68 THE FROG chap. 



to rectum, and certain portions of it are finally ejected from 

 the vent in the form of faeces. 



It is not difficult to assure one's self that the weight of the 

 fseces passed during a certain time is very much less than 

 that of the food swallowed during the same time. Obviously 

 some constituents of the food have disappeared during its 

 progress through the enteric canal. The character of the 

 faecal matter, moreover, is very different from that of the food; 

 the only portions of the swallowed animals discoverable 

 in the rectum are bits of their hard parts ; for the rest, the 

 fceces form a pulpy, black mass. That this change is due 

 to certain definite chemical processes taking place in the 

 enteric canal may be inferred from the fact that the contents 

 of the stomach, as well as the walls of that organ, have 

 an acid reaction, and turn blue litmus paper red. On 

 the other hand the contents of the small intestine are, 

 to a greater or less extent, alkaline, restoring reddened 

 litmus paper to its original blue colour. 



It is also obvious that there must be some definite 

 mechanism for propelling the food from one end of the 

 enteric canal to the other ; its passage through so long, 

 narrow, and coiled a tube can certainly not be accounted 

 for by supposing it to be merely pushed onwards as fresh 

 food is swallowed. 



In order to understand the various processes connected 

 with digestion we must make a renewed and more careful 

 examination of the organs concerned, after removing them 

 from the body. 



The Digestive Organs.— Arising from the gall-bladder 

 and passing backwards to the duodenum is an extremely 

 delicate tube (Fig. i8, Dc, Dc^), the common bile-duct, which 

 opens into the duodenum. By gently squeezing the gall- 

 bladder a drop of greenish fluid may be made to ooze out 



