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Thedomesticpig,JuJscrofa,is a memberof the cla ssM a m malia- the mammals.A ll ma mma ls havetwo characteristics that setthem apartfrom otheranimals- hair and marnmary glands.Modern mammalsarealsodistinguished fromotherver tebratesby the presenceor absenceof scveraladdi liun,rl.lru, lur.-. On eJ n ,rde u, the lohar I.r\^. mammalshavea singledcntarybone,themandible, which articulates with the squamosal bone.Thcre arethreebonesin the middleearcavity themalleus (hammer),the incus (anvil),and the stapes (stirrup).Betweenthe thoraciccavityand thc abdominalcavitythereis a musculardiaphragm that aids in respiration.Most mammalshave sweat glands.There is no cloacapresentin thc adult, (theplatypus exceptin themonotremes hasa cloaca and layseggs).All mammals,exceptthe toothed whales,have heterodontdentition;that is, they havetwo or moretype-rofteeth,for example,rnolars andcanines. Thereareusuallytwo-sets ofteeth,a set of milk teethand a set of permanentteeth.The mammalianbone marrow producesenuc/eatec/ (withouta nucleus), round,red bloodcells(camels and llamasdo not have round RBCS). The right, fourthaorticarch of mammalsis much reuucecr; what little remainsis part of the rightsubctavtan artery.NearlyaJlmammalshavea speciatsounocollectin8 structure knownasthepinnaor outerear. Thelarynxishighlyspecialized. Perhaps mostnorable of all, at leastfrom a humanviewpoint,is the enlarSement of the cerebralcortex,allowingthe developmentof what we looselyterm ,,intelli 8ence. Pigsare membersof the order Artiodactyla,the eventoedungulates, whichalsoincludes cowsand deer.Pigsareomnivores theyeatboth plantsand animals.Thedomestic pigcanlive15 20years,but mostbecomeporkchops,bacon,andsausage long beforethat. Adultsmay reachweightsof up to 400 k8 (approximately 900 lbs.). Thelengthoftheestrous cyclein pigsis21days.lf

conception period,or ttmeoi occurs,the gestation development of a pig fetuswithin the uterus,is between112 115 days-At least40 percentof the embryos diebeforebirthandareusuallyreabsorbed by the body.Sowsnormallygivc birthro 7 '12oif sprinBper lifter. PigsareusefulsLrbjects for anatomyandphysiolo gy experiments because of theirsimilarities to manMany drugsusedin modernrnedicinehavebcen testedon pigs.Likewise,manysLfgicaltechniques were perfectedon pigs beforebeing performedon humans.In some instances, piB body partshave beenusedin humans.Thevalvesusedin surgical neart vaive replacement were, at one time, pig valves.Pigskinwasoftenusedasa temporary skin graftforhumanburnvictims.Thepig isanexceitent exampleof a typicalmammaland exhibitsmany similarities to humananatomy.

Orientation Betoreyou beginthe actualdissection process, you needto becomefamiliarwjth somecommonly usedanatomical terms(Fjg.1). Anteriorlctanial)

toward the head.

Posteriot(caudal)

toward the tail.

Dorsal(supetior) toward the back. Ventral(infetior)

toward the lower surfaceor belly.

[atera/- toward the side. Medla/- towardthe midline. Proximal-

nearortoward a pointof reference,e.g. the midlineof the body.

Dista/- ah/ayfrom a pointof reference, e.g. the m id linep/aneTransverse

perpendiculaf to the lorlgaxis.

sagitta/p/ane thatdividesthebody - a lonBsection intoriBhtand lelt portions. A midsa8ittal sectionhasequalrightand left halves. F r o n ta/p/an c-alongsectionthatdivide s t h e b o d y into dorsaland ventralportions.

Questions 1. Whataresomecharacteristics sharedby all ma mma ls ? 2. How are domestif'trlgsrelatedto tBiwild

;5***-ff*$; How long in humans? 5. Which is moredistalto the elbowwristor the hoof?

Epidermis Gland ducl

Sebaceousgland

pllimuscle

Taclile nerueending

Bloodvessel

HairbuLb nerveendng Papilla Press!re-sensilive Fi g ur e2. Diagr am o f rh e ,k i n .

Sweal gland

the

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Takesometime to studycarefullythe exteriorof thepig.(Toproperlylearnaboutyourpig,you need to examinethe exterioras well as the interior-) How old is yourpi8?To findout, measure it from thetip of itsnoseto the baseof itstail. (String works betterthan a ruler sinceit bendsand followsthe curvesof thebody.Measurethestringwith a metric ruler.)Usethechartprovidedbelowto determine its age.

length of specimen

f .1

Aq?ro\imat' d8( in days from fertilization

1 cm (% inch)

20 days

2 cm (7ainch)

35 40 days

3 cm (17ainches)

,19 50 days

4 cm (l 7, inches) 20 cm (77ainches)

75 days

25 cm (9% inches)

100 days

30 cm (11% inches)

'l12 1'15days(fullterm)

Perhapsthe most often overlookedorgan of the bodyisthe mostobviousone. In fact,if you lookat yourpig,youarelookingrightat it. lfyoulookin the mirror,you'll seethe sameor8an.lt's the largest organof the humanbodyas well. lt is the skin. job- lt isa barier Theskinperforms an enormous to ultravioletradiation,keepsout harmfulsubstances and disease-producinB organisms, helpsto preventthe loss of water from the body, aids in regulatingthe body temperature,is an excretory organ(sweat,etc.), servesasa receptorfor external stimulations throughthe many nerveendingsand sensorystructuresembeddedthere, providesstora8efor reserve nutrients, andisthesiteof vitaminD production. Theskinis separated intotwo layers

an outer layerknown asthe epidelmisand a deeper layerknownasthedermi5(Fig.2).Alsofoundin the skinare hair{ollicles,severalkindso{ glands,varrou\ \en.e organ\.blood r"..e1.. and nerte'. Thehoovesofthe piB(andthenailsof yourhands In huandfeet)aremodifications of the epidermis. mans,the whitishcrescent-shaped areaat the base of the nail is the activegrowingportion. The pi8 can be divided into four sections head,neck,trunk,andtail - eachofwhich hasits (Fig.3). own externalfeatures

The Head Firstlocatethesnout.Thisstructure endsin a flat, platelike tip containingcartilage.The pairedexternalnares(nostrils) locatedon theendof thesnout serveasthepassa8eway for air intothe nasalcavity, whereit is warmed,filtered,and moistened. You can easilyfind the mouthwith its f/eshy/ips.The upper lip is thick-rimmedwhile the lower lip is smallerandmorepointed-Lookintothe mouthand findthe tongueand possibly, on the age depending your of specimen, teeth.Theroolof the mouth,the haldpalate, consists of a thin layeroftissuecovering the a baseof bone and cartilagethat separates mouthcavityfromthe nasalcavity.Thisallowsfood to be chewedwhile breathing continues.Examine the eyesand the upperand lowereyelids.In addition,thereisa thirdeyelid,thenictitatingmembrane that coversthe anteriorportion of the eye and helps to cleanit. You needto makea smallincisionand opcn thp othpr l\^o iid. to spe this. The inri.ior shouldbe forwardfrom the inner cornerof the eye. membrane is Only a smallvestigeof the nictitating presentin the innercornerof the humaneye. The externa/ears,ot pinnae, sefte to channel soundwavesinto the smallopeningknownas the externalauditorymeatus.A shortpassage, the audi tory cara/, leadsto the eardrumot tympanrcmem-

U rnbi l i cal cod

Forelegs

Hind egs

Figure3. External anatomyof the pig.

brane,fromwhich soundvibrations aretransmitted to specialized nerveendingsin the cochlea. The Neck Theneckof the pigisshortandthick.No external featuresare noticeable. The Trunk Thetrunkof mammalsis dividedintothe thorax and the abdomen.The thorax is the regionthat includesthe ribs-Belowthe neck,a singlepair of appendages,the forelegs, are connected at the shoulders. Notethatthepighasfourtoes,andthatit walkson the middletwo. On the foreleS,the foot bendsposteriorly at the w/ist.Moving proximally you alongthe limb, seethatit bendsanteriorly at the

On theabdomenaretwotowsolmammatypapil /ae, or teats.How many pairsare there?Thereare normallyfive to sevenpairsof teats.Both maleand femalehavetheseteats, buttheybecomeiunctional only in the female. Ihe umbilical cord provided the attachmentto the p/acenta.Theplacentaisa feta/orSanthatserves as an area of interactionbetweenthe circulatory systems of motherandfetus.Cutthe umbilicalcord 2 about cm (% inch)fromthe abdomen.Closeexof three openings aminationof the cut cordreveals bloodvessels._ one umbilicalvein andrwo umbil conTheyareembeddedin gelatinous icalartefies. rich The vein carries blood nectivetissue. umbilical in food and oxyBenfrom the placentato the dev e lL rp in le g t u . . lh e l\ ^ o u mb ili(d l d rle r i e .( d r r v waste ladenblood to the placenta,wherethewastes passacrossintothe mother'sblood;normallythere is no mixingof blood betlveenthe motherand fetus.

(Thelatexin your pi8'scirculatorysystemwas rn jectedinto the umbilicalarteriesand vein.) These Behindtheumbilicalcordarethehitd /-"9s. are very similarto the forelegs. The /eetbendanteriorlyat the ank/eandthe legbendsposteriorly at the kree. Beneath theta//istheanus,thecaudalopeningof the diSestive tract. lf you'vewonderedwhat sexyour pig is, now's lh e l i m * lu lrnd uul rl-i8.4 . In mdl... lh e rei. . rn openingimmediately caudalto the umbilicalcord. Thisisthe uroge,ita/ crpen in8and istheopeningior the penis.Also in males,you shouldfind the scro the tum,a sacof skinbcneaththeanusandbetu,een the hindle8s.In maturemalcs,thescrotum contains testes.lf you have a young pig, the testesrfay not have fully descended from the abdominalcavity into the scrotum. Femalesalso have a urogenita/opcningthat servesboth the reproductive and urinaryorgans. The/abraaresmallfoldson eithersideoi thc open

ing.Thesecometogether bcneath theanusto forrna genita/papiila. projection, Bethe smallspike-like neaththe papillais the c/itoris,an erectileorgan homoluguu\lo lhe mdlepeni.. B..Lrrero F\Jmine pigsof both sexes.

lJrcgenilalopening

Genilal papilla

genitalia FiSure 4. Viewof external of maleandfemale.

F

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Lungs

Oiaphragm

Slomach

Pancrca6

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To view the internalanatomy,it is necessary to openyourpig(Fig.15).Layyourpigon itsbackand tie the legswith strinSasyou did for skinninS. Take your scalpeland makean incisionalon8the mid ventralline of the body, startingat the neckand workingtowardthe tail. As you cut throughthe musclesofthe neck,use lightpressure. Be carefulnot to plungethe scalpel you needto intothe pigor you couldcut structures youshoulduse seelater.In fact,wheneverpossible upwardpressure whencufting,by tLrrning theblade over.Carefullycut throughthe muscles of the neck untilyou seea whitishtubethesizeof a pencil.This is the tracheaor windpipe.The thymus,an endo crineglandinvolvedin theimmuneresponse during infancy,lieson eithersideof the tracheaand ex tendsovertheheart.Youwill probablyseea brown, ovalstructure lyinSon top of thetrachea. Thisisthe thyroid.Removethe thymusand thethyroidat this ttme. Now lenSthen the incisionto includethethoracic cavity.You immediately The encounterresistance. structures causingthe problemarethe ribsandsternum. As this is a feta/pig, its ribsand sternumare stillcomposed ofcartilage. Cartilage canbecutby a sharpscalpelor a Boodpair ol di.\e(tinB\( i\\or\.

Useeitherof theseto cut throuShthe rib cageand exposetheorgansin thechestcavity.Thefirstorgan you seeis the heart,enclosed by a whitish,sac-like membrane,the pericadium. You can also seethe lungs. Ftee Ihe diaphragm,a bell-shapedmusclethat separates the thoracicand the abdominalcavities, by cuttingarounditsedgenearthe bodywall. This will keepit intactfor furtherstudy.Makea shallow incisionall the way to the umbilicalcord, then aroundthe cord (makingthe "U"-shapedcut for malesand cuftin8a circlefor females) to the pubic region(theareaimmediatelybetweenthe legs)and the anus.Cutabout3mm (7s")deep untilyou reach the pubis.As you are makin8this cut, use liSht pressure. The layerof muscleoverthe abdomenis very thin and you can easilycut into the internal organs.You havecut deeplyenouBhwhenyoLlsee peritoneum. the white, shiny semi-transparenl Whenyou reachthe pubicregion,makeyour incision slightlyoftcenter and deeper.The muscles overlyingthe pubic regionare roughlythe same thickness asthoseoftheneckregion,soyou haveto cut mLrch deeperthanin the abdominalcavity.You alsoneedto cut throughthe pubisto exposestruc turesthat lie beneathit.

26

m Unit 5'

F

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f

sPrRAroRt Epht

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Rlghtapicatlobe

Lett antgdortobe

Lett diaphragmattctobe

Rightdiaphragmatic lobe :ieur.

Intormediate tobe

t

Bronchiole

Capitaries Figure 17. Exploded viewof respiratory sysiem.

To studythe respiratorysystem,beginin the neck with the /arynx.Thisis the hard,enlarged regionof the trachea.Insidethe larynxarefibrouscordsthai vibrate when air passesover them, producing sounds.The tracheais the tube that connectsthe lun8swith the nasopharynx. lt is re'nforcedwith rinSsof cartilage thatkeepit fromcollapsing and is linedwith ciliatedcellsthattrapmucusandforeign particles and passthembackto the oralcavity.just dorsalto thetrachealiesa secondtube,theesophagus, a part of the digestivesystem. Followthe tracheato the /urgs,whereit branches intotwo tubes(Fig.16).Eachoftheseis knownasa bronchus(pl.: bronchi).lnsidethe lungs,thebronchial tubes further divide into seconclaryhrcrnchi, 2B

then into bronchio/es, and finally into very fine give tubesthat riseto microscopic airsacsknownas alveoli\FiB.17).Caseousexchange takesplacein the alveoli.ThelunSsarea pairof highlyvascularized orBans.Each lung is divided into sections known as /obestthereare four on the right sideand threeon the left. The lungsappearto "cradle"the heart.Ljftthe hear-t and you will seea junctionwherethe lungs and bloodvessels seemto merge.Thisiswherethe pulmonary ve/ri and arterier enter and leave the lungs. Thediaphragmis a bell,shapedmusclethat separatestheabdominalcavityfromthethoraciccavity. In the fetal pig, it is a sheetof muscleattached

arounditsperimeter to thebodywall.Thelungsand heartare anteriorto it, and the stomach,liver, and other abdominalvisceraare posteriorto it. Thediaphragmis responsiblefor the physicalmovements that we term breathing.When the diaphragrrrcontracts,it movesdownward,increasingthe sizeof the thoraciccavity.When it relaxes,it movesbackto its originalpositionand the thoraciccavitydecreases in size.Thisincrease anddecrease in thesizeof the thoraciccavityactsasa bellows,pullingairin when thecavityincreases in sizeandforcingairout when it decreases.

tracheacolla

S,{+eexhalingif ftere cartilagein itc \afis? 2. How does he blood?

3. Whymustthe lu blood vesselsi 4. What

5. Why is a

29

Lefi medial lobe

Righl medial lobe

Gallbladder Lell laleral lobe

Righl laleral

Figure19. View of the liver and gall bladderremovedfrom the body cavity.

bloodsupplyto the liveris a system of veinsknown po{a/ portal asthe circulation, blood system.In the from the digestiveorgans,the stomach,and both largeandsmallintestines the enters theliverthrouBh polta/vein;thiscanbe identified ifthe liveris lifted. The portalvein branchesrepeatedly into smaller andsmallervessels thatfinallyfeedintotinyvessels c a l l ed ca p illa r ies.The capillariescome b a c k togetherto form hepatic veins that then join the postcava/vern,which returnsthe bloodto the heart. This systemof veins-capillaries-veins enablesthe cellsof the liverto performmanyo{ theirfunctions. Locatedin a depressionon the dorsalsudaceof the rightmediallobe is the ga//b/adder,a smali, saclikestructure. Thegallbladder,which appears as a greensac in the adult, servesasa storageorgan juicethat is producedby the for bile, the digestive liverandthatemulsifies fat.Emer8in8 fromoneend 3)

of thegallbladderisthecysticduct,whichserves as joins passageway the for bile. The cysticduct the hepaticduct as it emergesfrom the liver. Together theyformthecommonbrleduct,whichcarriesbile to the smallintestine. Theliverhasotherfunctionsbesides digestion. lt filtersout and breaksdown the toxins ingestedor produced durin8proteinmetabolism andproduces, stores, and releases a carbohydrate used BlycoSen, by muscles asa sourceof energy.Theliverisalsoa majorstorage Removethe liverby sitefor nutrients. cuttinBthepostcaval vein,theportalvein,thecommonbilederct, andtheconnective tissueholdinBthe liver.Be carefulnot to cut throughthe esophaBus leadingto the stomach. When you removethe liver, you will seethe stomach(Fi8.20) lyinSin the upperleftsideof the abdomen.Clandsin the lininBo{ the stomachpro-

Diaphragm

Descending aorta

Stomach

S m allinles lin e

inlestine

Umbilicalvein

lJmbilical arleries Figure20. Abdominalorgan5with liverremoved.

d u ce a n d secretedigestivejuices, p rin c ip a lly hydrochloric acidandpepsin,whichareinvolvedin breakingdown food. Foodfirstentersthestomachin the cardiacregion(Fig.21).At the junctionof the stomachand theesophagus is a ringof muscle,the cardiac sphincter.This contractsto preventfood f rompassing Thestomach is backuptheesophagus. a large,bean-shaped, muscular, The sac-like orSanwall of the stomachis composedof a liningthat juices,suroundedby three produces the digestive layersof muscle.Eachlayeris orientedin a different direction:the outer layerrunslongitudinally, the middlelayeris circular,and the innerlayeris 33

oblique.On the dorsalsurfaceof the stomachyou ancl mayseea pocket.Thisis calleda diverticulum iscommonin pigsbut rarein humans.Thestomach of yourpig mayseemto be full of food-lf you open the stomach,you'll find that it is full of fluid and g re e n d e b ris . F e t a l ma mma ls a re c o n s t a n t l y swallowin8 amniotic//uid.This,alongwith mucLrs andcellsfromtheskinanddigestive tract,iswhatis in your pig'sstomach.Thecombinedmaterials are calledmeconium. At the lower end of the stomach,the pyloric end, is anothersphincter,the pyloric sph/ncter.This sphincter servesas the junctionbetweenthe stom

Partof diaphragm

Cardiacregion

Esophagus

Cardiacsphincler

py oricregion Figwe 21. Ventralview ol ihe stomachremovedfrorn the body .avity

ach and the smallintestine and allowsfoodto pass throuBhfor furtherdigestion. Next to the stomachare the pancreasand the sp/een(FiB.20).Thespleenisa brown,oblong,firm organfoundalongthe greatercurvatureofthe stom and isnota ach.lt is a partof the lymphaticsystem, organis digestiveorgan.This hiBhlyvascularized of the designed to filterthe blood.Theoutersurface tissue spleenis coveredby a capsuleof connective and smoothmuscle.The interiorof the spleenis tissue, filledwith a complexnetworkof connective nerves,bloodvessels, and the highlyvascularized splenicpulp.Afterbirth,the spleenfunctionsin the production of whitebloodcellsin removing old red production of antibodies. In bloodcells,and in the asthemajorsrteoi red thefetus,thespleenf unctions blood cell production.Removethe stomachand l4

spleenby cuttingtheesophagus abovethe stomach justbelowthepyloric andcuttinSthesmallintestine sphincter. Thepancreasfitsintothe"c shaped"curveoithe whereit joinsthe stomach(FiB.20). smallintestine granular Thepancreas isanelongated, structure that lighter in than either the intestines or the is color stomach.Liftthe mainportionof thesmallintestine closely.Thepancreas in orderto viewthe pancreas is a dualjunctionorgan.lt is both an endocrine orgdndndJn c\u' rineor8Jn.A. dr c\o, rrncorgdn it is concernedwith the secretionof pancreatic juice. This juice containsa nLrmberof enzymes into essentialforthe digestion offoodandisreleased duct.As the smallintestine throughthe pancreatic three an endocrineorgan,the pancreasprodLrces hormones thatregulate bloodsugarlevels./t5u/inis

essentialfor normal carbohydratemetabolism. Whenthe levelof sugarin the bloodrises,insulinis secretedinto the blood stream,resultingIn an Increasein the uptakeof bloodsugarby musclecells anda decrease in thelevelofbloodsu8ar.C/ucagot hasthe oppositeeffect.When there is a decline in the levelof bloodsugar,8lucagonissecreted, causinBan immediate but short-lived increase in blood sugar.Insulinand Slucagontogetherregulatethe levelof blood su8ar.The third hormone,somato-

stalin,inhibitsthe releaseof both glucagonand in s u lin . Thesma//intestineis a slendertube that emer8es from the pyloricend of the stomach(Fig.20). lt continues as a long,coiledtubeuntil it reaches the largeintestine. In the fetalpi8, the smallintestine may be bet\,veenone and tlvo meters(threeto six feet)long.Therearethreedivisions the duodenum,thelelunum,andthe i/eum.Theduodenumis the portionof the smallintestine that is adjacentto Smallinlesline{ileum)

Spkal colon

colon Descending

Figure22. Diagramof the regionsof the large intestine. l5

Longitudina muscle

Figure23. Scction oi the smallintcstine showing the circular io ds andvilli. the stomach.lt is the shortest portionof the smalJ intestine.Followingthe duodenumis the longest portionof the small intestine,the jejunLrm.The terminalportionof the srnallintestine is the ileum, (Fig.22)-Materialis whichjoinsthe largeiniestine prevented from passingprematurely into the large intestine by a sphincter knownasthei/eocecal va/ve at thejunctionof the ileumand the largerntestrne. Takea momentto cutopena sectionof thesmall intestine and lookinside.Noticethatit hasa velvety appearance. Thisisdueto manythousands of circularfolds,and the hundreds of thousands of vil/i on the folds(Fig.23). Thesestructures providea very largesurfaceareafor absorptionof digestedlood, w h ichor r u r ' L lrl) in lhp -mdllInte.tine. Mostdigestion alsooccursin the smailIntesttne. The stomachprovidesfor mixingof gastricjuices juicesandfood.Fats,proteins, withthesalivary and starchesare brokendown predominately by the digestive and the iuicesof the liver,the pancreas, smallintestine. lust beyondthe junctionof the small intestine with the /afgeirtestineis the cecum(Fi8.22)- ln herbivorous mammals,the cecumoftenpiaysan

36

important rolein digestion, but in the pig thisis not the case. In man the cecum tcrminatesIn rne appendixRemovethe small intestineby cuttin€iit at its junciionwith the largeintestine abovethe cecum, c a re f u llyc le a rin g a wa y t h e c o n n e c t i v et i s s u e (mesentery) holdingthe intestinein place,and lifting the smallintestine and pancreas as a unit from the abdomen.The largeintestinewill be clearly v is ib le . Thelargeintestine isdividedintotheipira/coror, the ascendirigco/on,the descendingco/on,the rectum, and the anuJ.The spiralcolon is the longest portionof the largeintestine andresernbles a coiled sprinS.lt is foundon the leftsideof the abdomen. Thenextportionis the ascending colon,foundjust to the rightof the midlineof the abdomen.As the namesuggests, theascendinS colonrunsanteriorly. posteriorly beneath the stomach h turns and lust becomes the descending colon,whichconnects to the rectumin the mostcaudalregionof the abdo men.Therectumis a dark,straight tubethatopens to theoutsidethroughtheanus.Thelargeintesttne ts p rima rily c o n c e rn e dwit h c o n s o lid a ti n gw a s t e md le rid l. I nd o in B . o .rl re mu e \ . mo . lo f lher . m d r n in8water.A IarBevarietyof bacteria arefoundIn the larSFinte