558 VERTEBRATE LIFE AND ORGANIZATION 



of the circulatory svsten. where the actual exchanges occur. Barclay F'^-nkHn and 

 PrichLd s 7/t Foelal Circulation describes the brilliant experimental work that led 

 o our under anding of the fetal circulation. A fine discussion of the activity of he heart 

 .nd the facto s of safety that enable it to continue operating, even though partially im- 

 paired b>clnary diseLe, can be found in an article by Wiggers, Tke Heart, published 

 in the Scientific American. 



