THE RABBIT. 



self-sufficing for full life) are called protozoa. All other 

 animals are called metazoa the rabbit is an instance and 

 their bodies are built up of cells and cell-products arranged to 

 form tissues. In such a body, each cell is not self-sufficing, 

 but all are mutually inter-dependent. They may be compared 

 to the different members of a human community, carrying 

 on different trades and professions. In other words, there 

 is a differentiation of function or physiological differentiation. 

 And correlated with this is a differentiation of form and 

 structure or histological differentiation. 



13, Tissue Metabolism. Every cell in every tissue 

 performs in a greater or less degree, according to its 

 relatively active or passive character the characteristic 

 functions of assimilation, respiration and excretion, or in 

 other words anabolism and katabolism, as does an amoeba ; 

 but with certain important differences. Firstly, the food 

 brought to it by the blood has already undergone the pro- 

 cesses of digestion, etc., which bring it to a soluble 

 condition, and only the final stages of anabolism take place 

 in the individual cell. Secondly, all katabolism involves 

 two things a conversion of the potential energy of proto- 

 plasm into some form of kinetic energy, and the production 

 from the substance of the protoplasm of simpler (and, usually, 

 more oxidized) compounds. But in some cells it is the 

 conversion of energy (i.e. the doing of some work) which is 

 the primary function of the cell, and the material products 

 are mere waste e.g. muscle and nerve cells. In other 

 cases it is the formation of definite products that is the 

 main function of the cell, the energy set free taking pro- 

 bably the form of heat and being wasted e.g. connective 

 tissue-cells. Lastly, some cells seem to have a purely passive 

 function, and their rate of metabolism must be extremely 

 slow. 



The products of katabolism that are nearest to protoplasm 

 in composition and whose production involves the least 

 degree of katabolism are mostly those that are retained to 

 form part of the tissues themselves. These may be termed 

 plastic products : when retained within the protoplasm of 

 the cell they are termed endoplastic, when deposited outside 



