FCETAL CIRCULATION. 297 



directly, and partly into the liver, which is, throughout 

 foetal life, exceedingly large. There are, therefore, four kinds 

 of blood mixed in the vena cava inferior above the level 

 of the liver, namely, vitiated blood from the lower limbs, 

 oxygenated blood from the placenta, blood which has passed 

 through both placenta and liver, and venous blood from the 

 portal system, acted on by the liver, but not oxygenated. This 

 mixture of blood, entering the heart behind the Eustachian 

 valve, is directed on through the back of the right auricle 

 and the foramen ovale, into the left auricle, and so into the 

 left ventricle and arch of the aorta, to supply the head, neck, 

 and upper limbs; but it is prevented by a fold in the aorta 

 from passing down into the descending part. Returning 

 from the head and upper limbs by the superior vena cava, 

 the blood enters the anterior part of the right auricle, and 

 mixing but little with the stream crossing through the back 

 part of that cavity, passes into the right ventricle, and is 

 propelled through the pulmonary artery, ductus arteriosus, 

 and descending aorta, partly to supply the lower parts of the 

 body, but in larger part to proceed again to the placenta by 

 the hypogastric arteries. Thus, curiously enough, there is 

 no part of the foetus nourished with blood the whole of which 

 has been purified. The whole of the blood returned from the 

 lower limbs is mixed with the current to the head and arms; 

 and the trunk and lower limbs are entirely supplied with 

 blood which has previously circulated in the head and arms 

 and been filled with further impurities. 



Immediately on birth, the circulation through the placenta 

 ceases, the impulse to breathe begins, and with the first 

 inspiration, not only air but blood is drawn into the expand- 

 ing lungs; the right and left pulmonary arteries are filled, 

 and blood ceases to pass through the ductus arteriosus; the 

 current in the arch of the aorta is continued on into the 

 lower parts of the body; and a fold of membrane, the valve 

 of the foramen ovale, which springs from the back of that 

 opening, and projected into the left auricle, now occludes the 

 opening, and in a short time becomes firmly bound down 

 to the walls. 



217. The period of gestation in the human species is about 

 two hundred and seventy days, but is undoubtedly liable to a 



