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The Digestive Process

food, mouth, saliva, pharynx, stomach, tongue, windpipe and juice

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THE DIGESTIVE PROCESS We have considered the character and struc ture of the digestive apparatus, the tract along which the food is conveyed, and the various glands in connection with it; and we have seen that theie glands pour various juices into the canal to mix with the food, namely the saliva, gastric juice, bile, pancreatic juice, and intes tinal juice. Let us now consider the changes undergone by the food as it passes along the digestive tract, and the part these juices and other agencies play in the change.

Mastication is the first part of the process to which the food is submitted. By this is meant the breaking down of the food by means of the teeth. In some animals, such as the tiger, the lower jaw is movable on the upper in one direction mainly, an upward and a downward movement that permits of tearing the food. In other animals, such as the cow, the movement is from side to side, the grind ing teeth being specially employed. In man, however, owing to the shape of the joint between the lower and upper jaws, the lower is movable in many directions, so that a cut ting or tearing and a grinding movement as well is permitted. While the food is being thus broken down it is moved about and mixed by movements of the tongue and cheeks, so that every part of it may come under the operation of the teeth. The advantage of this is obvious. The more completely the food is separated up into small portions the more easily will the digestive fluids reach every particle of it, and the more thorough and speedy will digestion be. The disadvantage, then, of "bolting the food" ought to be ap parent. If the breaking-down process is not accomplished in the mouth it must be per formed by the stomach, and the stomach has no apparatus for such a purpose. Children in particular, who are very prone to swallow their food almost without chewing it, should be trained to take time to do this part of their eating thoroughly.

chewing is going on, the saliva is streaming into the mouth, and is being intimately mixed with the food, and to this the term insalivation is given. The saliva is a colourless fluid, without smell or taste. It contains in solution very few saline matters, only about 5 parts in 1000. Its principal element, besides water, is a substance called ptyalin, which is a ferment, and possesses the remarkable property of being able to convert starch into sugar. As much as 98 ounces of saliva may be poured into the mouth daily. Thus in the mouth, and while chewing is being performed, another process is going on which has for its purpose the conversion of starch, which cannot dissolve in water, and cannot pass through an animal membrane, into sugar, which can do both. Besides effecting this

object, saliva also moistens the food so as to enable it to be made up into a consistent mass fit for swallowing. Saliva aids also in speech by moistening the mouth.

food having been thor oughly broken down and mixed with saliva is in a proper condition for deglutition or swal lowing. The tongue gathers it up into a bolus or mass and forces it backwards through between the pillars of the faeces into the pharynx, by whose muscles it is grasped. Now, having reached the pharynx, the food, it is easy to see, might be forced in various direc tions by the contractions of the constricting muscles of the pharynx. Reference to Fig. 101, p. 195, will explain how this comes about. Thus it might be forced back again into the mouth. This is prevented by the contraction of the front pillars of the fauces and the back ward pressure of the tongue. The food might pass upwards and get through the openings of the nasal cavity behind. This is prevented by the back pillars of the fauces contracting and the soft palate being raised to bar the way. It might pass down into the windpipe, but this is also prevented by the box of the windpipe being quickly raised up under cover of the root of the tongue. The elevation of this part of the windpipe anyone may feel by putting a finger on the front of the neck and then swallowing. At the same time as the windpipe is thus raised, its lid, the epiglottis (ep, Fig. 101), is lowered so as to cover the entrance. The food has thus only one pathway, namely, down the gullet. When it has been forced into this tube the walls contract in a wave-like fashion, and thus propel the food onwards to the stomach. The food does not then fall into the stomach from the mouth. It is swept along the gullet by the muscular wave. This is why a horse can drink though its mouth be below the level of its gullet, and why a man can drink stand ing on his head. Some of the movements that have been described are under the control of the will, some are involuntary. The forcing of the food backwards is voluntary ; but as soon as it has entered the pharynx all the other movements will occur in due order whether we will it or not. They are accomplished by a reflex nervous action (p. 132).

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