98 



THE HUMAN MECHANISM 



forms an important part of the feces. Other indigestible 

 substances are the outer skin of animals (for example, the 

 skin of fowls), and certain portions of the connective tissue 

 of meat. 



12. Animal and vegetable foods. The classification of foods 

 into animal and vegetable not only describes the origin of 

 foods from the two great kingdoms of living things, but 

 also defines important differences between them with refer- 

 ence to digestion. These differences may be summed up as 

 follows : Animal foods are generally rich in proteins and poor 



FIG. 51. Part of the seed of the bean 



Showing the larger starch granules and 



the finer protein granules inclosed 



within the cellulose cell walls 



FIG. 52. Section of potato 



Showing starch granules inclosed 

 within the cellulose cell walls 



in carbohydrates, while vegetable foods are generally poor in 

 proteins and very rich in carbohydrates, especially starch. 

 In the second place, animal foods contain relatively little 

 indigestible material, while vegetable foods, as they occur 

 in nature, contain large amounts of indigestible cellulose. 

 In the third place, the digestible materials of vegetable 

 foods (the proteins, carbohydrates, and fats) are often con- 

 tained within a plant cell which is surrounded by a cellu- 

 lose membrane impermeable to the digestive juices; before 

 they can be digested this membrane must be ruptured in 

 one way or another. In the case of many animal foods, on 



