I0 88 REPRODUCTION 



respects, indeed, the foetus may be considered, especially, perhaps, 

 in the first stages of its development, as a part of the mother, an 

 additional, although very complex, organ rather than an independent 

 organism. 



The blood of the umbilical artery, although far from the level of 

 the ordinary arterial blood of the mother as regards its gaseous 

 content, is yet the best the foetus ever gets; and by a series of con- 

 trivances it is assured that this best should go first to the most 

 important parts the liver, the heart, and the head while the legs 

 and most of the abdominal organs have to put up with an inferior 

 supply. This is brought about mainly by the existence of three 

 short-cuts for the blood, which disappear in the adult circulation, the 

 ductus venosus, the ductus arteriosus, and the foramen ovale. 



The blood of the umbilical vein, rich in oxygen for foetal blood, 

 passes partly through the circulation of the liver, but a part takes 

 the route of the ductus venosus, and empties itself into the inferior 

 vena cava. The latter gathers up the more or less vitiated blood 

 from the inferior extremities and the renal and hepatic veins, and* 

 pours its mixed, but still fairly oxygenated, contents into the right 

 auricle. By means of the Eustachian valve, the jet coming from 

 the mouth of the inferior vena cava is directed into the left auricle 

 through the foramen ovale in the inter-auricular septum. There 

 it is joined by the trickle of blood which is creeping through the 

 unexpanded lungs. The left ventricle propels its contents through 

 the aorta, and thus a large part of this comparatively pure or 

 second-best blood is sent to the head and upper extremities. It 

 returns in a vitiated state by the superior vena cava into the right 

 auricle, and owing to the position of the Eustachian valve and the 

 direction of the current, it flows now, not through the foramen ovale, 

 but into the right ventricle. Thence it rs driven through the pul- 

 monary artery, but only a small quantity of it finds its way through 

 the lungs; the main stream is short-circuited through the ductus 

 arteriosus, and mingles with the contents of the thoracic aorta 

 below the origin of the cephalic and brachial vessels. 



We may now give something more of precision to the statements 

 that different parts of the body receive blood of different quality; 

 and it is possible roughly to divide the organs in this respect into 

 four categories: (i) The liver, which partakes both of the best and 

 the worst, the purified blood of the umbilical veins and the vitiated 

 blood of the intestines and spleen; (2) the heart, head, and upper 

 limbs, which receive the blood from the inferior extremities and 

 kidneys, mixed with the pure blood of the venous duct; (3) the 

 legs, trunk, intestines, and kidneys, which are fed chiefly by the 

 off-scourings of the cephalic end, mitigated, however, by a pro- 

 portion of mixed blood from the inferior cava; (4) the lungs, which 

 receive only a feeble stream of unmixed venous blood. 



