CHAPTER III 



THE DIGESTIVE ORGANS 



The substances which occur in an animal's food ma}' be 

 divided for our present purpose into the following main groups : 



(1) Proteins which contain the five elements, carbon, oxygen, 

 hydrogen, nitrogen, and sulphur combined together to form 

 molecules of very large size. Albumen or egg-white, gluten 

 found in wheat flour, casein found in milk, and myosin which 

 is an important constituent of lean meat, are examples of proteins. 



(2) Amides which like proteins are nitrogenous substances 

 but of relatively simple composition. They are of little im- 

 jDortance as constituents of food. 



(3) Carbohydrates which are compounds of carbon, hydro- 

 gen, and oxj'gen, the U\o latter elements being always present 

 in the same proportion as in water. Starch (CeH^jOg), grape sugar 

 (CgHigOg), cane sugar (CigHgaO^i) and cellulose (CgHjoOg) are 

 examples. 



(4) Fats which contain the same elements as carbohydrates, 

 but with the carbon and hydrogen in very great excess over the 

 oxygen. 



(5) Salts which thoiigh not strictly foods since they are not 

 energy producers, are yet essential to the organism since they 

 supply certain necessary elements. The chlorides and phos- 

 phates of sodium, potassium and calcium are examples. 



(6) Water which though not a food is absolutely essential to 

 the organism, being present in every living cell. 



In the present chapter it is proposed to describe the organs 

 which are concerned with digesting, or rendering fit for assimi- 

 lation, the different classes of foods. 



The relative importance of the food stuffs for the well-being 

 of the animal is dealt with later in the volume on nutrition. 



The Mouth. The organs emplo3'ed by animals for convejnng 

 the food to the mouth are the lips, tongue, and teeth, but 

 the precise part played I)}' each varies in the different species. 

 A horse when feeding in a manger for example, employs its lips 



