274 



BIRDS 



on to the stomach the food is further acted on by the diges- 

 tive juices of this organ. In the gizzard the food is masti- 

 cated, as it were, and more thoroughly mixed with the 

 digestive juices trickling into the gizzard from the stomach. 

 In the absence of teeth, pebbles are swallowed and retained 



Brain 



Caeam 

 Intestine 

 Pancfea§ 



Heart 

 Liver 

 Breastbone 



Fig. 187. — Internal structure of a bird. 



in the gizzard to aid that organ in reducing the food. 

 In the duodenum the food receives the final digestive 

 ingredients, the bile from the liver and the pancreatic 

 juice. 



The circulatory system. — The heart of the sparrow is 

 four chambered and the left side is completely separated 

 from the right side ; therefore there is a double circulation, 

 that is, one set of blood vessels carrying impure (venous) 

 blood and another set carrying pure (arterial) blood. 

 Birds are very active animals, and the circulation is much 



