68 THE FROG CHAP. 



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

 the faeces 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 dis- 

 appeared 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 faeces 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 con- 

 nected 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. 18, DC, Dc l ), 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 of the end of the duct (Z)c 2 ) into the intestine ; 

 this fluid is the bile. 



Very careful dissection shows that the common bile- 



