710 MANUAL OF PHYSIOLOGY. 



branches of the aorta (the hypogastric arteries) to the placenta, 

 where it is aerated and purified, etc. 



It is evident, then, that, as the placenta is the great renovating 

 organ of the blood of the foetus, the blood in the umbilical 

 vein is the most arterial in the foetal circulation. The blood in 

 the ascending vena cava and first part of the aorta is likewise 

 fairly arterial, but the blood in the descending aorta is of a mixed 

 character, as it contains blood which has nourished the head and 

 neck, besides blood which has come from the placenta by the 

 inferior vena cava through the right auricle, foramen ovale, left 

 auricle, and left ventricle. 



As the foetal lungs are not called into play until after birth, 

 but little blood passes to them in the foetus ; this state of things 

 is, however, completely altered at birth, when the lungs of the 

 child expand, the pulmonary arteries increase in size, and the 

 ductus arteriosus dwindles in a corresponding degree. 



The liver, which in the foetus is of relatively greater size than 

 in the adult, receives much blood coming from the placenta to 

 the heart, and is thought to contribute to it several essential con- 

 stituents. 



The head and brain, which are largely developed in the foetus, 

 receive well-aerated blood, namely, the placental blood, which 

 has passed through the liver, and, in the inferior vena cava, is 

 mixed with blood coming from the lower limbs. The rest of the 

 foetus receives blood that is less well aerated, as it is mixed with 

 that which is returned from the head and neck to the right side 

 of the heart, and which is sent through the ductus arteriosus to 

 join the general blood current in the aorta going to the viscera 

 and lower extremities. 



DEVELOPMENT OF THE EYE. 



The optic vesicles arise from the anterior cerebral vesicle at a 

 very early period, and their cavities are continuous with that of 

 the fore-brain. With the development of the rudimentary cerebral 

 hemispheres the optic vesicles become displaced downwards, and 

 their cavities open into the junction of the cavities of the cerebral 



