HIGH SCHOOL ZOOLOGY. 265 



ing of the plant or animal as a machine, its object being to 

 follow the transformations of matter and of energy within the 

 livin"' organism. Although it is a comparatively easy task to 

 trace how the energy locked up in fuel is made available by the 

 steam-engine, it is an infinitely more difficult one to trace how 

 the energy locked up in food becomes available as muscular 

 work ; yet there are other activities of the body which lend 

 themselves far less easily to measurement than does that of the 

 muscular system. 



It is, then, the various activities of the organism which 

 Physiology discusses. Function is the term employed for 

 the role or duty discharged by any part of a plant or animal, 

 and the part discharging a particular duty is termed an organ ; 

 these terms are thus used correlatively. We have seen that 

 different kinds of activity such as nutrition, locomotion and 

 rei)roduction may be performed by apparently undifferentiated 

 or little-differentiated protoplasm ; it is therefore necessary 

 to beware of supposing that organization or differentiation 

 into separate organs is necessary for the display of the activities 

 of life, although the term organism would seem to imply that. 



7. The various functions of plants and animals may be 

 grouped as follows : — 



(a) Those essential to the process of waste and repair refer- 

 red to in §4. The i)rocess of waste is mainly one of oxida- 

 tion and involves the formation of waste-products, partly gaseous, 

 partly fluid, which require to be separated from the tissues: 

 Respiratory and excretory organs are therefore rendered 

 necessary. The process of repair, on the other hand, not only 

 involves the ingestion of food, but its assimilation, and, there- 

 fore, the nutritive organs first render the food soluble in such 

 a way that it can be absorbed, and then elaborate it into forms 

 , suitable for repairing the waste. Circulatory organs are sub- 

 servient to all three systems. 



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