206 THE ENGINES OF THE HUMAN BODY 



how the paving epithelium of the bowel does its work, 

 and the manner in which that work is done will depend 

 upon whether or not we ask it to deal with a greater 

 amount of fuel than the body really needs. 



There is a contrivance used by gold-miners which 

 will help us to understand the essential mechanism of 

 the small bowel. The rock or quartz which is quarried 

 from the gold-bearing vein is ground to a powder by 

 powerful batteries of stamps. Near the batteries is set 

 up a tailrace through which a powerful stream of water 

 flows. The bottom of the tailrace is covered with a 

 carpet made of blanket, which we may look upon as 

 representing the lining membrane of the bowel. When 

 the crushed quartz is thrown into the head of the tailrace, 

 the sand and particles of stone are carried away by the 

 flow of water, but the grains of gold sink into the pores 

 of the blanket and are held there. After the washing is 

 over we find the blanket, towards the head of the tailrace, 

 full of heavy gold grains; further down the gold particles 

 are fewer and smaller ; at the end none are to be seen ; 

 all the available gold has been absorbed before the wash 

 reached the dump at the tail. Now, the bowel is a tail- 

 race, 20 feet or more in length. The chyme is washed 

 over it by the contraction waves which sweep down the 

 bowel ; it is continually stirred by the kneading waves. 

 By the time the chyme has passed over the whole length 

 of the bowel and reached the dump-head — represented 

 by the caecum — all the fuel which can be extracted from 

 the chyme by ordinary digestive means has been removed. 

 The residue which passes into the caecum represents the 

 washed tailings of the intestinal tailrace. 



