Xii PREFACE 



The aspect of vertebrate physiology dealt with in this book is 

 ideal for illustrating a corollary of this — that the study of a 

 given system needs to take into account the function of many 

 other systems. Respiration involves a consideration of the 

 animal's environment as a source of oxygen as well as the bio- 

 chemistry of the cell. From the evolutionary point of view the 

 major structural and physiological changes are dominated by 

 the origin of the land habit which has profoundly affected both 

 the respiratory and circulatory systems. 



Many aspects of physiology considered here are 'Homeo- 

 static' in that they tend to maintain constant within limits some 

 feature of the animal's internal environment, usually by some 

 feedback mechanism. In a sense this book also represents a 

 *feedback' because my interest in the respiratory physiology of 

 fishes arose from demonstrating to Professor Sir James Gray's 

 course at Cambridge. The stimulus to research in this field was 

 a valuable one and I only hope that others may feel the desire 

 to follow up some of the many gaps which exist in our know- 

 ledge. 



In writing this book I have tried to include material which 

 will be useful for 'S' level candidates in Zoology as well as intro- 

 ductory courses at universities. Recent views have been given in 

 many instances and where they differ from common acceptance 

 this has been noted. Some of the diagrams are new and it is 

 hoped that they will summarise those parts of the text which 

 they illustrate. Unfortunately errors are bound to be present 

 for which I apologise both to the reader and to authors whose 

 views I may have unwittingly misrepresented. 



It is a pleasure to record my thanks to Mr W. H. Dowdes- 

 well, the editor of The Scholarship Series not only for inviting 

 me to write this book but for his very helpful comments on the 

 typescript. 



Magdalene College, G.M.Hughes 



Cambridge 



June 1962 



