CHAPTER XIV.
PHYSIOLOGY OF THE DIGESTIVE ORGANS.
259. Substances received into the stomach as food, must necessarily undergo many changes before they are fitted to form part of the animal body. The solid portions are reduced to a fluid state, and those parts that will nourish the body are separated from the waste material.
260. The first preparation of food for admission into the system, consists in its proper mastication. The lips in front, the cheeks upon the side, the soft palate, by closing down upon the base of the tongue, retain the food in the mouth, while it is subjected to the; process of mas-ti-ca´tion, (chewing.) The tongue rolls the mass around, and keeps it between the teeth, while they divide the food to a fineness suitable for the stomach.
261. While the food is in process of mastication, there is incorporated with it a considerable amount of sa-li´va, (spittle.) This fluid is furnished by the salivary glands, situated in the vicinity of the mouth. The saliva moistens and softens the food, so that, when carried into the pharynx. it is passed, with ease, through the œsophagus into the stomach.
262. When the food has been properly masticated, (and in rapid eaters when it is not,) the soft palate is raised from the base of the tongue backward, so as to close the posterior opening through the nostrils. By a movement of the muscles of the tongue, cheeks, and floor of the mouth, simultaneous with 125 that of the soft palate, the food is pressed into the upper part of the pharynx.
259–272. Give the physiology of the digestive organs. 259. What is necessary before food can nourish the body? 260. Describe how mastication is performed. 261. Of what use is the saliva in the process of mastication? 262. How is the food pressed into the pharynx?
263. When in the pharynx, the food and drink are prevented from passing into the trachea by a simple valve-like arrangement, called the ep-i-glot´tis. The ordinary position of this little organ is perpendicular, so as not to obstruct the passage of air into the lungs; but in the act of swallowing, it is brought directly over the opening of the trachea, called the glot´tis. The food, being forced backward, passes rapidly over the epiglottis into the œsophagus, where the circular band of muscular fibres above, contracts and forces the food to the next lower band. Each band relaxes and contracts successively, and thus presses the alimentary ball downward and onward to the stomach.[8]