CHAPTER IX. 



NUTRITION AND METABOLISM. 



No steam-engine can continue to work without fresh supplies 

 of fuel; no organism can maintain its vital activity without 

 fresh supplies of food. It is the combustion of the fuel which 

 gives to the engine its energy or power of doing work ; and in 

 the animal it is to a kind of slow combustion that the main- 

 tenance of the vital energy is due. But the chemical processes 

 in the latter case are vastly more complex, and are not confined, 

 as in the engine, to a special part of the machine \ for the 

 animal is a cunningly wrought piece of mechanism, and the most 

 efficient machine we know. But we must not forget that the 

 processes which go on in the animal, though they are chemical, 

 physical, and mechanical, are at the same time something more. 

 They are vital or organic ; and this, not through any mysterious 

 addition of something from without, but through more com- 

 plex combination of the self-same materials and energies. 



In this chapter we must consider shortly some of the metabolic 

 processes (as the chemical processes within the organism are 

 termed) upon which the continued vitality of the organism 

 depends. 



The Alimentary Canal. The alimentary canal is a continuous 

 more or less coiled tube running through the body from the 

 mouth to the vent, and for a considerable part of its course 

 suspended in the body-cavity. It does not in any way com- 

 municate directly, that is, by any opening or canal, with the 

 blood-vessels. And when we remember that its lining mem- 

 brane is continuous at the lips and vent with the epidermis of 

 the skin, we shall see that the contents of the canal, although 

 12 



