304 FIRST YEAR COURSE IN GENERAL SCIENCE 



other digestive organs, so that further digestion may be 

 accomplished. From the small intestine the contents are 

 pushed into the large intestine, where digestion is com- 

 pleted. In this part of the canal,, the water which is 

 no longer needed for solution is absorbed through the 

 walls. 



355. Indigestible Residue. Whatever parts of the food 

 have not been digested before reaching the end of the alimen- 

 tary canal are expelled through the anus, or posterior open- 

 ing. These parts are not waste products but an indigestible 



' a /b ' 

 FIG. 161. THE ALIMENTARY CANAL OR FOOD TUBE OF A FISH 



Observe the continuous passage from the mouth (a), through the 

 stomach (c) and the intestine (d), to the anus (e). b shows the gills and 

 gill rakers. 



residue. Though they have passed through the body, they 

 have never been a part of the body. Grains of sand might 

 enter the mouth with other food, but being insoluble, would 

 pass unchanged through the stomach and intestines and be 

 expelled. 



Animals living mainly upon plant food take, with the 

 nutritious part, much that is indigestible. This is not an 

 injury but a benefit, because it prevents the close packing 

 together of the substances which must be acted upon by the 

 solvents. Pure starch is not so good a food as a potato, 

 whose bulk is mainly pith cells containing starch. After 

 mastication, the starch is mixed with the covering of the 



