.INTRODUCTION 7 



of the living kingdom, where we can detect only a speck of structureless 

 material containing a few granules, of which one or more, in consequence of 

 its reaction to stains, is distinguished by the name of a nucleus, in the 

 higher members this organisation becomes more and more marked. This 

 increased complexity of organisation, which we often speak of as histological 

 differentiation, runs parallel with increasing range of power of adaptation, 

 and with increasing efficiency of adaptive reactions attained by the setting 

 apart of special structures (organs) for the performance of definite functions. 

 This parallelism between the development of function and structure justifies 

 us in the assumption generally, though often only tacitly, made by physio- 

 logists, that the structure is the determining factor for the function. We 

 might regard the histological differentiation as representing merely a con- 

 tinuation of the increasing molecular complexity, which we assumed must 

 accompany and determine every widening in the range of the adaptive 

 power of the organism. . 



To sum up : our objects in the study of physiology include the descrip- 

 tion of the chief reactions of the body to changes in its environment, the 

 analysis of these reactions into the simpler reactions of which they are made 

 up, and the assignment to each differentiated structure of the organism its 

 part in every reaction. We must determine the conditions under which 

 each reaction takes place, so that we may learn to evoke any part of it at 

 will by application of the appropriate stimulus, i.e. by effective change of 

 environment. 



A reaction involves expenditure of energy, and this can be derived only 

 from chemical change in the reacting organ, and ultimately from the dis- 

 integration or oxidation of the foodstuffs. Our next task must be, there- 

 fore, the analysis of the energetic and material changes, with a view to 

 determining the whole sequence of events, from the occurrence of the 

 external exciting change to the finished reaction, which will alter in tne 

 direction of protection the relation of the organism to its environment. 

 In short, it is the office of physiology to discover the routine sequence of 

 events in the living organism under all manner of conditions. In attacking 

 this problem our methods cannot differ fundamentally from those of the 

 physicist and chemist. In every case our experiments will consist in the 

 observation and measurement of movements of one kind or another which 

 we shall interpret in terms of mass or energy. Physiology, if it could be 

 completed, would therefore describe the how of every process in the body. 

 It would state the sequence of events and would summarise these as so-called 

 ' laws.' These laws would, however, no more explain the phenomena 

 of life than does the ' law of gravitation ' explain the fact that two masses 

 tend to move towards one another with uniform acceleration. Nor can we 

 hope to explain physiological phenomena by reference to the laws of physics 

 and chemistry, since these themselves are only expressions of sequences, 

 and not explanations. With every growth in science, however, its generalisa- 

 tions become wider and its laws summarise ever more extensive groups of 

 phenomena.' We have no reason for asserting that, in the course of research, 



