricle 

 into the lungs and from the lungs into the left auricle. 

 The foramen ovale between the auricles is a valvular arrange- 



