tween the crown and the root is the neek of the tooth, which is surrounded by the gums. The internal structure of the tooth is well shown in a vertical section (Fig. 30). The covering of enamel is thickest over the top of the crown; it becomes thinner down the exposed sides, and disappears in the neck region. The largest part of the tooth is composed of bony dentine. In the central part is the pulp cavity. This region is well supplied with nerves and blood vessels, which enter through a small opening at the end of each root. The blood furnishes the teeth with new building material. 119. Care of the teeth. Too much stress cannot be laid on the importance of caring for the teeth, since decaying teeth are frequently painful, are always unsightly, are usually the cause of an ill-smelling breath, and often lead to indigestion. Immediately after eating, one should remove any bits of food from between the teeth by using a wooden toothpick, dental floss, or thread. Pins, knife-blades, or other metallic implements should never be used for this purpose. The teeth should then be brushed thoroughly on all sides, and warm water and a little castile soap or reliable tooth powder should be used. The sides of the teeth should be brushed from the gums toward the crown in order to avoid pushing the gums away from the neck of the tooth. Since the enamel that covers the crown of the tooth is composed entirely of mineral matter, it cannot, of course, decay. If, however, food is allowed to decompose on or between the teeth, the acids formed by the action of the bacteria gradually dissolve the enamel until a cavity is formed. When the dentine is reached, the bacteria directly cause this part of the tooth to decay, since it contains living matter. The teeth ought never to be used to crack nuts or to bite hard substances, for while the enamel is a very hard substance, it is also brittle and may be cracked or broken off by such treatment. If once lost, it will not grow again. It is evident, therefore, that it is very essential to protect this outer layer, both from the action of acids, and from mechanical injuries. Some people seem to think that the loss of natural teeth is not a very serious matter, and that false teeth are just as effective as those teeth provided by nature. Experiments have shown, however, that the power to crush food with false teeth is only about one fifth that of the power exerted by a normal set of teeth. Hence, loss of teeth is very likely to result in imperfect mastication of food, with consequent ill-health resulting from indigestion. If, however, one has been unfortunate enough to have lost one or more teeth, the gaps should be promptly filled by bridge work. The teeth should be examined by a dentist at least twice a year so that any cavities found may be promptly filled. In short, everything possible should be done to secure and preserve a beautiful and effective set of teeth. 120. Importance of the digestion of starch. In 47 of "Plant Biology" we proved that starch can not pass through the walls of cells, and we likewise showed in 49 how this food substance is made ready by the process of digestion to pass through membranes. Many of the foods we eat contain large percentages of starch. We are now to show experimentally how starch is digested in the human body. 121. Does saliva digest starch?-Laboratory demonstration. Prepare some starch paste by boiling in a test tube of water an amount of arrowroot starch (or corn starch, if the arrowroot cannot be obtained) equal to half the size of a pea. 1. Pour a small amount of the starch paste into a test tube, add some Fehling's solution, and boil. Is grape sugar present? How do you know?. 2. Put some saliva into a clean test tube. Test it with Fehling's solution as you did the starch. Does this saliva contain grape sugar? How do you know? 3. In another clean test tube pour some saliva into some of the starch paste, shake the mixture, and warm it gently for a few moments to the same temperature as that of the mouth. Now test with Fehling's solution, as in 1 above. a. State what was done, the result, and the conclusion. b. What, therefore, is the effect of saliva on boiled starch? c. Name several foods already studied that could be partially digested by saliva. 4. (Optional home work.) Take some popped corn or shredded wheat into the mouth and chew it thoroughly. Can you detect any sweet taste at first? Can you after chewing for a time? What does this experiment teach you as to one advantage of thoroughly chewing the food? 122. Position and action of the salivary glands. In addition to the mucus given out by the mucous membrane (113) the mouth receives another secretion called saliva. At the sight or smell of tempting food "the mouth waters." Saliva is secreted by the salivary glands. Two of these glands (the parotids, from Greek, meaning "beside the ear") are located near the back of the lower jawbone just beneath and in front of the ear. Any one who has had the mumps can readily locate these organs, for mumps is a disease in which these glands swell. From the parotid gland of each side a duct conveys saliva along through the walls of the cheek. This duct opens at the top of a small elevation, which may be felt with the tip of one's tongue opposite the upper second molar teeth. Two other pairs of glands (the submaxillary, Latin, sub neath + maxilla jawbone, and the sublingual, Latin, = = be sub = beneath lingua = tongue) lie in the muscular floor of the mouth cavity, and the ducts from these glands open in the floor of the mouth under the tongue. 123. Uses of saliva. (1) The saliva aids the mucus in keeping the mouth moist, and thus we are enabled to talk easily. (2) It moistens the food for swallowing. The importance of this function is appreciated when one tries to hurry in swallowing the crumbs of dry cracker. (3) Saliva helps to 'dissolve sugar and salt,' thus enabling us to taste them. If the tongue is wiped dry and a piece of sugar is placed upon it, we have no sensation of taste until the sugar has been partially dissolved by the mixture of saliva and mucus that is poured upon it. (4) Besides the three mechanical functions of saliva that we have just enumerated, this secretion digests cooked starch, as we have already shown. This digestive action is due to a ferment known as ptyalin (pronounced ty'alin) which acts in the same manner as the diastase found in plants. III. THE THROAT CAVITY AND GULLET AND THEIR FUNCTIONS -- 124. Structure of the throat and gullet. The cavity of the throat is behind the mouth. If one holds a mirror in front of the mouth opening and presses down upon the tongue with a spoon, one sees hanging down a small, fingerlike extension of the soft palate, called the uvula. When food is swallowed, this little tongue of the soft palate is shoved backward into a horizontal position, where it helps to separate the throat cavity from the nose cavity. The lower part of the throat narrows into the gullet. This tube traverses the length of the chest cavity, and as it nears the stomach, it passes through the diaphragm. Like all other parts of the alimentary canal it is lined with mucous membrane, which furnishes a 1 See 130, A, 1. soft, moist surface for the passage of food. Outside the mucous membrane are rings of circular muscle running around the gullet. 125. Functions of the throat and gullet. - The food is quickly forced out of the throat cavity into the gullet, and is pushed slowly down the gullet by the successive contractions of the rings of muscle just described. After being swallowed from the throat, the food does not drop into the stomach, for the walls of the gullet are pressed together by surrounding organs, except when this tube is opened by the passing food. In fact, after practice, one can swallow when standing on one's head, and most quadrupeds (horse, dog, cow), when feeding, hold the head below the level of the stomach. IV. THE STOMACH AND ITS FUNCTIONS 126. Position, size, shape. - The stomach is a curved muscular pouch, which lies about midway between the upper and lower ends of the trunk, with its larger end lying toward the left side of the body, where it communicates with the gullet (Fig. 26). When moderately filled, this organ holds about three pints. The small intestine is continuous with the right end of the stomach, the communication between the two (known as the pylorus, from Greek, meaning gatekeeper) being controlled by a ring of muscle. opening 127. The lining of the stomach and the gastric glands. If one examines with a lens the mucous lining of the stomach, a countless number of small openings are seen which look like pin pricks. These are the pores through which a digestive fluid known as gastric juice is discharged from the gastric glands (Fig. 31). This digestive fluid is composed of water FIG. 31. Three gastric glands. |