PREFACE 



The volume here presented contains the substance of the 

 series of lectures delivered at Swarthmore in the spring of 

 1939, but a careful revision has been made and the text has 

 in several places been amplified and added to. The manu- 

 script was completed within that year, but the difficulties of 

 communication have delayed publication and thrown a great 

 burden of proofreading, correction, revision of literature, and 

 preparation of name lists and general index upon my friends 

 at Swarthmore and elsewhere. I am deeply grateful to 

 Dr. Laurence Irving, Dr. Per F. Scholander, Dr. Edgar C. 

 Black, and Miss Louise Boyden for the pains they have taken, 

 and I desire also to express my appreciation of the splendid 

 work done by the University of Pennsylvania Press under 

 very trying circumstances. 



As I have had an opportunity of going over the page proofs, 

 I am personally fully responsible for the errors and omissions 

 which no doubt remain. 



August Krogh 



The Laboratory of Zoophysiology, 

 Copenhagen University, 

 October, 1940 



