THE DEVELOPMENT OP THE EMBRYO ITSELF. 125 



all the constituents of a normal diet are provided, easy of diges- 

 tion. 



Bile, inspissated and mixed with the dead and cast-off epi- 

 thelium of the alimentary tract, is abundant in the intestine at 

 birth ; but bUe is to be regarded perhaps rather in the light of 

 - an excretion than as a digestive fluid. The skin and kidneys, 

 though not functionless, are rendered unnecessary in great part 

 by the fact that waste can be sind is withdrawn by the placenta, 

 which proves to be a nutritive, respiratory, and excretory organ ; 

 it is in itself a sort of abstract and brief chronicle of the whole 

 physiological story of foetal life. 



All 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 sup- 

 pose that the amoeboid 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 de- 

 part 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 it- 

 self may, perhaps, be regarded as the first completed tissue. We 

 are, accordingly, led to inquire how this river of life is distrib- 

 uted; in a word, into the nature of the foetal circulation. 



Foetal Circulation. — The blood leaves the placenta by the 

 umbilical vein, reaches the inferior vena cava, either directly 

 (by the dvictiis venosus), or, after first passing to the liver (by 

 the vence advehentes, and returning by the vence revehentes), 

 and proceeds, mingled with the blood returning from the lower 

 extremities, 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 arriv- 

 ing at the right auricle,being dammed back by the Eustachian 

 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 corre- 

 sponding ventricle, and thence into the pulmonary artery ; but, 

 finding the branches of this vessel unopened, it takes the line of 



