大臼齿






人类的第三大臼齿（智齿）


大臼齿，简称臼齿，俗称磨牙（客語：撞牙），是人类和其它哺乳动物的一种牙齿。大臼齿位于口腔后方，因上端扁平而且主要用来研磨和咀嚼食物而得名。


臼齿由于随使用而被磨损，可以被用来判定生物个体或其遗骸的年龄。




目录






  • 1 人类臼齿


  • 2 哺乳动物的臼齿


    • 2.1 三尖理论




  • 3 参见





人类臼齿




人类的大臼齿（最下面三颗）


人类的乳牙包括8颗乳臼齿，左右上下各2颗，位于乳犬齿之后，从前向后称作第一乳臼齿和第二乳臼齿。恒牙则有12颗大臼齿，左右上下各3颗，位于前臼齿之后。依次称作第一大臼齿（一般在5-7岁萌出）、第二大臼齿（一般在10-14岁萌出）和第三大臼齿(17-25岁萌出，不但时间最晚而且常见经常萌发不全，又称智齿)。



哺乳动物的臼齿


一般来说，食草动物因为要咀嚼大量纤维性食品，臼齿（以及前臼齿）比较发达。食肉动物的臼齿则不发达，如成年猫一共只有4颗臼齿。


哺乳动物的大臼齿按其状态可以分很多类：



  • 高冠齿（hypsodont）: 齿冠的高度大于齿根高度，多见于需要咀嚼坚硬食物的动物，例如马。

  • 低冠齿（brachyodont）齿冠的高度低于齿根高度者。

  • 三尖齿（tribosphenic）: 见于一些食虫动物和鸭嘴兽幼体（成体没有牙齿）。上臼齿有三个尖，下臼齿有两个尖以及位于侧面的第三个尖。

  • 方形齿 （quadrate）: 见于人类等。臼齿上有呈长方形排列的四个尖，也有五个尖的。

  • 丘状齿（bunodont）: 臼齿上的尖圆滑呈丘状，整个臼齿上有珐琅质覆盖。见于一些杂食动物如猪、熊和人类等。

  • 横堤齿（lophodont）: 表面有与颌骨走向垂直的嵴。

  • 半月齿（selenodont）: 表面有半月形的嵴。

  • 斜纹齿（loxodont）:表面有斜走向的嵴。非洲象的拉丁文名字Loxodonta（菱形）即来自此。



三尖理论


1907年，H.F.Osborn以原始有胎盘类（Eutheria）的臼齿为原型，提出了哺乳动物臼齿（特别是
脊齿型的草食动物）的三尖解剖命名方式，目前仍被广泛采用：


上臼齿：位于舌面的原尖（protocone）和位于唇面的前尖（paracone）和后尖（metacone）。原尖和前尖之间的小突起称为前小尖（paraconule），原尖和后尖之间的小突起称为后小尖（metaconule）


下臼齿的三个尖与上臼齿的三尖方位相反，以便咀嚼。下原尖（protoconid）位于唇面，下前尖（paraconid）和下后尖（metaconid）位于舌面。此三尖以外又有延伸的较低部分，称为下跟座（talonid），上有二尖，下次尖（hypoconid，外面），下内尖（endoconid，内面）。



参见



  • 前臼齿

  • 门齿

  • 犬齿




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