CHAPTER IV 



THE DIGESTIVE SYSTEM 



THE ALIMENTARY CANAL 



The digestive system is represented by a tube about 100 feet 

 long, open at both ends, looped on itself many times, dilated at 

 intervals along its course, and provided at various places with 

 accessory glandular organs that pour their secretions into the 

 tube to aid digestion. The anterior opening is for the reception 

 of food; the posterior, for the expulsion of the unabsorbed por- 

 tion and wastes. The tube is commonly known as the alimentary 

 canal. 



Mucous membrane lines the alimentary canal throughout its 

 length. This is a moist, velvet-like tissue with an inner or 

 free surface which is usually thrown up into folds. These folds 

 provide a means of increasing the area through which absorption 

 of the products of digestion occurs. They become effaced with 

 the distention of the walls of the organs with food or gas. At 

 the natural body openings the mucous membrane is continuous 

 with the external skin. It has been called the "internal skin." 

 Its color, thickness, and other characters vary in different organs, 

 depending on the functions which it has to perform. The con- 

 tents of the canal are not to be considered as within the body 

 but only in contact with a part of its surface. 



External to the mucous lining is found a double layer of invol- 

 untary muscle-fibers, composed of an inner circular and an outer 

 longitudinal layer (see Fig. 24). Through the alternate wave- 

 like contraction and relaxation of these muscle-fibers the food is 

 thoroughly mixed with the digestive juices, and conveyed along 

 the canal from organ to organ. 



A serous membrane covers the alimentary canal in the greater 

 part of its course. This is a thin, transparent, shiny, and smooth 

 tissue which is reflected from the walls of the body cavities. It 

 serves as a covering for the soft organs in these cavities and acts 

 as a lining for their walls. Its free face is always in contact with 

 itself. From it is secreted a serous fluid that keeps the membrane 

 moist and allows the viscera to glide freely upon each other. 



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