298 THE HUMAN BOD Y. 



and for which effects they are taken. In other words, their 

 influence as stimuli in exciting certain tissues to liberate 

 energy, or as inhibitory agents checking the activity of 

 parts, is more marked than their direct action as force gen- 

 erators. As examples, we may take condiments: mustard 

 and pepper are not of much use as sources of energy, al- 

 though 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, again, the active principle of tea and coffee, is 

 taken for its stimulating effect on the nervous system, rather 

 than for the amount of energy which is yielded by its own 

 oxidation. 



Conditions which a Food must Fulfill. (1) A food 

 must contain the elements which it is to replace in the 

 Body: but that alone is not sufficient. The elements leav- 

 ing the Body being usually derived from the breaking down 

 of complex substances in it, the food must contain them 

 either in the form of such complex substances, or in forms 

 which the Body can build up into them. Free nitrogen 

 and hydrogen are no use as foods, since they are neither cxi- 

 dizable under the conditions prevailing in the Body (and 

 consequently cannot yield it energy), nor are they capable 

 of construction by it into its tissues. (2) Food after it 

 has been swallowed is still in a strict sense outside the 

 Body; the alimentary canal is merely a tube running 

 through it, and so long as food lies there it does not form 

 any part of the Body proper. Hence foods must be capa- 

 ble of absorption from the alimentary canal; either directly, 

 or after they have been changed by the processes of diges- 

 tion. Carbon, for example, is useless as food, not merely 

 because the Body could not build it up into its own tissues, 

 but because it cannot be absorbed from the alimentary 

 canal. (3) Neither the substance itself nor any of the 

 products of its transformation in the Body must be inju- 

 rious to the structure or activity of any organ. If so it is 

 a poison, not a food. 



Alimentary Principles. What in common language we 



