THE DIGESTIVE APPARATUS.
The Mouth is a cavity formed by the lips in front, cheeks at the side, tongue below, and palate above. The roof of the mouth derives its bony part from the upper jaw-bone on each side and the palate bones behind (see p. 60); the bone is covered by the mucous membrane (c, Fig. 101) of the mouth. Reference to Fig. 101 will show that the bone (a) forms only the front por tion of the palate—the hard palate, as it is called, —for the bone stops short at b, and the con tinuation is effected by mucous membrane and muscular substance as the soft palate, of which the uvula (u) is a part. The hard and soft palates form not only the roof of the mouth but also the floor of the cavity of the nose (a, Fig. 101), so that they partition off the nose from the mouth. Now sometimes this partition is not properly developed, and a cleft exists in the soft palate. The cleft may extend forwards some distance and may involve the hard palate, so that the partition is incomplete and an opening more or less wide permits of an unusual means of com munication between the month and nose. The result is a very serious defect of speech, the air being sent up into the nose. The mouth is con tinued behind into the throat, the separation between mouth and throat being marked by a narrowing, called the isthmus of the fauces. This constriction is formed of fleshy pillars—the pillars of the arch up from the sides to form the soft palate or velum ; and from their meeting-place in the middle of the arch there hangs down a portion, termed the uvula(u). At each side, where the pillars begin to arch up, is an almond-shaped body—the tonsil, which ought not to be prominent at all, but which swells and projects towards the middle line in inflamed throat, threatening often to block the way from the mouth into the throat. The tonsil is seen in the figure partly covered by the uvula. The whole mouth is lined with mucous membrane. This membrane is really in its essence the same as the skin which lines all the external parts of the body. On referring to the section on the SKIN it will be seen to be of two layers--a deep one of a fibrous structure, rich in blood-vessels and nerves, and one on the surface of this, consisting of cells only, with no vessels or nerves. In mucous membrane the same two layers are found, only they are more delicate and soft. At all the openings of the body the skin becomes modified into mucous membrane, which takes its place and lines all the cavities and channels of the body which communicate with the exterior. The mucous membrane of the mouth is, therefore, continuous with that of the throat, gullet, stomach, and bowels. It is beset with little glands which
pour out a fluid to moisten the mouth. A view of the throat, tonsils, uvula, &c., in colour, will be found in Plate XV.
The Teeth are embedded in sockets in the upper and lower jaw-bones. Each tooth consists of a crown, the visible part, and one or more fangs buried in the socket. The teeth are thirty-two in all, sixteen in the upper, and the same number in the lower jaw. They differ in form from one another, and have different names according to their use. Thus the four central teeth of each jaw have chisel-shaped crowns with sharp cutting edges, and are called, on this account, incisors; they have but a single fang. On each side of these central four is one tooth with pointed extremity, the tooth de veloped in dogs and other animals for holding and tearing--the canine tooth (Latin, mils, a (log). The upper two are also called eye-teeth. Behind the canine teeth there follow on each side two bicuspid teeth, teeth with two cusps or points instead of one, and having often double fangs ; and succeeding them are the molars or grinders, three on each side, broad teeth with four or five points on each, and with two or three fangs.
The following table shows the teeth in their order:— The upright line indicates the middle line of the jaw, and shows that on each side of each jaw there are eight teeth. These are the per manent set, which succeeds the milk-teeth. At about the sixth year of a child's life the milk teeth begin to fall away and give place to the permanent teeth, which appear in the following order:— The first of the permanent set to appear is thus the first grinding tooth ; and it appears above the gum behind the farthest back of the milk-teeth. These first molar teeth of children decay very fast, and if they are extracted early, say at the age of 9 or 10, before the second molar has appeared, the jaw will become smaller than it ought to be and contracted. Parents should always do what they can to preserve this tooth till the second molar has appeared above the jaw. If, therefore, it is much de cayed early, it should be cleaned and filled rather than extracted. When the second molar appears, or is about to appear, the first may then be removed, for by that time the jaw will have grown, and the second molar with, in time, the wisdom tooth will gradually fill the space. The last to appear are the last grinding teeth, which, owing to their lateness of arrival above the gums, have been called the wisdom teeth. In some people they never appear above the gum at all.