CHAPTER IX. 

 DIGESTION. 



Foods are heterogeneous 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. Before the nutritive principles can be utilized they 

 must be dissociated from the non-nutritive material. Even when 

 consumed in the free state, the food principles are seldom in a condi- 

 tion 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 nutri- 

 tive 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 temL^jgggt^n has been given: 



The digestive apparatus comprises the entire alimentary or food 

 canal and its various appendages: the teeth, the tongue, the mouth, 

 the gastric and intestinal glands, the pancreas, and the liver (Fig. 59). 

 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, esoph- 

 agus, 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 pyramidal-shaped structure 

 about five inches in length, which in turn is followed by the esoph- 

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

 passes through the diaphragmTt 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 intes- 

 tine is that portion of the alimentary canal extending from the end 

 of the stomach to the beginning of the large intestine; owing to its 

 length, about twenty-two feet, it presents a very convoluted appear- 

 ance in the abdominal cavity. Embedded in its walls are the intes- 

 tinal glands which open on its surface and secrete the intestinal 



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