ALIMENTATION AND DIGESTION 



97 



changed, or digested, before they can be absorbed for use in 

 the body. The same thing is true of water. 



10. Composition of some common foods. The following table 

 gives the percentage composition of some of the more common 

 foods (see also p. 238). 



11. Indigestible material in food. When we say that "a food 

 is digestible we generally mean that when taken into the 

 alimentary canal, if not already in solution, it is chemically 

 acted upon by the digestive juices so as to be dissolved and 

 made capable of being absorbed into the blood. The greater 

 part of the food we eat consists in this sense of digestible sub- 

 stances, but many foods contain a certain amount of indigest- 

 ible material, and some contain a very considerable amount. 



The most conspicuous example of such material is cellu- 

 lose, a member of the same group of carbohydrates to which 

 starch belongs. It occurs in almost all vegetable foods ; and 

 since, in the human alimentary canal, cellulose is for the 

 most part unaffected, it cannot be absorbed and necessarily 



