20 
INTRODUCTION. 
soluble, tbe 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 Radrices, 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 
coeca, 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 
c oe cum, like that of the mammalia, but small. 
