118 



THE DIGESTIVE SYSTEM 



system, the part of the food that cannot be used is separated 

 from the nutrients. This process may be called a refining pro- 

 cess. The waste or indigestible 

 part of the food is ultimately 

 expelled from the body. The 

 nutrients are made ready for 

 absorption by the action of 

 various juices. In this diges- 

 tive process the nutrients are 

 reduced to simpler and simpler 

 organization or certain standard 

 forms to enable them to pass 

 through the walls of the blood 

 vessels. For example, the pro- 

 teins in eggs, milk, and meat 

 are reduced to standard protein 

 products ; the starch in bread 



nasal cavity.. 

 mouth Cavity 



thoracic cavity--/-- 



abdominal 



Cavity 



and potatoes and the sugar in 

 fruits, candies, and carrots are 

 all reduced to a standard car- 

 bohydrate product. The pro- 

 cess of digestion really consists 

 Man is a collection of tubes and cavities. o f refining, digesting, and stand- 



The spinal column is a bony tube, the nerve 



cord a solid rod made of delicate tissue, and the ardizing processes. 



food tube a long, continuous, pipelike canal. .- 



Digestive organs. The diges- 

 tive system may be said to consist of two groups of digestive 

 organs, those making up the alimentary canal or food tube in 

 which foods are actually digested, and the accessory organs, the 

 glands, which make or secrete the juices for digestion. A gland is 

 a collection of epithelial cells that secretes a juice. Some of the 

 digestive glands are large organs outside the food tube, such as 

 the pancreas and liner, and others are minute structures in the 

 lining of the food tube, as the gastric, peptic, and intestinal glands. 



