46 BIOLOGY 



There is a greater frequency of the arch arrangement, in 

 which the bones form the sides of the arch and the 

 muscles act as the tie-beams. But many of the bones 

 have also to withstand a tearing or bending force, and 

 in such cases they are arranged more or less after the 

 pattern of the hollow column ; this is exactly on the 

 same lines as the skeleton in the majority of plants. 



CHAPTER IX 



FOOD 



IF we inquire into the meaning of the continuous 

 activity of living organisms, remembering all the time 

 that every phase of activity is an expenditure of energy 

 and results in oxidation and finally in molecular trans- 

 formations, we find that it can be explained only 

 through the self-sustaining power of the organism, and 

 this self- sustaining power is the power of introducing 

 new matter to replace what has been used up. 



Such introduced matter constitutes the food of the 

 organism, and so we may define food as any substance 

 from which a living organism is able to derive material 

 for its sustenance. This is food in its widest sense, and 

 under this definition all the inorganic compounds could 

 be safely included. But we must remember one fact 

 too often glossed over, that no protoplasm, whether of 

 plant or of animal, can assimilate such inorganic sub- 

 stances until they are united into the complex sub- 

 stances known as organic compounds. Now organic 

 compounds are found in nature only as the products 



