DIGESTION, ASSIMILATION, NUTRITION 219 



feed nutrients are classed as protein, carbohydrates, fat, 

 and ash. They must be present in the feed in the right 

 proportion if the body is to be properly nourished. 



THE DIGESTIVE TRACT IN BIRDS 



The digestive tract is made up of many different organs 

 and groups of organs, each with its own function to perform 

 and each constructed especially for the work which it has 

 to do. 



Division of the Work of Digestion. — The digestive tract 

 performs four different kinds of work, and the organs of 

 digestion are so designed and constructed that each one is 

 especially fitted for the work assigned to it. One group of 

 organs, called the mouth parts, is for seizing the feed. An- 

 other organ, known as the gizzard, is used for grinding the 

 feed, and takes the place of the teeth in most animals. The 

 upper part of the digestive tract performs, to a greater or 

 less extent, the actual work of digesting the feed, while, as 

 was previously mentioned, the lower part is especially 

 designed and adapted to promote the assimilation of the 

 digested material into the blood (Fig. 124). 



Mouth Parts. — The mouth parts are hard, horny growths 

 which project outside of the body. In land birds, the mouth 

 part is called the beak, in water birds, the bill. The beak of 

 the land bird is sharp and pointed, the upper and lower 

 parts closing tight together. They are designed to pick up 

 grains and to cut blades of grass. In water fowls the bill is 

 wide and blunt, and is especially designed for scooping in 

 the mud or digging in the wet soil. 



The Gizzard. — The function of the gizzard is to grind the 

 feed, and this is quite necessary since the bird has no teeth 

 with which to chew it. Birds subsist chiefly upon hard grains, 

 like corn and wheat, which must be subjected to consider- 

 able grinding before they are reduced to fine particles. The 

 gizzard looks very much like a large ball, slightly flattened, 



