18 INTRODUCTION. 



and catch 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 oaso- 

 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 



