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牙齒是由什么組成的？
[ 2007-03-21 12:37 ]
先來回答一個(gè)問題：人身上最硬的部位是什么？是牙齒。正因有有了堅(jiān)硬的牙齒，我們才能吃到各種美味。那么，牙齒是由什么組成的、才會這么堅(jiān)硬呢？且看下文分解。
 Teeth have to be very hard to withstand all the chewing and crunching of food. The hard material of the tooth is composed of calcium, phosphorus, and other mineral salts. The material in the majority of the tooth is calleddentine. The hard, shiny layer that you brush is called theenamel.
Teeth have two basic parts, a root to anchor the tooth to the jaw and a crown above thegumline. The root is covered with a hard material calledcementum. At the center of each tooth is an area with nerves, arteries and veins called thedental pulp.
Humans have four different types of teeth, each with a different function:
Incisorsfor cutting off bites of food.
Cuspids(sometimes called canines because of their long sharp points) for tearing food
Bicuspids(with two points) to tear and crush food
Molarswith large relatively flat surfaces to crush and grind food.
Our four different types of teeth together are called collective dentition and they allow humans to be omnivores (eating both meat and vegetables). Most animals have more specialized teeth. Carnivorous (meat eating) animals have long sharp tearing teeth (like our incisors and cuspids). Grazing animals, like cows and horses have large flat teeth (like our molars) for grinding grass and other vegetation.
dentine: 牙本質(zhì)
enamel: 琺瑯質(zhì)
gum: 牙齦
cementum: 牙骨質(zhì)
dental pulp: 牙髓
incisor: 門牙
cuspid: 犬牙
bicuspid: 臼齒 （with two points）
molar: 臼齒 （with large relatively flat surfaces）
（來源：coolquiz.com 英語點(diǎn)津 Annabel 編輯）
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