98 MANUAL OF PHYSIOLOGY. 



form an essential part of the food of the latter. They may be 

 divided into three groups 



i. Carbohydrates bodies so called from the presence of 

 hydrogen and oxygen in the proportion to form water ; 

 e. g. : 



Starch, C 6 H 10 O 5 = C 6 (H 2 O) 5 , 

 Grape sugar (dextrose), C 6 Hi 2 O 6 = C 6 (H 2 O) 6 , 

 Cane sugar (sucrose), C 12 H 22 O U = C, 2 (H 2 O),i. 

 ii. Hydrocarbons compounds of carbon and hydrogen with 

 a less proportion of oxygen than Division i, as oils and 

 fats 



Olein (principal constituent of olive oil), C 57 H 1(H O 6 . 

 iii. Albuminous bodies which contain nitrogen in addition 

 to carbon, hydrogen, and oxygen. These are of very 

 complex composition, and, as yet, cannot be repre- 

 sented by chemical formulae. 



Animals, on the other hand, cannot thrive on the simple forms 

 of food obtainable from the inorganic kingdom, which suffice for 

 the nutrition of a plant. They require the materials for their 

 assimilation to be nearly allied in chemical composition to their 

 own tissues. In short, they require as food the very organic sub- 

 stances which the plants spend their lives in making : viz., 

 starches, fats, and albuminous bodies. These substances must, 

 therefore, be supplied to animals ready made, as they are pro- 

 duced by plants. Directly or indirectly, through the medium of 

 other animals, all these complex substances, which form fuel so 

 useful to our economy, are derived from the work done by vege- 

 table life. 



The chief acts of animal protoplasm are really oxidations, a 

 slow burning away of its substance, which results in the produc- 

 tion of inorganic materials like those used by plants as food. 



Plants, then, use simple food stuffs, and from them manufac- 

 ture complex combustible materials, and thus store up the energy 

 of the sun's rays in their textures. 



Animals use complex food! stuffs to renew their tissues, which 

 they are constantly oxidizing, and by this means the energy for 

 the performance of their various active functions is set free. 



