Thewholebodyiscoveredwithskin.Thesurfaceoftheskinisdead(likethehairandthenails),butitismadefromalivinglayer(层)justunderthesurface.Theoutsidedeadlayerswearoff(磨损)allthetime,withnewonesreplacingthem,andthebodyformsatotallynewskineverythreeweeks. Theskinprotectsthebodyfromtheoutsideworldanditpassesinformationfromtheoutside.Ithelpstokeepwaterinthebodyandtokeepthebodyatthesametemperature.Becteria(细菌)can’tgetinthroughtheskinunlessitisdamaged. Iftheweatheriscold,thebloodvessels(血管)intheskinsqueezedown,orcontract(收缩),sothataslittlebloodaspossiblecomesnearthesurface.Thismeansthatlesswarmthislosttotheoutside.Thismakestheskinlookpale,oreven“blue”.Atthesametime,thehairsontheskinarepulleduprightbytinymuscles. Whenitishot,thebloodvesselsexpand(扩大),orgetwideragain,toallowplentyofbloodtorunnearthesurfaceoftheskin.Thishelpsthebodytoloseheat,andtheskinmaylookred.Atthesametime,thesweatglands(汗腺)producemoresweat,whichcoolstheskin. Ifyourskinhaseverbeennumb(麻木的),youwillrealizehowimportantthefeelingfromtheskinis.Nerves(神经)intheskinaresensitive(敏感的)toheatandcold,sharpness,hardnessandsoftness,wetnessanddrynessandpain.Ifoneisnotabletofeelthesethings,whichcanactaswarnings,heislikelytobeburned,cutorhurtinotherways. Thecolouroftheskindependsonspecialcolourcells(细胞).Thesearejustbelowthetoplayersoftheskin.Theyareneededasprotectionfromsomeharmfulrayofthesun.Thestrongerthesunis,themoreimportantitistohaveadarkskin.Peoplewithpaleskinsmay“tan”(晒黑)inthesun.Thishappenswhenthebodyproducesextracolourcellsintheskinforprotection.Inthesun,theskinproducesavitamin(vitaminD)whichisneededforstrongbones. 小题1:WhichofthefollowingstatementsisNOTtrue? |