io AN INTRODUCTION TO ZOOLOGY 



energy that is, as it were, stored up waiting for some stimulus to 

 release it. A body possesses potential energy by virtue of the 

 previous expenditure of kinetic energy upon it. Thus, to take a 

 simple example, if we take a weight from the floor and place it on 

 a high shelf, kinetic energy has to be utilised to do this. While 

 on the shelf the weight manifests no activity, but it possesses 

 potential energy as a result of its position, and if we remove the 

 shelf from under it this stored energy becomes released as potential 

 energy, and the weight starts to move. The work it is capable of 

 performing may be utilised by connecting it with a machine, or 

 else it will be transformed into heat when it strikes the ground. 

 In a somewhat analogous way, the complex compounds have used 

 up a supply of kinetic energy in their formation, and under suitable 

 conditions they can be induced to break down into simpler substances 

 and release a certain amount of kinetic energy. 



The maintenance of life and the exhibition of vital activities 

 require the expenditure of a certain amount of energy, and this is 

 obtained by the oxidation of certain constituent parts of the living 

 organism itself. These compounds under suitable conditions 

 combine directly or indirectly with the oxygen supplied by respira- 

 tion and break down into simpler compounds, releasing, as they do 

 so, the necessary kinetic energy. In many cases these break-down 

 products are substances that are of no further use, and so need to 

 be eliminated. The production of kinetic energy therefore is the 

 result of katabolic changes, and it takes place throughout every 

 minute particle of the whole of the protoplasm. This wastage 

 needs to be made good and stores of potential energy built up for 

 future use, and this is brought about by anabolism. Thus we see 

 that, from the chemical point of view, protoplasm is an intensely 

 active substance, the seat of unceasing destructive and constructive 

 changes ; it is never quite the same for two consecutive moments. 



When viewed under the microscope living protoplasm is 

 found to be a semi-viscous, almost transparent, granular substance 

 usually in a state of motion. When examined under the highest 

 powers a certain amount of structural organisation can be made 

 out, but this is relatively simple, and certainly gives no hint of its 

 wonderful complexity. 



All animals are composed of this substance. At the com- 

 mencement of their existence they consist practically exclusively of 

 protoplasm, but in the higher animals other non-living products 

 appear as a result of its activities ; in ourselves, for example, hair, 

 nails, and the inorganic matter of bone and teeth. Such animals 

 do not consist of protoplasm alone, but the other lifeless substances 

 in them have been formed by it. Then, too, we find that the 



