318 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



vies), a reservoir for storing and softening the food by the 

 saliva; (2) the proventriculus, which furnishes the peptic 

 juice; and (3) the gizzard, or ventriculus, or gigerium, 

 which triturates the food and mixes it thoroughly with 

 saliva and gastric juice furnished in the parts above. The 



gizzard consists of two very 

 powerful muscles, provided 



Fig. 202. — Digestive apparatus of a Fig. 203. — The gizzard of a goose 



granivorous bird : cr, crop ; fv, cut open : tp, triturating pad. 



proventriculus ; g, gizzard ; i, in- (After Owen.) 

 testine. 



each with a cushion or pad, which rub against one an- 

 other (Fig. 203). The cavity is small and lined with a 

 hard, almost horny skin. Gravel is taken as grinders 

 to this mill, and renewed as required. The whole pro- 

 cess is briefly as follows: The food is first stored and 

 insalivated and softened in the crop. The crop acts as 

 a hopper to the gizzard mill, dropping little by little as 

 required. The digestive juice secreted by the proven- 

 triculus is also added little by little as required. The 

 triturated and digested material finally passes on to the 

 intestines. 



Evolution of this Apparatus. — This elaborate ap- 

 paratus is most perfect in grain-eating birds ; but all 

 grades approaching it may be found in birds from the 

 simple thin sac of the flesh-eating to the powerful mill 

 of the grain-eating. 



