英文科學(xué)讀本 第五冊·Lesson 44 Teeth of Mammals

Lesson 44 Teeth of Mammals

Speaking generally, the teeth of most mammals are arranged on similar lines to those of man; that is to say, we find, as a rule, the same kinds of teeth—incisors, canines, and molars. Every order of the class, however, is distinguished by the special development of one or other of these, to suit the habits of the individuals. Let us examine them one by one.

In the Carnivora, or flesh-eating mammals, the special development is in the direction of the four canine teeth, which are always remarkable for their size, sharpness, and prominence. The main object of these teeth is to seize and hold the prey and to tear their flesh. There is very little work for the incisors, hence they are always small, like those of the cat. The molars are well developed, but their surfaces are specially furnished with sharp, jagged ridges, which work against each other at every movement of the jaw, like the sharp edges of a pair of scissors. The peculiar development of the teeth in these animals is accompanied by a corresponding modification in the movement of the lower jaw. The flesh-food, on which they live, cannot be ground up as other food can. The molars, instead of doing the grinding work of a mill, become mere cutting or chopping instruments; in consequence, the jaw requires only a simple up-and-down movement.

A remarkable divergence may be seen in the case of some of the so-called Carnivora, such as the bears and the domestic dog, which vary their flesh diet with more or less vegetable food. In all these the sharp cutting ridges of the exclusively flesh-eating animals begin to disappear, and give place to the ordinary, roughened surface of the grinders, while, more remarkable than this, the jaw begins to take the double movement—sideways as well as up and down—which is wanting in the true flesh-eaters.

The greater the mixture of food, the greater the modification, both in the teeth themselves and in the movement of the jaw. In the dogs, for instance, only the two back molars are changed; in most of the bears three or four are affected, the bears being further advanced as vegetarians than the dogs. The teeth of the aquatic flesh-eaters, such as seals and otters, are deserving of notice. Their prey consists of the slimy, slippery inhabitants of the water, and not only are their canine teeth well developed, but all the teeth are furnished with sharp, saw-like edges, to serve the double purpose of holding the victim, and cutting through its flesh.

In the walrus, one of the family, the canines of the upper jaw are very largely developed, and form great tusks, often measuring 2 feet in length, and serving as weapons of attack and defence. It has neither canines nor incisors in the lower jaw.

The Insectivora include the shrews, hedgehogs, and moles, in addition to the bat family. The former find their prey on and in the earth, the latter feed mainly on the insects that people the air, and this explains why they are flying mammals.

In this order the special feature about the teeth is that the molars bristle with sharp points, specially fitted to crush the hard coverings of the insects on which they feed. Only one movement—an up-and-down one—is required for this work. Another remarkable modification is seen here. Certain of the bats live on fruits in preference to insects. In these the molars are true grinders, with the ordinary, flat, roughened surfaces, and no sharp points, while the jaw has the double movement for mill-work.

In the order of Rodents we find the incisors specially developed. There are never more than two in each jaw, but they are long and broad, and have sharp, chisel-shaped, cutting edges, specially fitted for gnawing purposes. These animals live on vegetable food, which they gnaw or nibble with their chisel-shaped incisors. The remarkable provision about these teeth is that they never lose their sharpness—the very work of gnawing tends to sharpen them.

Like all other teeth, they are composed of the two substances, dentine and enamel. The front of the teeth is hard enamel, the rest is the softer material dentine. The gnawing work of the animal wears away both substances, of course; but as the dentine wears faster than the enamel, the edge is always left sharp.

There is one other remarkable fact in connection with these teeth. From the nature of their work they wear away faster than the teeth of any other animal; indeed, they would soon wear away completely unless some provision were made to prevent it. Look at Nature's provision in this case. The incisors of all these gnawing animals are entirely without roots, such as other teeth have, and they grow from below as quickly as they are worn away at their edges.

Just one thought more before leaving this order. There are no canine teeth, but there is a considerable space between the incisors and the molars, where the canine teeth should be. The molars themselves, too, are deserving of notice. Instead of irregular projections, these teeth have a number of parallel ridges running crosswise. The movements of the jaw, moreover, are most remarkable. In addition to the up-and-down and the sideway movements, it has a peculiar movement of its own: backwards and forwards. This helps the molars to do their work by a sort of rasping process.

The Ruminants, or cud-chewers, are distinguished, as regards their teeth, by one characteristic—the want of incisors in the upper jaw, their place being supplied by a pad. One member of the order, however, the camel, is a departure from the rule, as he has incisors in both jaws. The whole order, being exclusively vegetarians, either have very small canine teeth, or are entirely devoid of them. The molars are always large and well developed.

The teeth of the horse present the remarkable peculiarity of a space between the canines and molars. This is known as the bar. The bit of the bridle fits into it. The canines, as might be expected, are very small; the molars are largely developed, and are twenty-four in number, and there are six incisors in each jaw.

In the elephant and wild boar the canine teeth are developed to an enormous size and form tusks, which serve as weapons of attack and defence.