18 
INTRODUCTION. 
and catcli it in a position favourable for swallowing, unless 
it may have been at first seized in a suitable manner. 
The oesophagus has a width proportionate to the objects 
which pass through it. Thus, in Snipes, which feed on small 
worms, it is slender ; in Crows, which swallow objects of va- 
rious sizes, moderately wide ; in Gannets, Guillemots, and 
Auks, which seize fishes, and are incapable of tearing them 
in pieces, of extreme width. In birds which usually obtain 
a large prey, but only at long intervals, or whose supply is 
precarious, the oesophagus is dilated into a pouch, intended 
as a reservoir, in which they can stow away a quantity of 
provision for future use ; as in Vultures and Eagles. Some- 
times, where the objects obtained are small or moderate, and 
the time of feeding limited, the oesophagus has no dilatation, 
but is uniformly wide, and the stomach is also dilated, as in 
Owls and Goatsuckers. The largest dilatations of the oeso- 
phagus are in birds which feed on vegetable substances, as 
seeds, twigs, leaves, stems, and roots, which require to be 
gradually pounded, and which must be supplied by the re- 
servoirs to the grinding organ ; as in Pigeons, Grouse, and 
the like. 
Generally, the oesophagus merely affords a passage to the 
food, without acting upon it ; and the crop merely contains 
and moistens its contents ; but, when the food is of such a 
form as to be incapable of entering the stomach at once, as 
in the case of a fish of large size, part of the oesophagus has 
a solvent action upon it. The glandules placed in the walls 
of the proventriculus, or lower part of the oesophagus, se- 
crete exclusively the solvent fluid, as is evident from the 
fact that food in general undergoes no change until it arrives 
there, and is always found to be acted upon when mixed 
with the pro ventricular fluid ; which, however, in the case of 
fish-eating birds, extends some way upwards into the oeso- 
phagus. Generally, the width of the proventriculus is not 
greater than that of the oesophagus ; but in many fish-eating 
