210 THE ENGINES OF THE HUMAN BODY 



Having dismissed our bismuth friend for good, we 

 now set out to ascertain the nature of the chemical opera- 

 tions carried on in the great bowel and the particular kind 

 of body-fuel which is there manufactured. Something is 

 known about these matters, but much remains to be dis- 

 covered. We have seen that it is merely the " tailings " 

 of the food which enter the caecum by the ileo-caecal 

 orifice. The refuse shot from the tailrace of the gold- 

 mine, although all the pure gold has been successfully 

 removed from it, may yet contain gold in chemical com- 

 bination which can be extracted only by the application 

 of special chemical means. That was a discovery which 

 gold-miners made ; the refuse heaps of old workings 

 suddenly became of value. At an early point in the 

 evolution of vertebrate animals, a discovery of a similar 

 kind was lighted on. The tailings of the small bowel, 

 after running the long gauntlet of the small bowel, still 

 retained certain valuable materials which could not be 

 reduced and extracted by ordinary digestive juices. 

 Such juices could remove almost the whole of the useful 

 fuels contained in all kinds of flesh food in an animal's 

 diet ; but in fruit, roots, vegetables, and particularly in 

 the husks of grains, there was a large food element — 

 particularly cellulose — which passed without being acted 

 on by ordinary digestive juices. Cellulose husks have 

 to be dissolved before the valuable kernels they enclose 

 can be extracted and absorbed. These husks, even straw, 

 hay, and wood, disappear if left exposed to the weather. 

 They are digested and dissolved by bacteria and their 

 solutions washed away by rain. Bacteria, then, were the 

 means which Nature selected for dealing with the tailings 

 from the bowel ; they are often given easy access to the 

 alimentary canals of animals by being carried in with 

 the food eaten. The hinder part of the bowel became 

 altered in construction and established as a special 

 laboratory in which the digestive operations of bacteria 

 might be carried on. That is how the great bowel came 

 to be established. The new method was cheap and 

 effective. The production of digestive juices is costly ; 



