THE NOURISHMENT OF THE BODY 253 



transported to the distant cells. That part of the body in 

 which food is digested is called the alimentary canal. The 

 simplest digestive system is a mere sac or stomach into which 

 food is taken, the available material selected, and the refuse 

 thrown out. The more complex animals, including man, have 

 a tube extending through the body along which the food 

 slowly moves while substances are absorbed from it. During 

 its passage through the alimentary canal, digestive juices are 

 poured over the food, thus rendering it soluble and capable 

 of being absorbed. The movement of the food is caused by 

 rhythmic contractions of the walls of the alimentary canal 

 which forces the contents of the canal onward. 



217. Structure of the Alimentary Canal. The alimentary 

 canal in general consists of an outer layer of muscles and con- 

 nective tissue and a lining of mucous membrane, the latter so 

 called because it secretes a glairy liquid, called mucus, which 

 lubricates it. This membrane is much like the outer layer of 

 the skin in structure and originates from the same tissue. 

 There are considerable differences in the digestive organs of 

 the different groups of animals, due in part to the kind of food 

 they take and the kind of structures to be nourished, but all 

 are essentially alike. In the vertebrates, there is first a 

 mouth for taking the food, usually equipped with teeth for 

 tearing or grinding it into small pieces, and a tongue for 

 moving it about during the process of chewing. The mouth 

 opens into the throat from which a tube, the esophagus, leads 

 to the stomach, the latter largely a storage organ. Beyond the 

 stomach is an intestine in which the greater part of the food 

 is digested and from which it is absorbed. The stomach lies 

 crosswise of the body just below the diaphragm. Ordinarily 

 it holds about three pints but it may be distended to hold more. 

 In man, the intestine is divided into two regions, the small 

 intestine and large intestine respectively. The small intestine 

 begins at the stomach on the right side of the body and owing 



