JS(54 



PHYSIOLOGY, 



cause an enormous enlargement of the mammary glands of the 

 rabbit, showing that the sympathetic enlargement of the mammary 

 glands in pregnancy is due to some chemical agent, a hormone, act- 

 ing through the blood, and not by the nervous system. 



Fig. 393. The Foetal Circulation. (GRANDIN and JARMAN.) 



Placenta. The placenta is the nutritive, excretory, and respir- 

 atory organ of the foetus from the third month to the end of preg- 

 nancy. It is discoid in shape, one side being attached to the uterine 

 wall, the other becoming attenuated, to end in the umbilical cord, 

 which is the medium of exchange between the placenta and the 

 fcetus. The villi of the chorion frond osum dip down into the 



