CHAPTER 4 



PHYSIOLOGY OF THE VERTEBRATE ANIMAL: 



METABOLISM 



In the preceding chapter it was shown that, structurally, ver- 

 tebrate animals are made up of systems of organs. These systems 

 are named according to the functions they perform, and it is to an 

 account of these functions that we must now turn our attention. 

 Since structure and function, morphology and phj^siology, are so 

 closely correlated, the student must keep clearly in mind the facts 

 of morphology as the discussion of physiology progresses. In this 

 account we shall deal with the physiology of the human body in 

 particular, although other forms will be mentioned in special cases, 

 and the discussion in general is applicable to all vertebrates. 



The Nature of Protoplasm. — It is necessary, in order to under- 

 stand and appreciate the significance of the functioning of the 

 animal as a whole, that we should look more closely at its structure. 

 One of the fundamental generalizations of biological science is 

 that all living organisms are composed of cells, which are the units 

 of structure and function, and of cell products. This is known as 

 the Cell Doctrine. Cells are microscopic masses of protoplasm. 

 A detailed account of their grosser structure and variations in kind 

 will be given in Chapter 6. At this point it is necessary to gain 

 some ideas concerning the protoplasm that occurs in all types of 

 cells. The physico-chemical nature of protoplasm is very com- 

 plex, but upon its organization depend the reactions that charac- 

 terize living organisms. 



Physical Nature of Protoplasm. — After many attempts to 

 explain protoplasm as a " living compound," investigators are 

 now agreed that, physically, protoplasm is a colloidal system. 

 The tenn colloid is derived from colla, meaning glue, and the name 

 was first applied to types of solutions which do not diffuse through 

 parchment or similar membranes, and which, upon evaporation, 

 yield formless masses of material. Such mixtures differ from crys- 



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