AVES. 



285 



a state suitable for digestion. The gullet, after leaving the crop, 

 shortly opens into a second cavity, called the " proventriculus," 

 which is the true digesting stomach, and is richly supplied with 

 glands which secrete the digestive fluid or gastric juice (p). This, 

 in turn, opens into a muscular cavity which is called the " gizzard " 

 ig), and which leads into the commencement of the small intestine. 

 The characters of the gizzard vary with the nature of the food. In 

 the Birds of Prey, which live on an easily digested animal diet, the 

 walls of the gizzard are thin and membranous. In the grain-eating 



;. 205.— Digestive system of the common Fowl (after Owen), o Gullet ; c Crop ; 

 p Proventriculus ; g Gizzard ; sm Small intestine ; k Intestinal cEeca ; ; Large 

 intestine : cl Cloaca. 



Birds, such as the Fowls, whose hard food requires to be crushed 

 before it can be properly digested, the walls of the gizzard are 

 extremely thick and muscular, and the inner lining is hard and 

 horny. In these Birds the gizzard constitutes a kind of grinding 

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

 compared to the " hopper " of the mill, since it supplies to the 

 gizzard " small successive quantities of food as it is wanted " (Owen). 

 The grinding action of the gizzard is further assisted by the small 

 pebbles and gravel which, as is well known, so many birds are in 

 the habit of swallowing. These pebbles take the place of teeth, 



