RECEPTACLES FOR RETAINING FOOD. 



131 



300 



much enlarged, and very active in preparing their peculiar 

 secretions: this is remarkably the case in the Pigeon (Fig. 



300,) which, instead of a single 

 sac, is provided with two (seen 

 at c, c, Fig. 300,) one on each 

 side of the oesophagus (o.) The 

 pouting pigeon has the facul- 

 ty of filling these cavities with 

 air, which produces that distend- 

 ed appearance of the throat from 

 which it derives its name. Birds 

 of prey have, in general, very 

 small crops, their food not re- 

 quiring any previous softening; 

 but the Vulture, which gorges large quantities of flesh 

 at a single meal, has a crop of considerable size, form- 

 ing, when filled, a visible projection in front of the chest. 

 Birds which feed on fish have no separate dilatation for this 

 purpose, probably because the great width of the oesophagus, 

 and its having the power of retaining a large mass of food, 

 render the farther dilatation of any particular part of the 

 tube unnecessary. The lower portion of the oesophagus ap- 

 pears often, indeed, in this class of animals, to answer the 

 purpose of a crop, and to effect changes in the food which 

 may properly be considered as a preliminary stage of the 

 digestive process. 



