CHAPTER XII 



THE ALIMENTARY CANAL 



The Alimentary Canal is the great route by which 

 nutritive matter reaches the interior of the body. It is 

 the most universal organ in the animal kingdom, and 

 the rest are secondary or subservient to it. In the 

 higher animals, it consists of a mouth, pharynx, gullet, 

 stomach, and intestine. 



It is a general law, that food can be introduced into 

 the living system only in a fluid state. While plants 

 send forth their roots to seek nourishment from without, 

 animals, which may be likened to plants turned outside 

 in, have their roots (called absorbents) directed inward 

 along the walls of a central tube or cavity. This cavity 

 is for the reception and preparation of the food, so that 

 animals may be said to carry their soil about with them. 

 The necessity for such a cavity arises not only from the 

 fact that the food, which is usually solid, must be dis- 

 solved, so as to make its way through the delicate walls 

 of the cavity into the system, but also from the occur- 

 rence of intervals between the periods of eating, and 

 the consequent need of a reservoir. For animals, unlike 

 plants, are thrown upon their own wits to procure food. 



The Protozoa, as the amoeba and Infusoria, can not be 

 said to have a digestive canal. The animal is here 

 composed of a single cell, in which the food is digested. 

 The jelly like amoeba passes the food through the firmer 

 outer layer (ectosarc) into the more fluid inner part 

 (endosarc), where it is digested (Fig. i). The Infusoria, 



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