640 THEORY OF FffiTAL 



oxygen combines immediately with the blood, 

 and enables it to excite the various tissues. But 

 if oxygen were the animating principle, it ought 

 to excite the heart and other muscles to contract 

 when removed from the body, which is not the 

 fact; for when reduced to the temperature of 32, 

 it produces no more effect upon them than so 

 much carbonic acid. And although electricity 

 excites contractions for a short time, its influence 

 is exceedingly partial, compared with the same 

 agent in the form of caloric. 



Moreover, it has been fully established by re- 

 cent researches into the phenomena of generation 

 and growth of the embryon, that in no case is 

 arterial blood transmitted from the mother to the 

 foetus through the vessels of the placenta, which 

 serves chiefly as a reservoir of nourishment, sup- 

 plied by the mother, absorbed by the vessels of 

 the foetus, and converted into blood that is wholly 

 different from that of the mother, in short, that 

 the embryon is nourished by the transudation of 

 arterial blood from the vessels of the mother. But 

 we have yet to learn in what way the blood of 

 the foetus is arterialized in the placenta, and 

 whether oxygen is essential to the process.* This 

 much, however, may be asserted with confidence, 

 that it is not by means of oxygen, or any other 

 ponderable gas, that the proximate constituents of 



* Nor is there a single anatomical preparation in the Hunterian 

 Museum, shewing whether the colour of the venous blood in the 

 foetus differs from that of the arteries. 



