44 CLASSIFICATION 



are absolutely essential to life, for without them life would be 

 impossible. Every manifestation of vitality by an organism, such 

 as movement, production of digestive or other secretions, sen- 

 sations, and mental operations, involve the breaking down or 

 waste of material, without which the necessary energy would not 

 be available. Broadly speaking, energy is the power of doing 

 work, and a distinction is drawn between potential or stored 

 energy and actual or kinetic energy. Complex chemical sub- 

 stances, such as high explosives, represent a store of energy, and 

 ^ PROTOPLASM when these break 



.^'S^^^-f—^ STORED I 1 __^^^^_^ down into simpler 



/^ "^"^ I'^-^ g^fr,.. substances, z.e. ex- 



Fig. 20. — Diagram of chemical changes in Body 



f.o<^(^"' """--a^ I — ^^T^ plode, a large amount 



of actual energy is 

 liberated, which, in 

 the case of the ex- 

 plosives alluded to, 

 can be made to do various kinds of useful work. Protoplasm and 

 the other complex body-substances are, so to speak, of explosive 

 nature. The final results of these down -breaking changes are 

 waste products, which have to be eliminated or excreted from 

 the system. The accompanying diagram (fig. 20) illustrates the 

 matter. It represents a double staircase, with a shorter ascending 

 stair on the left, and a longer descending stair on the right. Food 

 is represented as being on the level of the lowest step of the left- 

 hand stair, the successive steps of which represent the stages by 

 which more and more complex substances are built, till the most 

 complex of all, protoplasm, is produced. The breaking down of 

 this into simpler and simpler substances is represented by the 

 successive steps of the descending stair, at the bottom of which 

 are the waste products. These are represented at a lower level 

 than the food, because they are of simpler chemical constitution. 

 They are mainly — water (HjO), carbonic acid gas (CO2), and 

 compounds of nitrogen. 



The organs which get rid of waste are (i) skin, (2) lungs, 

 (3) kidneys, and (4) liver. 



I. The Skin, among its other functions, eliminates a large 

 amount of water and small quantities of the other kinds of waste, 

 here constituting the excretion called sweat or perspiration. 

 Imbedded in it are innumerable sweat-glands, in the form of 



