VERTEBRATA: AVES. 



585 



ventriculus. The two chief forms of gizzard are exhibited 

 respectively by the Raptorial birds, which feed on easily- 

 digested animal food, and the Rasores and some of the Nata- 



d 



Fig. 326. Digestive System of the common Fowl (after Owen), o Gullet : c Crop ; 

 p Proventriculus ; g Gizzard; sm Small intestine; k Intestinal caeca; / Large in- 

 testine ; cl Cloaca. 



tores, which feed on hardly-digested grains. In the birds of 

 Rapine the gizzard scarcely deserves the name, being, as a 

 rule, nothing more than a wide membranous cavity with thin 

 walls. In the granivorous birds, whose hard food requires 

 crushing, the gizzard is enormously developed ; its lining coat 

 is formed of a thick, horny epithelium, and its walls are ex- 

 tremely thick and muscular. This constitutes a grinding 

 apparatus, like the stones of a mill ; whilst the " crop " or 

 cesophageal dilatation may be compared to the " hopper " of 

 a mill, since it supplies to the gizzard " small successive quan- 

 tities of food as it is wanted" (Owen). Supplementing the 

 action of the muscular walls of the gizzard, and acting in the 

 place of teeth, are the small stones or pebbles, which, as is so 

 well known, so many of the granivorous birds are in the habit 

 of swallowing with their food, or at other times. In fact, there 

 can be no doubt but that the gravel and pebbles swallowed 

 by these birds are absolutely essential to existence, since the 



