392 THE NUTRITION OF THE EMBRYO. [BOOK iv. 



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 appears 

 not to mingle largely with that of the inferior vena cava, but to fall 

 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 umbili- 

 cal 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 respiration of the foetus is con- 

 cerned, 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 

 much more a simple excretory 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 appears to be 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 dis- 

 tinctly 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 Jrom the left ventricle round the 

 aortic arch falls into the umbilical arteries and so reaches the 

 placenta. The foetal circulation then appears to be so arranged 

 that, while the most distinctly venous blood is driven by the right 

 ventricle back to the placenta to be arterialized, the most distinctly 

 arterial (but still mixed) blood is driven by the left ventricle to 

 the cerebral structures, which, we may conclude, 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, accordingly, that greater thickness of 

 the left ventricular walls, so characteristic of the adult, does not 

 become marked until close upon birth. 



963. 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 

 intra-uterine life the amount of oxygen in the blood passing from 

 the aortic arch to the brain is sufficient to prevent any inspiratory 

 impulses being originated in the bulbar respiratory centre. This, 

 during the whole period elapsing between the date of its struc- 

 tural establishment, or rather the consequent full development 



