英文科學(xué)讀本 第五冊·Lesson 39 Teeth

Lesson 39 Teeth

Our very earliest lessons led us to pay special attention to the teeth of each animal that passed under our notice, because we were taught to regard the teeth as a sure indication of the food, life, and habits of the individual. The lessons on digestion made the reason for this clearer to us, by showing the important work which falls to the lot of the teeth in the proper mastication of the food.

We are now in a position to study the teeth a little more closely, and as we shall best understand those of the various classes of mammals by referring to our own, we will confine ourselves today to an examination of the teeth of man.

You all know that your own teeth are of different shapes and sizes. This is not an accident, for each kind of tooth was designed for a special purpose.

Let us commence with the long, broad, flat teeth in front of both the upper and the lower jaw. There are four of them in each jaw, and they have sharp, cutting edges. We use these teeth for biting through our food. It is with these teeth that a boy takes a bite out of an apple or a slice of bread and butter.

In our earlier lessons we were content to call these the chisel-teeth, because their sharp, cutting edges bear some kind of resemblance to a carpenter's chisel. We must now learn to recognise them by their scientific name— incisors—a word which means simply cutting-teeth.

Next to the incisors, on either side, and in both jaws, is a long, rounded, conical tooth. There are thus four of them altogether. Some of you may keep a dog at home. If so, look in his mouth, and you will find that he has four teeth like these, but they are very large, sharp, and prominent. We speak of these four conical teeth of ours as canines, or dog-teeth, because they are like those of the dog. Canis is the Latin name for dog.

Behind the canines, in each jaw, and on either side, are a number of large, square, broad-crowned teeth, which differ from both the incisors and the canines. The working surfaces of their crowns are rough and uneven, resembling in this respect the roughened millstones of a mill. Indeed, they form the mill for grinding the food; we often speak of them as the grinders. Even their scientific name—molars—points to the same thing, for the word molar comes from a Latin word signifying a mill.

The full number of molars for an adult is twenty—that is, there are five on each side of both jaws. But boys and girls have not so many. Indeed, there was a time when we had no teeth at all; we required none, for our food consisted entirely of milk, although we ourselves cannot recollect that time.

Teeth begin to make their appearance when the infant is about six months old, and the cutting of this first set is usually completed by the end of the second year. By this time the child's digestive organs have grown strong enough to enable it to give up its milk diet for more solid food, which the teeth are now ready to masticate.

It is easy from this to see why these are called the milk-teeth. There are twenty in the complete set, viz. four incisors, two canines, and four molars in each jaw—that is to say, two incisors, one canine, and two molars in either half of the jaw.

These milk-teeth fall out when the child is about seven years of age, to make room for the new teeth. There are more than twenty teeth in the permanent set, as they are called.

The complete permanent set contains thirty-two; but the last of the molars do not make their appearance till adult age.

It will assist us in the proper understanding of what is to follow, if we now examine briefly the structure of a tooth. The bulk of the substance of all teeth is a hard, bony matter, called dentine, but the whole of that part of the tooth which appears above the gum is coated with an exceedingly hard substance, known as enamel.

The tooth is lodged in the gum by one or more roots or fangs, and the blood-vessels for nourishing it, as well as certain nerves, enter the tooth at the extremity of the fang. There are no nerves or blood-vessels in the hard enamel coating on the outside.

Good teeth are such an important factor, not only as regards our personal appearance, but also in our general health and well-being, that it behoves us to take every care in preserving them. It should not be forgotten that, hard as this enamel is, it is extremely brittle. It will not stand too much rough usage, and when once it is cracked or broken, it lays bare the softer dentine underneath, which soon begins to decay, and all the pleasure of life is destroyed by the racking pains of toothache. Very hot, as well as very cold liquids should be avoided, as both of them have a tendency to crack the enamel. Regular use of the tooth-brush night and morning, careful avoidance of nut-cracking, too many sweets, and also of that silly practice of picking the teeth with pins and needles, will do much towards keeping them sound and healthy.