212 



PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



neck of the bird, while its width and dilatability depend upon the nature 

 <>t" the food. In granivorous birds we meet with the first indication of 

 the development of the oesophageal pouches for the retention and macer- 

 ation of food,— organs which are identical in function with the first three 

 pouches of the mammalian ruminant stomach. The locality and character 

 of these pouches vary in different birds. 



In the granivorous birds this organ, which is termed the crop, is 

 located at the lower part of the gullet. It may be double, as in the case 

 of the pigeon, and distinctly arrests the food and retains it in contact 

 with fluids to enable it to become macerated before being passed down 

 to the digestive orgsins proper. In flesh-eating birds, such as the pelican, 



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Fig. 71.— Horizontal Section of Gizzard of Goose, after Garrod. 

 A, in contraction ; B, in relaxation. 



the pouch is located higher up in the digestive canal, ordinarily below 

 the lower jaw, and here seems to be more of a reservoir for storing food 

 than as a distinct commencement of the digestive apparatus. Fruit- and 

 insect-eating birds are not supplied with anj- such reservoirs, while the 

 turkey, ostrich, goose, swan, and most of the waders, have a highly- 

 developed crop; the pigeon, as before stated, having two, one on each 

 side of the oesophagus. The stomach in birds differs according as their- 

 diet is vegetable or animal. Granivorous birds have a small, straight, 

 dilatable stomach, called the proventriculus, communicating above with 

 the gullet and below with a highly muscular organ, the gizzard (Fig. 

 71). lined with horny epithelium, usually containing gravel or sand, 



