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 subsequently 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, 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. 59). 



The alimentary canal is a musculo-membranous tube about eleven meters 

 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 twelve centimeters in length, which in turn is 

 followed by the esophagus or gullet, a tube about twenty-two centimeters 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 recep- 

 tion and retention of the food for a varying length of time. The small in- 

 testine 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 eight meters, 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 por- 

 tion of the small intestine, within twelve centimeters of the stomach, there 

 is an orifice, the outlet of a small pouch, the Ampulla of Vater, into which 



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