284 VERTEBRATE ANIMALS, 
membranous. In the grain-eating birds, such as the fowls, 
whose hard food requires to be crushed before it can be pro- 
perly 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 tothe 
“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 
Fig, 153-—Digestive System of the common Fowl (after Owen). 0 Gullet ; ¢ Crop ; 
# Proventriculus ; ¢ Gizzard ; s#z Small intestine; & Intestinal ceca; / Large 
intestine ; cZ Cloaca, 
pebbles and gravel which, as is well known, so many birds are 
in the habit of swallowing. These pebbles take the place of 
teeth, and there can be no doubt that they are in many cases 
essential to health, the bird being otherwise unable to triturate 
its food properly. The intestinal canal extends from the giz- 
zard to the cloaca (c/), and is comparatively short. The secre- 
tions of the liver and pancreas’ are poured into the commence- 
ment of the small intestine. The commencement of the large 
