118 ANIMAL PHYSIOLOGY. 
spiratory, and excretory organ; it is in itself a sort of abstract 
and brief chronicle of the whole physiological story in foetal life. 
All of the foetal organs, especially the muscles, abound in an 
animal starch (glycogen), which in some way, not well under- 
stood, forms a reserve fund of nutritive energy which is pretty 
well used up in the earlier months of pregnancy. We may 
suppose that the amceboid cells—all the undifferentiated cells 
of the body—feed on it in primitive fashion; and it will not 
be forgotten that the older the cells become, the’ more do they 
depart from the simpler habits of their earlier, cruder existence ; 
hence the disappearance of this substance in the later months 
of foetal life. 
In one respect the foetus closely resembles the adult: it 
draws the pabulum for all its various tissues from blood which 
itself may be regarded as the first completed tissue. We are, 
accordingly, led to inquire how this river of life is distributed ; 
in a word, into the nature of the foetal circulation. 
Fetal Circulation.—The blood leaves the placenta by the um- 
_bilical vein, reaches the inferior vena cava, either directly (by 
the ductus venosus), or, after first passing to the liver (by the 
vene advehentes, and returning by the vene revehentes), and 
proceeds, mingled with the blood returning from the lower ex- 
tremities, to the right auricle. This blood, though far from 
being as arterial in character as the blood after birth, is the 
best that reaches the heart or any part of the organism. After 
arriving at the right auricle, being dammed back by the Eus- 
tachian valve, it avoids the right ventricle, and shoots on into 
the left auricle, passing thence into the left ventricle, from 
which it is sent into the aorta, and is then carried by the great 
trunks of this arch to the head and upper extremities. The 
blood returning from these parts passes into the right auricle, 
then to the corresponding ventricle and thence into the pul- 
monary artery; but, finding the branches of this vessel un- 
opened, it takes the line of least resistance through the ductus 
arteriosus into the aortic arch beyond the point where its great 
branches emerge. It will be seen that the blood going to the 
head and upper parts of the body is greatly more valuable as 
nutritive pabulum than the rest, especially in the quantity of 
oxygen it contains; that the blood of the foetus, at best; is rela- 
tively ill-supplied with this vital essential; and as a result we 
find the upper (anterior in quadrupeds) parts of the foetus best 
developed, and a decided resemblance between the mammalian 
foetus functionally and the adult forms of reptiles and kindred 
