牙齒是由什么組成的？
[ 2007-03-21 12:37 ]
先來(lái)回答一個(gè)問(wèn)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(huì)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 called dentine. The
hard, shiny layer that you brush is called the enamel.
Teeth have two basic parts, a root to anchor the tooth to the jaw and a crown
above the gum line. The root is
covered with a hard material called cementum. At the center of each tooth is an area
with nerves, arteries and veins called the dental
pulp.
Humans have four different types of teeth, each with a different function:
Incisors for 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
Molars with 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(mén)牙
cuspid: 犬牙
bicuspid: 臼齒 （with two points）
molar: 臼齒 （with large relatively flat surfaces）
（來(lái)源：coolquiz.com 英語(yǔ)點(diǎn)津 Annabel 編輯） |
