CowEyeDissection

IntroductoryDiscussion:Tellthestudentsthatwewillbelearningaboutwhateyesaremadeofandhowtheyworkbydissectingacoweye.Talkaboutwheretheeyecomesfrom,andhowweshouldberespectfulandmindfulwhenitcomestousinganimalsforlearning.Rulesofthedissection:

- Don’ttouchanythingunlessthetutorspecificallyasksyouto.- Don’ttouchanythingwithoutgloves.- Dissectionswillbeperformedonthedissectiontrayontopofpapertowels- Studentsmayusethescissors,butnotthescalpel

Onlinedissection,forkidsabstaining:http://www.pbslearningmedia.org/resource/lsps07.sci.life.stru.coweye/cows-eye-dissection/

Dissection:Thehumaneyeissimilarinstructuretotheeyeofothermammals,suchasacow’s.Acow’seyeislargerthanahuman’s,butithasallthesameparts.Followthedirectionstodissectamammalianeyeandlearnhowyousee.Havestudentsfillouttheworksheetaboutthepartsoftheeyeasyougoalong.

1) Examinetheoutsideoftheeye.Seehowmanypartsoftheeyeyoucanidentify.Lookforthewhites(orsclera(2)),thetough,outercoveringoftheeyeball(youmaynotbeabletoseethisuntilyoucutawaysomefatandmuscle).Youshouldbeabletoidentifythefatandmusclesurroundingtheeye.Youshouldalsobeabletofindthecoveringoverthefrontoftheeye(thecornea(1)).Whenthecowwasalive,thecorneawasclearbutitbecomescloudywhentheeyeispreserved.Youcanlookthroughthecorneaandseetheiris,thecoloredpartoftheeye(youmayhavetogentlysqueezetheeyetoinflatethecorneatobeabletoseetheiris).Atthebackoftheeye,youshouldbeabletoseetheopticnerve(3).Howitworks:Whenyoulookatsomething,youreyeseesthelightthatisbeingreflectedofftheobject.Lighttravelsthoughthecornea.Theimagesyouseearetransferredtoyourbrainthroughtheopticnerve.Ifyoureyeseesthelightthatisbeingreflectedoffanobject,whatdoyouthinkyouseeifthereisnolightbeingreflectedoffanobject?

2) Cutawaythefatandmuscle(10-15mins).Thismaybethemosttimeintensivepartoftheactivity.Allowstudentstocutusingthescissors.Youmaywishtostartattheeyelidflapandpeelawayfromthere.Onceyoustarttocutawaythefatandmuscle,youshouldbeabletobetterseethesclera.Youmaynotgettocuttingawayallthefatandmuscle,butaslongasyouhaveexposedtheoutercurvatureoftheeyesoitcanbecutinhalf(asseenbelow),youshouldbeabletocontinuewiththedissection.

3) Tutors-Usethescalpeltomakeanincisioninthecornea.Cutuntiltheclearliquidunderthecorneaisreleased.Thatclearliquidistheaqueoushumor.It’smostlymadeofwaterandkeepstheshapeofthecornea.Youmayallowthestudentstogentlysqueezeortiptheliquidoutoftheeye.

4) Tutors–Usethescalpeltomakeanincisionthroughthesclerainthemiddleoftheeye.Donotcuttoodeeplyintotheeyeballorsqueezeittootightly,oryoumaydamagetheinterior.Usethescissorstocutaroundthemiddleoftheeye,cuttingitinhalf.Youmayallowthestudentstodothiswiththescissorsoncetheincisionhasbeenmade.Youshouldendupwithtwohalves.

5) RemovethevitreoushumorWhentheeyeiscutinhalf,youwillnoticeaclear,jelly-likesubstancecalledthevitreoushumor.Itmaybeattachedtothefronthalfoftheeye.Allowthestudentstousethewoodenprobeorthescissorstocarefullyremoveallthevitreoushumor.Whatdoyouthinkthefunctionofthevitreoushumoris?Howitworks:Thevitreoushumorisamixtureofproteinandwater.It’sclearsolightcanpassthroughit.Italsohelpstheeyeballmaintainitsshape.

6) Takeoutthelens.Fromthefronthalfoftheeye,studentsshouldbeabletoremovethelens(5)withtheirfingers(youmayhavetocarefullyflipthehalfoftheeyeballinsideouttodothis).Thelensisaboutthesizeandshapeofasquashedmarble.Itshouldfeelsoftontheoutsideandhardinthemiddle.Ifyoulookthoughit,youshouldbeabletoseelightthroughit(tutorsmayhavetousethescalpeltopeelbacktheouterlayersofthelenstobeabletobetterseelightthroughthelens,aspreservationmakesthelenscloudy).Howitworks:Thelens,whichisprotectedbythecornea,focuseslightthatcomesintotheeye.Thelensinyoureyeissimilartotheoneinahandlens.

7) Peelofftheiris.Locatetheiris(6),ablackareathatthelenssitson.Studentscancarefullypullouttheirisandlayitflatonthetray.Itshouldcomeoutinonepiece.Noticetheholeinthemiddleoftheiris.Thisiscalledthepupil(7).

Howitworks:Theirisopensandclosestoallowmoreorlesslightintotheeye.Inoureyes,theirisisthecoloredpartandthepupilisthedarkcircleinthemiddle.Whenitisdark,youririswidensthepupiltoallowmorelightin,makingiteasiertosee.Whentheenvironmentisbright,youririscontractstoallowlesslightin.Ittakesaminuteforthischangetooccurandthatiswhyyoucannotimmediatelyseeclearlywhenyouwalkintoadarkareafromabrightone(ortheotherwayaround).

8) (Optional)Allowstudentstousethescissorstocutthecornea.Cuttingthecorneawithscissors,youmaybeabletohearacrunchingnoise.That’sthesoundofcrunchingthroughlayersoftissue.

Howitworks:Thecow’scorneahasmanylayerstomakeitthickandstrong.Whenthecowisgrazing,bladesofgrassmaypokethecow’seye,butthecorneaprotectstheinnereye.

9) Lookatthebackhalfoftheeyeball.Ontheinsidebackhalfoftheeyeball,youcanseesomebloodvesselsthatarepartofathinfleshyfilm.Thatfilmistheretina(8).Beforeyoucuttheeyeopen,thevitreoushumorpushedagainsttheretinasothatitlayflatonthebackoftheeye.Itmaybeallpushedtogetherinawadnow.Useyourfingertopushtheretinaaroundandremoveallthegoopiness.Howitworks:Thereinaisalayerofcellsthatcoversthebackoftheinsideoftheeye.Lightthatcomesthroughthelensisprojectedontotheretina,similartohowamovieprojectorcreatesanimageonascreen.Theretinaismadeupofnervecellscalledrodsandcones.Thesenervecellssendsignalstothebrainsoyouknowwhatyouareseeing.Rodcellshelpyouseeclackandwhiteimagesandconesareusedforseeingcolorimages.Didyouknowtheimagesthatareprojectedontoyourretinaareactuallyupside-down?Whenthelightcomesthroughthelens,theimagesgetflippedandlandontheretinaupsidedown.Fortunately,ourbrain“corrects”theimage,soeverythingappearsright-sideup.

10) Lookatthetapetum.Afteryouhaveremovedtheretina,youshouldseeshiny,blue-greenstuffunderneath.Thisisthetapetum.Itreflectslightfromthebackoftheeye.Howitworks:Thetapetumreflectslightfromthebackoftheeye.Haveyoueverseenacat’seyesshiningintheheadlightsofacar?Cats,likecows,haveatapetum.Acat’seye

seemstoglowbecausethecat’stapetumisreflectinglight.Ifyoushinealightatacowatnight,thecow’seyeswillshinewithablue-greenlightbecausethelightreflectsfromthetapetum.

11) Findtheblindspot.Useyourfingerstogentlyliftthetapetumupandseethereitisattachedtothebackoftheeyeatjustonespot.That’stheplacewherethenervesfromallthecellsintheretinacometogether.Thisiscalledtheblindspot(9).Howitworks:Theblindspotiswherethenervecellsoftheretinaconvergetoformtheopticnerve.Itisnamedthisbecausetherearenorodsorconesinthisarea(lightsensitivecells).Therefore,anyimagesthatfallintothisareacannotbeseen.Thismeansthereisalwaysonetinyspotinourfieldofvisionthatwedonotactuallysee.Ourbrain,however,compensatesand“fillsin”themissingopticalinput.

12) Pinchtheopticnerve.Lookattheothersideofthebackoftheeyeattheopticnerve(3).Toseetheseparatefibersthatmakeuptheopticnerve,pinchthenervewithapairofscissorsoryourfingers.Ifyousqueezetheopticnerve,youmaygetsomewhitegoop.Thatismyelin,thefattylayerthatsurroundseachfiberofthenerve.Howitworks:Theopticnerveishowtheeyesendssignalstoyourbrain.

Funactivity:Locateyourblindspot

Tofindtheblindspotinyourowneye,holdthediagrambelowinfrontofyou,withyourarmstraight.Closeyourlefteyeandlookdirectlyatthe“X”withyourrighteye.Youshouldbeabletoseeboththe“X”andthesquare.Whilestaringdirectlyatthe“X”,slowlymovethepagetowardsyouropeneye.Assoonasthesquaredisappears,stop.Whathappenedtotheimageofthesquare?

Cleanup:Placethetrayandanyusedglovesintothebiohazardbag.Studentsmaywipetheirhandswiththewipes.Keepthehandlensandanythingelsethatmightbeusefulforafuturelesson(butwipethemoffwithethanolfirst).

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