CHAPTER I 



Introduction 



HE SCIENCE OF PHYSIOLOGY is the analysis of function in living organ- 

 isms. One of the prerequisites for its study is a knowledge of morph- 

 ology. Physiology is a synthetic science which applies physical and 

 chemical methods to biology. 



The Fields of Physiology. For practical purposes, physiology can be 

 divided into three categories, as follows. 



Comparative Physiology. Comparati\'e ph\siology treats of organ function 

 in a wide range of groups of organisms. Comparative animal physiology in- 

 tegrates and coordinates functional relationships which transcend special 

 groups of animals. It is concerned with the ways in which diverse organisms 

 perform similar functions. CeneticalK' dissimilar organisms may show striking 

 similarities in characteristics and response to the same en\ ironmenial stimulus. 

 By the same token closely related animals frequently react very differently to 

 their surroundings. While other branches of physiology use such variables as 

 light, temperature, oxygen tension, and hormone balance, comparati\e physiol- 

 ogy uses, in addition, species or animal type as a variable lor each 1 unction. 

 The generalizations which emerge from this experimental approach are im- 

 portant as biological principles which often aid in solving problems of cellular 

 and special group physiology. 



Physiology of Special Groups. The physiology of special groups of organ- 

 isms treats of functional characteristics in particular kinds of plants and ani- 

 mals. Traditionally, the basic animal physiology is buman and mammalian 

 physiology, and this science provides the rational basis for much ol medicine 

 and animal husbandry. The physiology of higher plants is equalK' specialized 

 and important as a basis for plant agriculture. Insect ph\siology is rapidly 

 becoming a special group physiology. 



Cellular Physiology. Cellular or general physiology treats of those basic 

 characteristics common to most living organisms. A vast amount of biochemi- 

 cal evolution occurred in protoplasm before multicellular organisms appeared, 

 and cells are exceedingly complicated in their functional organization. In any 

 cell-yeast, muscle fiber, or leaf parenchyma cell-the fundamental properties 

 of differential permeability, oxidative enzyme activity, genie control of cyto- 

 plasm, and many other properties are much the same. At the cellular level all 

 organisms have more in common than in difference, and this basic similarity 

 should form the starting point for evolutionary theory. Cellular specialization 

 has led to some diversity of cell types and has often brought with it the loss 

 of one function with emphasis on another. In this sense one may speak of a 

 comparative physiology of the cells in one organism. 1 he characters treated 

 in cellular physiology are nearly universal and are extremely stable with 



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