REPRODUCTION 1015 



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 is 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. 



These peculiarities of the embryonic circulation are in obvious 

 correspondence with the physiological events taking place in the 

 foetal body. The liver is not only the greatest gland in the embryo, 

 as it continues to be in the adult, but its activity seems to dwarf 

 that of all the other glands put together, and is in striking contrast 

 with the functional torpor of the lungs. From the third month of 

 intra-uterine life the secretion of bile begins and the intestines 

 gradually fill with meconium, of which the principal constituent is 

 bile. Accordingly the liver is most lavishly supplied with blood, 

 while the lungs are stinted. And since the liver has, as we have 

 already learnt, other and, in the adult at least, even more important 

 labours than excretion, a large portion of the blood it receives 

 is of the best quality : it enters the gland comparatively rich in 

 oxygen, and passes out comparatively poor ; while the lungs, which 

 have to be nourished only for their own sake, and are of no use 

 whatever till the child is born and respiration has begun, must be 

 content with the poorest fare with the crumbs that fall from the 

 table of foetal nutrition. The full-fed cephalic end of the embryo 

 grows far more rapidly than the half -starved inferior extremities, 



