92 



DIGBSTIOIT 



the lower animals, such as the common earthworm, this 

 tube is almost straight and of the same diameter throughout 

 the body. In man it is very much twisted and doubled 

 upon itself, so that its entire length in our body is over 

 thirty feet. Its internal diameter in man varies at different 



points, forming at such points 

 large cavities (mouth, stomach, 

 etc.), which are connected by 

 narrower portions (gullet and 

 intestines). At certain points 

 it gives out growths of various 

 sizes called glands (liver pan- 

 creas, salivary glands). The 

 main bulk of this tube and its 

 outgrowths is packed and coiled 

 in the abdominal cavity (see 

 plate I) of the body. 



The tube is made up of vari- 

 ous kinds of tissue, muscular, 

 connective, epithelial, blood, and 

 nerve, arranged in such a way as 

 to form layers of varying thick- 

 ness. It is lined throughout its 

 entire length with a layer of epi- 

 thelial tissue called the mucous 

 membrane. The various parts of 

 the tract have been given spe- 

 cial names, and each part performs a distinct action in 

 the preparation of food for the body. 



While there are many of these parts each having a special 

 kind of activity, their actions admit of classification under 

 general headings. For example, the parts known as glands 



Fig, 23 — The alimentary canal. 



