7 6 



THE BOOK OF NATURE STUDY 



being toothless. In the majority of cases it is swallowed whole ; 



but many birds, as hawks and eagles, tear their prey in pieces 



when too large to be swallowed whole. 



The solid and often large pieces of food thus necessarily 



swallowed are passed, in a large number of birds, into a dilatation 



of the gullet known as the " crop " 

 (Model D. 8). Here it is often 

 retained in large quantities before 

 being passed on to the stomach, 

 in order that it may be softened, 

 or because of the necessity of 

 taking in, at once, more food 

 than can be immediately dealt 

 with by the stomach. 



This crop is very thin walled, 

 and while in some birds it takes 

 a spherical form, as in the game 

 birds (Text, Fig. 24), in others, as 

 in pigeons, it is bilobed. From the 

 crop the food passes on to the 

 " proventriculus " (Model D. 8). 

 This is distinguished from the 

 gullet chiefly by its thickened 

 walls, which are crowded with 

 glands secreting the digestive 

 juices. This proventriculus is to 

 be regarded as the anterior end, or 



FIG. 24-The crop and gizzard of a fowl, first portion ^ Q f a comp lex stomach. 



For the hinder end of this organ, in 

 grain-eating birds, has been modi- 

 fied to form what is known as a 

 " gizzard " (Model D. 15). Herein the walls are of great thick- 

 ness, and very muscular, while the cavity they enclose is lined 

 with a thick, horny layer, thrown into folds, or provided with 

 dense opposing surfaces in the form of pads, as in the common 

 fowl and swan (Text, Fig. 25). Here grain and other hard food 

 is ground into minute pieces by means of small stones swallowed 

 by the bird for this purpose. 



to show their general form and rela- 

 tions. W, trachea or windpipe ; 

 G, gullet or cesophagus ; P, proven- 

 triculus ; G 1 , gizzard ; I, intestine. 



