FOODS. 317 



building and repair must be supplied in the food, as well as 

 the fuels, or oxidizable foods, which yield the energy the 

 machine expends; and while experience shows us that even 

 for machinery construction oxidizable matters are largely 

 needed, it is nevertheless a gain to replace them by non-oxi- 

 dizable substances when possible; just as if practicable it 

 would be advantageous to construct an engine out of mate- 

 rials which would not rust, although other conditions deter- 

 mine the use of iron for the greater part of it. 



Definition of Foods. Foods may be defined as substances 

 which, when taken into the alimentary canal, are absorbed 

 from it, and then serve either to supply material for the 

 growth of the Body, or for the replacement of matter which 

 has been removed from it, either after oxidation or without 

 having been oxidized. Foods to replace matters which have 

 been oxidized must be themselves oxidizable; they are force- 

 generators, but may be and generally are also tissue-formers; 

 and are nearly always complex organic substances derived 

 from other animals or from plants. Foods to replace matters 

 not oxidized in the Body are force-regulators, and are for the 

 most part tolerably simple inorganic compounds. Among 

 the force-regulators we must, however, include certain organic 

 foods which, although oxidized in the Body and serving as 

 liberators of energy, yet produce effects totally dispropor- 

 tionate to the energy they set free, and for which effects they 

 are taken. In other words, their influence as stimuli in excit- 

 ing certain tissues to liberate energy, or as inhibitory agents 

 checking the activity of parts, is more marked than their 

 direct action as force -generators. As examples, we may take 

 condiments: mustard and pepper are not of much use as 

 sources of energy, although they no doubt yield some; we 

 take them for their stimulating effect on the mouth and 

 other parts of the alimentary canal, by which they promote 

 an increased flow of the digestive secretions or an increased 

 appetite for food. Thein and caffein, the active principles of 

 tea and coffee, are taken for their stimulating effect on the 

 nervous system, rather than for the amount of energy yielded 

 by their own oxidation. 



Conditions which a Pood must Fulfil. (1) A food 

 must contain the elements which it is to replace in the Body: 

 but that alone is not sufficient. The elements leaving the 

 Body being usually derived from the breaking down of com- 



