196 THE ENGINES OF THE HUMAN BODY 



the ileum ; the ileo-caecal sphincter is an efficient door- 

 keeper and prevents any reflux ; once the chyme is in the 

 caecum, there is no turning back, unless the mechanism has 

 become disordered. The sphincter marks the boundary 

 line between two distinct alimentary factories ; the one 

 we are now considering ceases there. 



It is very likely that if we had examined the csecal 

 region of our friend's body at midday, by which time the 

 stomach had discharged its contents, we should have seen 

 the advance guard of the bismuth breakfast then reaching 

 the caecum. The intestinal journey of 20 feet takes about 

 four hours — 240 inches in 240 minutes, an average pace 

 of an inch a minute. If we have patience and continue 

 our examination we shall see the rear-guard of the meal 

 reach the caecum somewhere between 4 and 5 o'clock — 

 about tea-time — if our friend's system is regulated as in 

 most healthy people. By tea-time the bismuth breakfast 

 — or what is left of it — will have reached the caecum. 

 What happens to it there we shall discover in another 

 chapter. Meantime our business is to see what changes 

 have taken place in the bismuth meal during its four or 

 five hours' sojourn in the intestinal factory. 



Before proceeding to examine the complex mechanism 

 of the human intestine it will repay us to spend a moment 

 on the simple intestinal machinery of a motor cycle. One 

 of our main aims is to follow out in detail the corre- 

 spondence of parts in a locomotive machine planned by 

 man and in one designed by Nature. We have identified 

 the feed-pipe which conveys fuel from the petrol tank 

 to the carburettor of the engine, as the part which serves 

 the same office in a motor cycle as the bowel does in the 

 human machine (fig. 1). We were careful to point out 

 that, as petrol is a prepared fuel and already fit for 

 internal combustion, the feed-pipe had merely to play a 

 passive mechanical part. The feed-pipe which could 

 truly be compared to the bowel would be one designed 

 for a motor cycle which was built for the consumption of 

 a bituminous shale, one containing the raw materials from 

 which a combustible oil could be manufactured. A 





