38 PRINCIPLES OF ANIMAL BIOLOGY 



try because of their simplicity. The examples therefore need hardly be 

 remembered if the ideas they represent are mastered. The principles 

 have been kept at a minimum but should suffice for a fair understanding 

 of the simpler operations of protoplasm. Living things are essentially 

 chemical and physical laboratories, with this distinction, that the chemical 

 substances are not limited to a few reagent bottles on the shelves nor 

 the physical apparatus to a few resistance boxes and potentiometers in 

 the cabinets; instead these things constitute most of the building itself. 

 Changes are going on in them everywhere and all the time. It is of 

 these chemical and physical processes that life consists. As explained 

 in other parts of this book, the common physiological processes of 

 digestion and respiration are chemical reactions and physical phenomena 

 that are fairly well understood. Not so well known but assuredly 

 chemical and physical are muscular contraction and elimination of wastes. 

 Even growth, the development of the embryo or young stages, and the 

 conduction of impulses by nerves must be largely physicochemical. 



It is important to know, in connection with all these life processes, 

 that substances react as they do because of their electronic structure. 

 This structure is, in most protoplasmic substances, enormously com- 

 plicated by radicals of complex design. Their reactions and structure 

 are for this reason not easy to discover, but there is every reason to 

 assume that their physiological behavior is quite as dependent upon their 

 architecture as are the reactions of the simplest inorganic compound. 

 Valence determines the proportions of different substances which will 

 unite in protoplasm as certainly as in the salts. Electric phenomena 

 result from electronic reactions in living things just as in batteries. 

 Energy, one of the most important requirements of animals and plants, 

 flows from chemical combination as abundantly and as certainly in 

 protoplasm as in a test tube or an engine. It seems likely that life 

 consists entirely of physical and chemical changes. 



With this equipment of elementary knowledge in a pair of sister 

 sciences, and an understanding of the extent to which these sciences 

 underlie all knowledge of biology, we may notv return to the operations 

 of cells. 



References 



Partington, J. R. A Textbook of Inorganic Chcnustry. 5tli Ed. The Macmillan 



Company. (Pp. 428-430; 446-453; 466-473.) 

 Smith, A. W. The Elements of Physics. 4th Ed. McGraw-Hill Book Company, 



Inc. (Chaps. 60, 61 ; structure of atom, nuclear physics.) 

 TiMM, J. A. An Introduction to Chemistry. McGraw-Hill Book Company, Inc. 



