20 INTRODUCTION. 



soluble, the intestine is short and narrow ; when it consists 

 of fish, it is also often short and slender ; when of various 

 substances, animal and vegetable, it is of moderate length 

 and width ; and when of comparatively innutritious vegetable 

 matter, it is very long and wide. It is in the duodenum, or 

 first fold of the intestine, that digestion is perfected, by the 

 aid of the pancreatic juice ; and a little farther on, that, on 

 being mixed with the bile, the chyle is deposited on the vil- 

 lous surface of the intestine, whence it is absorbed. 



At the commencement of the rectum, which is analogous 

 to the colon and rectum of the mammalia, are placed two 

 lateral blind-guts, or coeca, which vary extremely in size. 

 They receive their greatest development in Grouse, which 

 feed on comparatively innutritious vegetable matter, and are 

 smallest in the flesh-eating birds, whose food is most nutri- 

 tious. In the Radriees, in some of which the coeca have a 

 capacity as great as that of the intestine, sometimes even 

 greater, the finer particles of the mass of food which have 

 not been sufficiently acted upon in their course, enter the 

 cceca, and are subjected to a second digestion and absorp- 

 tion. This is also the case in the Cribratrices, which feed 

 on similar substances. In most other birds, the coeca are 

 small and secrete a mucous fluid only, but do not admit the 

 food. It is very remarkable, that in Owls, whose food is 

 like that of Hawks, the coeca are large, and act upon the 

 food, while in these birds they are merely rudimentary. The 

 reason may be, that while Hawks prey by day, and can fill 

 not only their stomach but their crop also, so that the assi- 

 milative function requires no special care, owls, which 

 prey by night and have no crop, require to have their com- 

 paratively scanty food better husbanded, and thus submitted 

 to a more special action. Goatsuckers have the same rela- 

 tion to Swifts. Some birds have no coeca, as Kingfishers, 

 Woodpeckers, and Hummingbirds. Others, as Herons, have 

 no organs precisely similar, but are furnished with a single 

 ccecum, like that of the mammalia, but small. 



