678 F(ETAL CIRCULATION. [BOOK iv. 



auricle, passing from which into the left ventricle it is driven into 

 the aorta. Part of the umbilical blood, however, instead of passing 

 directly to the inferior cava, enters by the portal vein into the 

 hepatic circulation, from which it returns to the inferior cava by 

 the hepatic veins. The inferior cava also contains blood coming 

 from the lower limbs and lower trunk. Hence the blood which 

 passing from the right auricle into the left auricle through the 

 foramen ovale is distributed by the left ventricle through the 

 aortic arch, though chiefly blood coming direct from the placenta, 

 is also blood which on its way from the placenta has passed 

 through the liver and blood derived from the tissues of the lower 

 part of the body of the foetus. The blood descending as foetal 

 venous blood from the head and limbs by the superior vena cava 

 does not mingle with that of the inferior vena cava, but falls into 

 the right ventricle, from which it is discharged through the ductus 

 arteriosus (Botalli) into the aorta, below the arch, whence it flows 

 partly to the lower trunk and limbs, but chiefly by the umbilical 

 arteries to the placenta. A small quantity only of the contents 

 of the right ventricle finds its way into the lungs. Now the blood 

 which comes from the placenta by the umbilical vein direct into 

 the right auricle is, as far as the foetus is concerned, arterial blood ; 

 and the portion of umbilical blood which traverses the liver 

 probably loses at this epoch very little oxygen during its transit 

 through that gland, the liver being at this period a simple ex- 

 cretory rather than an actively metabolic organ. Hence the blood 

 of the inferior vena cava, though mixed, is on the whole arterial 

 blood; and it is this blood which is sent by the left ventricle 

 through the arch of the aorta into the carotid and subclavian 

 arteries. Thus the head of the foetus is provided with blood 

 comparatively rich in oxygen. The blood descending from the 

 head and upper limbs by the superior vena cava is distinctly 

 venous ; and this passing from the right ventricle by the ductus 

 arteriosus is driven along the descending aorta, and together with 

 some of the blood passing from the left ventricle round the aortic 

 arch falls into the umbilical arteries and so reaches the placenta. 

 The fcetal circulation then is so arranged, that while the most 

 distinctly venous blood is driven by the right ventricle back to the 

 placenta to be oxygenated, the most distinctly arterial (but still 

 mixed) blood is driven by the left ventricle to the cerebral struc- 

 tures, which have more need of oxygen than have the other tissues. 

 Contrary to what takes place afterwards, the work of the right 

 ventricle is in the foetus greater than that of the left; and, ac- 

 cordingly, that greater thickness of the left ventricular walls, 

 so characteristic of the adult, does not become marked until close 

 upon birth. 



In the later stages of pregnancy the mixture of the various 

 kinds of blood in the right auricle increases preparatory to the 

 changes taking place at birth. But during the whole time of 



