CHAPTER IV 



THE DIGESTIVE SYSTEM 



The digestive system, is made up of a tube about 100 feet long, 

 looped on itself many times, dilated at intervals along its course, 

 and provided with several accessory organs. The anterior opening 

 is for the reception of food; the posterior, for the expulsion of the 

 unabsorbed portion and wastes. 



The entire canal is lined with mucous membrane — a moist, 

 velvet-like tissue with an inner or free surface — usually thrown up 

 into folds. These folds become effaced with the distention of the 

 walls of the organs with food or gas. At the natural body openings 

 it is continuous with the skin. The character of the mucous mem- 

 branes varies in different organs, depending on the functions which 

 it has to perform. 



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

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

 longitudinal layer. 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. 



In the greater part of its course the canal receives another 

 covering in the form of a serous membrane. This is a thin, trans- 

 parent, 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. Serous membranes are very sensitive to 

 infection, for they rapidly absorb fluids. They are never so placed 

 as to come in contact with the external air. 



The peritoneum is the name given to the serous lining of the 

 abdominal cavity. The most satisfactory way to understand the 

 general disposition of it is to think of the cavity as empty and 

 lined by a single layer of peritoneum (Fig. 11). The organs may 

 be regarded as beginning to develop outside this membrane, and, as 

 they enlarge, gradually push into it and become enveloped by it. 



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