THE NUTRITION OF THE EMBRYO. 851 



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 con- 

 tents 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 excretory 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 around the aortic arch falls 

 into the umbilical arteries and so reaches the placenta. The foetal circula- 

 tion 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 dis- 

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

 cerebral structures, which have more need of oxygen than have the other 

 tissues. Contrary to what takes place afterward, the work of the right ven- 

 tricle 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. 



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 medulla oblongata 

 is sufficient to prevent any inspiratory impulses being originated in the 

 medullary respiratory centre. This, during the whole period elapsing be- 

 tween the date of its structural establishment, or rather the consequent full 

 development of its irritability, and the epoch of birth, remains dormant; 

 the oxygen-supply to the protoplasm of its nerve-cells is never brought so 

 low as to set going the respiratory molecular explosions. As soon, however, 

 as the intercourse between the maternal and umbilical blood is interrupted 

 by separation of the placenta or by ligature of the umbilical cord, or when, 

 as by the death of the mother, the umbilical blood ceases to be replenished 

 with oxygen by the maternal blood, or when in any other way blood of 

 sufficiently arterial quality ceases to find its way by the left ventricle to the 

 medulla oblongata, the supply of oxygen in the respiratory centre sinks, 

 and when the fall has reached a certain point an impulse of inspiration is 

 generated and the foetus for the first time breathes. This action of the 

 respiratory centre may be assisted by adjuvant impulses reaching the centre 

 along various afferent nerves, such as those started by exposure of the body 



