IX 



treatises on comparative physiology, copious details of alimentary or digestive 

 mechanisms will be found, but no discussion of the general nature of the 

 action of enzymes. 



In speaking of higher and lower organisms, it is well to make it clear that 

 110 invidious distinction is intended to be made. Both are equally well 

 adapted to their environments. The higher are so called because they are 

 affected by a greater variety of changes in their environment arid respond to 

 these ^iii a more complex manner. 



A certain amount of repetition is unavoidable, since the same process has 

 different aspects and, owing to the interaction and interdependence of the 

 phenomena observed in the more highly developed organisms, it is impossible to 

 avoid references in the general treatment to activities which are also described 

 as parts of complex actions in later chapters. The reader who is unable to 

 follow the meaning of the text in places in earlier chapters, owing to 

 reference to matters discussed in detail in later chapters, will usually find in 

 tlje index the pages on which this description occurs, and can make himself 

 familiar with them before proceeding further. A better course would be 

 to read the earlier chapters a second time, after the later pages have been 

 mastered. 



An elementary knowledge of physics, chemistry, and biology must be 

 assumed, unless the book is to become altogether unwieldy. It is indeed 

 impossible to insist too strongly on the importance of at least an elementary 

 knowledge of these three basal sciences for every one, much more for those 

 pursuing the study of any branch of science whatsoever. At the same time, 

 it has been thought useful to enter into some detail with respect to con- 

 ceptions with which the student of physiology frequently finds difficulty, 

 such as catalysis, the tension of gases, and some of the laws of hydrodynamics. 



Vital phenomena being essentially dynamic, the study of physiology 

 consists in the investigation of changes. As Jennings (quoted by von 

 Uexkiill, 1909, p. 30) says, " It is of the very greatest importance for the 

 understanding of the behaviour of organisms, to look upon them chiefly as 

 something dynamic as processes rather than as structures. An animal is 

 something that happens." The velocity of reactions and the conditions 

 affecting it, together with the energy changes involved, are, therefore, more 

 essential than the chemical structure or physical properties of the reacting 

 substances or the resulting products, although the knowledge of certain of 

 these properties is, of course, necessary. To use an illustration, inadequate as 

 it is, that of a petrol motor, the problem of the physiologist is analogous to 

 that of the investigation of the amount of fuel consumed in relation to the 

 work done, when the engine is working under various conditions. The greater 

 number of the chemical and physical properties of the materials used in the 

 construction of the engine are of no importance, such as the valency of the 

 iron or the smell of the lubricating oil, while others are fundamental, such 

 as the heat of combustion of the fuel and the insulation of the ignition 

 circuit. Even the exact chemical nature of the fuel is of subsidiary 

 importance, so long as it is sufficiently volatile, and capable of giving an 

 explosive mixture with oxygen. Moreover, the precise form of many parts, 

 such as the heads of bolts, is immaterial, just as many structural details of 

 living organisms or the precise chemical composition of connective tissue have, 



