CHAPTER X. 

 DIGESTION. 



Digestion is a process partly physical, partly chemic, by which the 

 nutritive principles of the foods are prepared for absorption. The reason 

 for these changes lies in the fact that the foods as consumed are hetero- 

 geneous compounds consisting of organic and inorganic nutritive principles 

 associated with a varying amount of non-nutritive material, such as the 

 dense parts of the connective tissue of the animal foods and the woody 

 fiber or cellulose of the vegetable foods, from which the nutritive prin- 

 ciples must be freed before they can be utilized; and in the further fact, that 

 even when consumed in the free state, the food principles are seldom in a 

 condition to be absorbed into the blood and assimilated by the tissues. 

 When foods are consumed in their natural state or after they have been 

 subjected to the cooking process, they are subjected while in the food canal 

 to the solvent action of various fluids by which they are disintegrated and 

 reduced to the liquid condition. The nutritive principles freed from their 

 combinations are changed in chemic composition and transformed into 

 substances capable of absorption. To all the physical and chemic changes 

 which foods undergo in the food canal the term digestion has been given. 



The digestive apparatus comprises the entire alimentary or food canal 

 and its various appendages: the lips, the teeth, the tongue, the salivary 

 glands, the gastric and intestinal glands, the pancreas, and the liver (Fig. 60). 



The canal itself is a musculo-membranous tube about thirty-two feet 

 in length, and extends from the mouth to the anus. It may be subdivided 

 into several distinct portions, as mouth, pharynx, esophagus, stomach, 

 small and large intestines. The mouth is provided (i) with teeth, by which 

 the food is divided, (2) with the tongue, and (3) with glands, by which a 

 solvent fluid, the saliva, is secreted. The glands, though situated for the 

 most part outside the mouth, are connected with it by means of ducts. 

 Posteriorly the mouth opens into the pharynx or throat, a somewhat py- 

 ramidal-shaped structure about five inches in length, which in turn is 

 followed by the esophagus or gullet, a tube about nine inches in length. As 

 the esophagus passes through the diaphragm it expands into the stomach, 

 a curved pyriform organ, which serves as a reservoir for the reception and 

 retention of the food for a varying length of time. The small intestine is 

 that portion of the alimentary canal extending from the end of the stomach 

 to the beginning of the large intestine in the right iliac fossa; owing to its 

 length, about twenty-two feet, it presents a very convoluted appearance in the 

 abdominal cavity. Embedded in its walls are the intestinal glands which 

 open on its surface and secrete the intestinal fluid. In the upper portion of 

 the small intestine, within five inches of the stomach, there is an orifice, the 

 outlet of a small pouch, the Ampulla of Vater, into which open the termina- 

 tions of the ducts of the liver and pancreas, organs which secrete the bile and 



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