180 THE VITAL FUNCTIONS. 



very small crops, their food not requiring any 

 previous softening ; but the Vulture^ which 

 gorges large quantities of flesh at a single meal, 

 has a crop of considerable size, forming, when 

 filled, a visible projection in front of the chest. 

 Birds which feed on fish have no separate dila- 

 tation 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 further dilatation of any particular part of 

 the tube unnecessary. The lower portion of the 

 oesophagus appears 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. 



Chapter VII. 



Digestion. 



All the substances received as food into the 

 stomach, whatever be their nature, must neces- 

 sarily undergo many changes of chemical com- 

 position before they can gain admission into the 

 general mass of circulating fluids ; but the extent 

 of the change required for that purpose will, of 

 course, bie in proportion to the diflference be- 



