下载后可任意编辑新概念英语第四册Lesson20~22课文及翻译新概念英语第四册Lesson20课文及翻译Snakepoison蛇毒Whatarethetwodifferentwaysinwhichsnakepoisonacts?Howitcameaboutthatsnakesmanufacturedpoisonisamystery.Overtheperiodstheirsaliva,amild,digestivejuicelikeourown,wasconvertedintoapoisonthatdefiesanalysiseventoday.Itwasnotforceduponthembythesurvival下载后可任意编辑competition;theycouldhavecaughtandlivedonpreywithoutusingpoison,justasthethousandsofnon-poisonoussnakesstilldo.Poisontoasnakeismerelyaluxury;itenablesittogetitsfoodwithverylittleeffort,nomoreeffortthanonebite.Andwhyonlysnakes?Cats,forinstance,wouldbegreatlyhelped;norunningfightswithlarge,fierceratsortussleswithgrownrabbits--justabiteandnomoreeffortneeded.Infact,itwouldbeanassistancetoallcarnivoresthoughitwouldbeatwo-edgedweaponwhentheyfoughteachother.But,ofthevertebrates,unpredictableNatureselectedonlysnakes(andonelizard).OnewonderssalivaintowhyNature,withrespectfromthatofothers,asotherontheblood.Intheconversionofsalivaintopoison,onemightsupposethatafixedprocesstookplace.Itdidnot;somesnakesmanufactureapoisondifferentineveryrespectfromthatofothers,asdifferentasarsenicisfromstrychnine,andhavingdifferenteffects.Onepoisonactsonthenerves,theother下载后可任意编辑ontheblood.Themakersofthenervepoisonincludethemambasandthecobrasandtheirvenomiscalledneurotoxic.Vipers(adders)andrattlesnakesmanufacturethebloodpoison,whichisknownashaemolytic.Bothpoisonsareunpleasant,butbyfarthemoreunpleasantisthebloodpoison.Itissaidthatthenervepoisonisthemoreprimitiveofthetwo,thatthebloodpoisonis,sotospeak,anewerproductfromanimprovedformula.Bethatasitmay,thenervepoisondoesitsbusinesswithmanfarmorequicklythanthebloodpoison.This,however,meansnothing.Snakesdidnotacquiretheirpoisonforuseagainstmanbutforuseagainstpreysuchasratsandmice,andtheeffectsontheseofviperinepoisonisalmostimmediate.JOHNCROMPTONThesnakeNewwordsandexpressions生词和短语下载后可任意编辑salivan.唾液digestiveadj.助消化的defyv.使不可能analysisn.分析preyn.被捕食的动物fierce下载后可任意编辑adj.凶狠的tusslen.扭打carnivoren.食肉动物vertebraten.脊椎动物lizardn.蜥蜴concoctv.调制下载后可任意编辑potencyn.效力conversionn.转变arsenicn.砒霜strychninen.马钱子碱mamban.树眼镜蛇cobra下载后可任意编辑n.眼镜蛇venomn.毒液neurotoxicadj.毒害神经的vipern.蝰蛇addern.蝮蛇rattlesnaken.响尾蛇下载后可任意编辑haemolyticadj.溶血性的viperineadj.毒蛇参考译文蛇是怎样产生毒液的，这是一个谜。蛇的唾液原来和我们人的消化液一样柔和，但经过漫长的时间，演变成了今日仍无法分析清楚的毒液。毒液不是生存竞争强加给它们的，它们也可以不用毒液捕捉动物而生存，就像今日成千上万的无毒蛇那样。毒液对毒蛇来说只不过是一种舒适生存的优越手段，它使蛇不用费多大力气就能捕获到食物轻咬一口即可。为什么只有蛇才有毒液呢?譬如说，假如猫有毒液，那对猫会大有帮助，它就不必再和又大又凶的老鼠边跑边博斗了，也不必再和大兔子扭斗了，只要咬一口就不必再费大力气。因此，任何食肉动物有了毒液，都能从中获益。不过，当它们相互撕打时，毒液就成了利弊参半的武，可以杀死对方，也可以被对方的毒液杀死。然而下载后可任意编辑在脊椎动物中，大自然神奇模测地只选择了蛇(还有一种蜥蜴)，人们弄不清楚大自然为什么在某些蛇的身上调制出如此高效的毒液来。人们可能认为，唾液转变成毒液，其中有固定的程序其实没有。有些蛇产生的毒液也在各方面与另外一些毒蛇产生的毒液不同，就像砒霜不同于马钱子碱一样。不同毒蛇产生的毒液产生的效果不同，一种毒液作用于神经，另一种毒液作用于血液。产生神经毒液的蛇有一种非洲...
