11.7: Teeth
Teeth seize, tear and grind food. They are inserted into sockets in the bone and consist of a crown above the gum and root below. The crown is covered with a layer of enamel , the hardest substance in the body. Below this is the dentine , a softer but tough and shock resistant material. At the centre of the tooth is a space filled with pulp which contains blood vessels and nerves. The tooth is cemented into the socket and in most teeth the tip of the root is quite narrow with a small opening for the blood vessels and nerves (see diagram 11.5).
In teeth that grow continuously, like the incisors of rodents, the opening remains large and these teeth are called open rooted teeth . Mammals have 2 distinct sets of teeth. The first the milk teeth are replaced by the permanent teeth .
Diagram 11.5 - Structure of a tooth
Types Of Teeth
All the teeth of fish and reptiles are similar but mammals usually have four different types of teeth.
The incisors are the chisel-shaped ‘biting off’ teeth at the front of the mouth. In rodents and rabbits the incisors never stop growing (open-rooted teeth). They must be worn or ground down continuously by gnawing. They have hard enamel on one surface only so they wear unevenly and maintain their sharp cutting edge.
The largest incisors in the animal kingdom are found in elephants, for tusks are actually giant incisors. Sloths have no incisors at all, and sheep have no incisors in the upper jaw (see diagram 11.6). Instead there is a horny pad against which the bottom incisors cut.
The canines or ‘wolf-teeth’ are long, cone-shaped teeth situated just behind the incisors. They are particularly well developed in the dog and cat families where they are used to hold, stab and kill the prey (see diagram 11.7).
The tusks of boars and walruses are large canines while rodents and herbivores like sheep have no (or reduced) canines. In these animals the space where the canines would normally be is called the diastema . In rodents like the rat and beaver it allows the debris from gnawing to be expelled easily.
The cheek teeth or premolars and molars crush and grind the food. They are particularly well developed in herbivores where they have complex ridges that form broad grinding surfaces (see diagram 11.6). These are created from alternating bands of hard enamel and softer dentine that wear at different rates.
In carnivores the premolars and molars slice against each other like scissors and are called carnassial teeth see diagram 11.7). They are used for shearing flesh and bone.
Dental Formula
The numbers of the different kinds of teeth can be expressed in a dental formula . This gives the numbers of incisors, canines, premolars and molars in one half of the mouth. The numbers of these four types of teeth in the left or right half of the upper jaw are written above a horizontal line and the four types of teeth in the right or left half of the lower jaw are written below it.
Thus the dental formula for the sheep is:
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- 0.0.3.3
- 3.1.3.3
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It indicates that in the upper right (or left) half of the jaw there are no incisors or canines (i.e. there is a diastema ), three premolars and three molars. In the lower right (or left) half of the jaw are three incisors, one canine, three premolars and three molars (see diagram 11.6).
Diagram 11.6 - A sheep’s skull
The dental formula for a dog is:
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- 3.1.4.2
- 3.1.4.3
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The formula indicates that in the right (or left) half of the upper jaw there are three incisors, one canine, four premolars and two molars. In the right (or left) half of the lower jaw there are three incisors, one canine, four premolars and three molars (see diagram 11.7).
Diagram 11.7 - A dog’s skull