INTRODUCTION 19 



Amoeba, we find organisms which consist of several cells, 

 and * specialization of function* begins to appear. Thus 

 the Hydra, the * common fresh-water Polyp ' of our ponds 

 and marshes, has an outer set of cells, the ectoderm, and an 

 inner set, the endoderm. Through the superficial portions 

 of the former it learns what is going on in the world ; by 

 the contraction of their deeply-placed processes it shapes its 

 life to its environment. As we mount in the animal scale, 

 specialization of structure and of function are found con- 

 tinually advancing, and the various kinds of cells are grouped 

 together into colonies or organs. 



The Functions of Living Matter. The peculiar functions of 

 living matter as exhibited in the animal body will form the 

 subject of the main portion of this book; and we need only say 

 here (i) that in all living organisms certain chemical changes go 

 on, the sum total of which constitutes the metabolism of the 

 body. These may be divided into (a) integrative or anabolic 

 changes, by which complex substances (including the living 

 matter itself) are built up from simpler materials ; and 

 (6) disintegrate e or katabolic changes, in which complex sub- 

 stances (including the living substance) are broken down 

 into comparatively simple products. In plants, upon the 

 whole, it is integration which predominates ; from sub- 

 stances so simple as the carbon dioxide of the air and the 

 nitrates of the soil the plant builds up its carbo-hydrates and 

 its proteids. In animals the main drift of the metabolic 

 current is from the complex to the simple ; no animal can 

 construct its own protoplasm from the inorganic materials 

 that lie around it ; it must have ready-made proteid in its 

 food. But in all plants there is some disintegration ; in all 

 animals there is some synthesis. (2) The living substance is 

 excitable that is, it responds to certain external impressions, 

 or stimuli, by actions peculiar to each kind of cell. (3) The 

 living substance reproduces itself. All the manifold activities 

 included under these three heads have but one source, the 

 transformation of the energy of the food. It is not, however, 

 upon the whole, peculiarities in food, but in molecular 

 structure, that underlie the peculiarities of function of 

 different living cells. A locomotive is fed with coal ; a 



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