6 A MANUAL OF PHYSIOLOGY 



the loss of its nuclear material. It must be remembered, how- 

 ever, that nuclear material may sometimes be present in diffuse 

 form in cells which do not show a nucleus in the histological sense. 



When we carry back the analysis of an organized body as far as 

 we can, we find that every organ of it is made up of cells, which 

 upon the whole conform to the type we have been describing, 

 although there are many differences in details. Some organisms 

 there are, low down in the scale, whose whole activity is con- 

 fined within the narrow limits of a single cell. The amoeba sets 

 up in life as a cell split off from its parent. It divides in its turn, 

 and each half is a complete amoeba. When we come a little 

 higher than the 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 con- 

 traction 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 continually advancing, 

 and the various kinds of cells are grouped together into colonies 

 or organs. In some organs and tissues the bond of union is 

 simple juxtaposition and similarity of function of the constituent 

 cells. But in others the union is protoplasmic, processes of the 

 cytoplasm actually passing from cell to cell. This is seen in 

 certain epithelial tissues, and conspicuously in the cardiac muscle. 



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 

 (b) disintegrate e or katabolic changes, in which complex bodies 

 (including the living substance) are broken down into com- 

 paratively simple products. In plants, upon the whole, it is 

 integration which predominates ; from substances so simple 

 as the carbon dioxide of the air and the nitrates of the soil the 

 plant builds up its carbo-hydrates and its proteins. 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 protein 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 ex- 



