DIGESTIOIf OF FOOD. 



The processes of digestion may be considered as having for 

 [ their end the preparation of food for entrance into the blood. 

 This is in part attained when the insoluble parts have been 

 rendered soluble. At this stage it becomes necessary to inquire 

 as to what constitutes food or a food. 



Inasmuch as animals, unlike plants, derive none of their 

 ; food from the atmosphere, it is manifest that what they take in 

 1 by the mouth must contain every chemical element, in some 

 '( form, that enters into the composition of the body. 



But actual experience demonstrates that the food of animals 

 must, if we except certain salts and water, be in organized 

 form — i. e., it must approximate to the condition of the tissues 

 of the body in a large degree. Plants, in fact, are necessary to 

 animals in working up the elements of the earth and air into 

 form suitable for them. 



Foodstuffs are divisible into : 



I. Organic. 



1. Nitrogenous, (a) Albumins; (6) Albuminoids (as gelatin). 



2. Non-nitrogenous, (a) Carbohydrates (sugars, starches) ; 



(&) Fats. 



II. Inorganic. 



1. Water. 



2. Salts. 



Animals may derive' the whole of their food from the bodies 



of other animals (carnivora) ; from vegetable matter exclusively 



(herbivora) ; or from a mixture of the animal and vegetable, as 



in the case of the pig, bear, and man himself {omnivora). 



I It has been found by feeding experiments, carried out mostly 



; on dogs, that animals die when they . lack any one o f the con- 



i stituents of food, though they Ijve longe r on the nitrogenous 



• I than any other kind. In some instances, as when fed on gela- 



I tin and water, or sugar and water, the animals died almost as 



