A THREATENED FACTORY 211 



their manufacture is a constant drain on the resources of 

 the animal machine. Bacteria, on the other hand, are 

 content to perform the work of digestion for a small 

 percentage of the gains which accrue from their labours. 

 How profitable this novel bacterial method of digestion 

 has proved may be judged from the success of the animal 

 forms which adopted it ; the three higher forms of verte- 

 brate animals — reptiles, birds, and mammals — have in- 

 stalled it as a regular part of their alimentary systems. 

 The new system, however, had certain disadvantages. So 

 long as only harmless cellulose-loving bacteria gained 

 access to the new laboratory all went well, but others 

 of a harmful kind could also gain admittance, and hence 

 an elaborate police system had to be established and 

 maintained in the lower bowel to save the body from 

 invasion. Putrefaction also was liable to occur in the 

 stagnant contents of the bowel, leading to the formation 

 of substances which might be absorbed and thus poison 

 the body. 



Unless we interpret the operations carried on in the 

 great bowel in the manner just outlined, the changes 

 which we see taking place in it have no meaning for us. 

 We are at a loss, otherwise, to explain the peculiar con- 

 struction of the caecum and colon. But when we regard 

 the great bowel as a chamber in which the refuse from 

 our food is submitted to a new kind of digestion we find 

 a key to its apparent mysteries. We then understand 

 why the ileo-caecal orifice — the threshold between the first 

 and second intestinal laboratories is guarded and regulated 

 by a sphincter mechanism (fig. 44B). The ileo-caecal 

 orifice marks the end of one digestive system and the 

 beginning of another. We explain the presence of the 

 caecum by looking upon it as the stomach of the great 

 bowel. We have seen that it is always partly filled with gas, 

 and that it retains part of one batch apparently to serve as 

 leaven for the next, appearances which suggest a bacterial 

 function. We see, too, why alimentary matter is retained 

 so long in the great bowel ; a fermentative change is 

 necessarily a slow one. There are the same two muscular 



