viii AUTHOR'S PREFACE 



the Department of Physiology in Edinburgh University to 

 medical students having completed a course of elementary 

 physiology and to science students taking an honours course 

 in zoology. I have in mind the same mixed audience as 

 readers : the advanced student in zoology who knov^^s very 

 little physiology, and the student v^ho, having passed through 

 a course in physiology designed to equip him for the pursuit 

 of the medical profession, may wish to acquire information 

 about branches of the subject that have at present no such 

 remunerative value. In doing this one has the feeling of falling 

 between two stools. The physiological critic will object to 

 dealing with topics which practitioners do not regard as the 

 business of the physiologist ; while zoologists will protest 

 against omission of reference to experimental work which seems 

 to them to be as important as much that has been treated as 

 physiological in the pages which follow. 



Since the objective of physiological inquiry is the quantita- 

 tive study of the relation between processes characteristic of 

 living organisms and properties of inanimate matter, in 

 attempting to treat the subject with reference to a coherent 

 theme one of two courses is open : to illustrate the known 

 properties of non-living matter by reference to their operation 

 in the processes of living organisms, or to consider what are 

 the characteristic properties of animate systems and inquire 

 how far it is possible to interpret each in terms of knovm 

 physico-chemical laws. 



Against the former course, it is sufficient to point out : 

 first, that this method of treatment has been adopted as 

 successfully and comprehensively as possible in the existing 

 state of knowledge in such works as those of Bayliss and of 

 Hoeber ; secondly, that if carried out consistently it necessitates 

 the elimination of all reference to some of the most character- 

 istic properties which distinguish living systems. Generally 

 speaking, those who restrict the scope of physiology to pheno- 

 mena for which ready-made physico-chemical explanations are 

 at hand, make an exception for the treatment of reflex action. 

 By some obscure convention this grace is rarely extended to 

 the phenomena of reproduction. I shall make no apology for 



