When you are approaching what is called the age of reason, (and this word implies a great deal, my dear child,) the real teeth, the teeth which are to serve you for life, begin to whisper among themselves, "Now, here is a little girl who is becoming reasonable, and who will soon, or else never, be fit to take charge of her teeth." No sooner said than done: other masons set to work in other cells, placed under the first set, and as the permanent teeth keep growing and growing, they gradually push out the milk-teeth, which were only keeping their places ready for them till they came.
This is just your case at present, and you now understand your responsibility, and how necessary it is to preserve those good teeth which have placed so generous a confidence in your care of them, and which, once gone, can never be replaced.
You have no loss by the exchange; you had twenty-four at first, you will now have twenty-eight. Twenty-eight, did I say? nay, you will have thirty-two; but the last four will come later still. The last molars on each side, above and below, in both jaws, will not make their appearance till you are grown up. They are a fastidious and timid set, and will not run any risks; and they are called wisdom-teeth, because they do not appear till we are supposed to have arrived at years of discretion. Some people do not cut them before they are thirty, and you will agree that, if they have not become wise by that time, they have but a very poor chance of ever being so!
There is much more still to be said about the teeth; but I think I have told you quite enough to teach you the importance of these little bony possessions of yours, which children do not always value as they deserve, and whose safety they endanger as carelessly as if they had fresh supplies of them ready in their pockets. If so many skilful contrivances have been devised for enabling us to masticate our food properly, it is clear that this process is not an unimportant one. Those, therefore, who swallow a mouthful after two or three turns, forget that they are thereby forcing the stomach to do the work the teeth have neglected to do, and this is very bad economy, I can assure you. You will see hereafter, when we speak about animals, that by a marvellous compensation of nature, the power of the stomach is always great in proportion to the _in_efficiency of the teeth, and that by the same rule, it is weakest when the jaws are best furnished. Now, no jaw is more completely furnished than the human one; it is clear, then, that it should do its own work and not leave it to be done by those who are less able: and the little girl who, in order to finish her dinner more quickly, shirks the use of her teeth, and sends food, half chewed, into her stomach, is like a man who, having two servants, the one strong and vigorous, the other feeble and delicate, allows the first to dawdle at his ease, and puts all the hard work on the other. He would be very unjust in so doing, would he not? And as injustice always meets with its reward, his work is sure to be badly done.
Now, the work in question consists in reducing what we eat into a sort of pulp or liquid paste, from which the blood extracts at last whatever it requires. But the teeth may bite and tear the materials as they please, they can make nothing of them but a powder, which would never turn into a pulp, if during their labors they were not assisted by an indispensable auxiliary. To make pap for infants what do we add to the bread after it is cut in little bits? Without being a very clever cook, you will know that it is water which is wanted. And thus, to assist us in making pap for the blood, Providence has furnished us with a number of small spongy organs within the mouth, which are always filled with water. These are called salivary glands. This water oozes out from them of itself, on the least movement of the jaw, which presses upon the sponges as it goes up and down. The name of this water, as I need scarcely tell you, is saliva.
When I call it water, it is not merely from its resemblance; saliva is really pure water with a little albumen added. Do not be afraid of that word—it is not so alarming as it appears to be. It means simply the substance you know as the white of egg. There is also a little soda in the water, which you know is one of the ingredients of which soap is made. And this explains why the saliva becomes frothy, when the cheeks and tongue set it in motion in the mouth while we are talking; just as the whites of egg, or soapy water, become frothy when whipped up or beaten in a basin.
But the albumen and the soda have not been added to the saliva, in our case, merely to make it frothy; that would have been of very little use. They give to the water a greater power to dissolve the food into paste, and thus to begin that series of transformations by which it gradually becomes the fine red blood which shows itself in little drops at the tip of your finger when you have been using your needle awkwardly.
When once minced up by the teeth and moistened by the saliva, the food is reduced to a state of pulp, and having nothing further to do in the mouth, is ready to pass forward. But getting out of the mouth on its journey downwards is not so simple an affair as getting into it by the front door, as it did at first. Swallowing is in fact a complicated action, and not to be explained in half a dozen words, and I think we have already chatted enough for to-day. I only wish I may not have tired you out with these interminable teeth! But you may expect something quite new when I begin again.