MUSCLES OF THE AVIAN HIP AND THIGH

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HOWEMLLu,scoleftsheArianHipandThigh

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MUSCLES OF THE AVIAN HIP AND THIGH

BY A. BRAZIER HOWELL

AN increaseof interestin the myologyof birdsis apparent. The more recentpapersonthesubjecth, oweverc, oncernthemselvewsith a comparisonof the musclesof differentavian groups,and pay but slight attention to homologics.Therehavebeenfew contributionstowardan understandingof the latter, and mostauthorsreferto Gadow'sworkof forty-sixyears ago (in Bronn's'Klassenund OrdnungendesTier-Reichs,'1891). Exceptionalin thisrespecthavebeentwo papersof Romer,one(Jour.Morph., 43: 347-385, 1927)on the developmenot f the musclegroupsof the pelvic limb of the chick, and another (Bull. Amer. Mus. Nat. Hist., 48: 533-552, 1923)that considertshe musclesof birdsratherincidentallywith thoseof (chiefly)Alligator. Hudson'spaper('Studieson the musclesof the pelvic appendagein birds,'Amer. Midland Nat., 18: 1-108, 1937) is a notable contributionto our knowledgeof interordinalvariationsof the musclesof thepelviclimbin birds,but unfortunatelyfor the morphologicaml yologist, he failed to considerthe innervations,and hencehis paper is of little use from this aspect.

RecentlyI havecompleteda studyof themusclesof hip andthighof all vertebrateclassesa, nd the resultsof this, as bearingon mammalianhomologies,are containedin a papernow in press,'Morphogenesiosf the architectureof hip and thigh.' This containsonly minorreferencetso the conditionsin birds,however,for Avesare usuallyconsideredw, hetherrightly or wrongly,as beingtoo specializedto have muchbearingon the morphogenesisof other vertebrateclasses. Accordinglyit appearsadvisableto offera separatepaperdealingwith birds. As the topographicarloyology of birdsiswellknown,descriptionasrereducedto the minimumcompatible with a clearpresentation.Gallusgalluswaschosenasthe subjectfor discussions,inceit is a type relativelyunspeeializeadnd is easilyprocurable. It is deemedunnecessaryto repeat the discussionosfferedin my paper referredto above;hencethe two papersshouldbe usedtogether. A few pertinentpointsregardingthe osteologyarementionedfor a better understandingof the discussions.

The postureof birds variesconsiderably. In some,as penguins,loons, grebest,he positionof the trunk, whenon land,is largelyvertical,in order to bringthe centerof gravity abovethe aeetabula,and may be likenedto that of man. In other sortsthe body is inclinedat an anglewhenat rest or while walking,but in many terrestrialtypes (as Gallus)the body is carriedwith its longaxisalmostparallelto theground,a positionassumed by no bipedalmammalnor extinctbipedalreptilewithouta heavytail for

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a counterbalanceT. his is madepossibleby a relativedecreasein the preaeetabulanr dan increasien postaeetabulwareight. Thesedifferent positionosf thebodyhavevariouseffectsuponthepelvicarchitecturaend its controllingmusculature.Of influencealsoisanotherfactorof moment. Asinman,theonlyexclusivelbyipedaml ammals, oin birdsw, hentravelling on land,the pelviclimbsarethe solesupporot f the body,andsupply exclusiveltyhe propulsivfeorce. Becausoef the shiftin the centerof gravityandthefactthat theabdomeinsinterposebdetweenthethighst,he latterarelessapproximatetdhanisusualin mammals.

Asis to beexpectedb,irdsin generahl avea modifiedo, r evenan accentuated,reptiliantypeof pelvisw, ith an iliumextendinfgor a relatively greatdistancbeothcaudaal ndcraniatlo thelevelof theaeetabuluma,nd fusedsolidlyt'o the saeralvertebrae.The bar-likeisehiumis directed largelyeaudallyandparallelto the ventralborderof theilium,whilethe pubis,alsobar-like,is morevariablein positionb, ut is directedeaudoventrally. Whetheror not thesebonesarelargelyfusedwith their neighbors,thereis alwaysa vacuitybetweenilium andisehiumfor thepassage of peroneaalndtibialnervebranchesa,ndbetweenisehiu?nandpubisfor thepassagoeftheobturatonr erveandmuscle.

In many,if not all, birds,includingGalhst., the rootsof the 'lmnbar' plexusemergferomthesaeravl ertebraet,husindicatingthat in thisclass the pelvishasexperienceadsecondarfyorwardmovementd, oubtlesisndueedby bipedahl abitsandtheconsequensht iftin thecenterofgravity.

All striatedmusclesarearrangedin pairsof groups,--primemoversand their antagonists,---althouignhthe courseof specializatioonf movements the distinetivenesosf individualunits of thesetwo groupsmay become obscured.Similarly,nervesto musclesare arrangedin corresponding dorsaal ndventralbranchesto, primitiveextensoarndflexormusclegroups, respectivelyE. xperiencheasabundantlyshownthat theinnervationof a muscleisthemostreliablecriterionofitsidentity;but nervesaresubjecto evolutionarychangesand not only mustotherfactors,as topographyin regardto othermuscleasndnervetrunks,beconsideredb,ut the evidence offeredby thenervesthemselvems aybesoobscuredasto misleadeventhe most carefulanatomist. On the whole,homologiesthat do not consider the innervationsare valuelessi;f correctthey chanceto be soon the basis of incompleteevidence.

Becausien tetrapodsi,ncludingbirds,the iliumhasthrustdorsalward betweenthe lumbarand saeralplexus,thereare four main nervegroups andfourcorrespondimngusclegroupsto thepelviclimb,onepairprozonal andthe othermetazonale, achpair comprisingonedorsaland oneventral element. The fernoral(dorsal)and obturator(ventral)are prozonalb, ut the latter has beenseeondarilyencompassebdy the pubis,as in other

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vertebratess,othatit nowpassetshroughthe'obturatorforamen,b' etween pubisandischiuma,ndthushasbecomme esozonaTl.heperonea(dl orsal) and tibial (ventral)nervesare metazonalp, assingbetweenilium and ischium.Eventhehighests, hortesbt ranchetso the deephipmusculature belongto oneor anotherof thesefournervegroups.

Thereis muchevidencien comparativaenatomythat in thebasictetrapodconditionthedorsanl ervecomponen(tfsernoraalndperoneali)nnervated a singleextensomr usclesheetin the thigh,with corresponding ventralnerveelements(obturatorandtibial). Thisarrangemenotf single musclesheetsi,n layers,duallyinnervateds,till occursin variousdegrees in lowertetrapodsa,ndthe generatlopographsytill suggestist in birds. It isnoteworthyh,owevert,hat in Avesthereissurvivalof originadl uality of innervationof but oneprimitivemusclecomplexw, hiletherehasbeen establishedbut onenewfusionof two unrelatedmuscleunits (bicepsand femorocrurala),nd this incompletely.Thus the primitive conditionin mammalsi,n whichthereappearsto havebeensegregatioonf all muscle unitsinto singlyinnervatedslips,is approached.

In spiteoftheextremespecializatioonfbirdsthereareonlya fewmuscles of hip andthigh(suchasfemorocruraliosb, turatori,nsertionof ambiens) that have becomemarkedlyaltered,and eventhese,for the mostpart, presentno great difficulty. The remainderw, ith few exceptionsa,re readilyhomologizewd ith the musclesof tetrapodswhen all pertinent factorsare considered. Within the class,however,there is considerable

variability in muscle pattern. Unfortunatelythe namesof the musclesof the vertebratesbelowmam-

malsarein a stateofgreatconfusionth; echiefreasonisthatmanyof them arecompoundosf mammalianunits. The termsusedherewitharethose basedonthe group(extensoorr flexor),originandinsertiono,r elsethe homologoumsammalianameswherethereis little or no doubtof the equivalence.Followingthe nameof eachmuscleI haveplacedin parenthesisthecomparablteermsusedby GadowandHudsonw, henthesediffer

from mine.

Do?sxL (rx'rrNsoa)?)tvtsto?'

Coxocruraelxtensocrsonsisot f a superficiadlualsheet(iliotibialis);an anteriorderivativeof this;an ambiensa; nd an extensoriliofibularis.

M. extensoirliotibialislateralishas originfrom the dorsalfascia. The insertionof thecranialparthasretreatedin Gallusandnowendsonfascia. Thecaudapl artinsertsonlyslightlyonthetibiaandmostlyuponthesurface of the vastuslateralis. Femoral and peronealnn.

M. extensoilriotibialisanterior(i. internusGadow)is an anteriorderivative of the last, from the dorsalmarginof the anteriorilium, inserting

roedial to the knee. Femoral n.

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M. ambiensarisesfrom the spineof the pubis,developsa slendertendon that passesdeepto the iliotibialisanterior, then lateralward deepto the pateIlar tendon,and is insertedin the complicatedmannercharacteristicof birds. It is undevelopedin manygenera. Fernoraln.

obt?r.

TEXT-FIG.1.--Lateral view of right hip and thigh of Gallus,with someof the musclescut to showdeeperdetails.

M. extensoirliofibularis(bicepsfemorisHudson).--Originis from the dorsalborder of the entire caudal half of the ilium, deep to iliotibialis lateralis. Insertionison a markedprocessof the fibularshaftmedialto the lateralgastrocnemiubsy, a longtendonpassingthroughthe usual(in birds) tendinoussling. It is very uniformin birds,and it is interestingthat in thisclassit passesmedialto the lateralgastrocnemiusb,ut lateralto that

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musclein mammals. In otherwords,in oneclassthis headhasdeveloped and extendedlateral to the iliofibularis,as well as to the peronealnerve, whilein theotherclass(andinReptilia)it passems edialto both. Peroneanl .

Asin reptiles,the morecranialpart of the lateral iliotibialis(peronealn.) is a gluteussuperficialisa,nd is homologouwsith the mammaliangluteus longus(femorococcygeuasn)d gluteusmaximus. The more cranialpart

TEXT-FIG.2.--Medial view of right hip and thigh of Gallus,with abdominalwall cut closeto pelvisto showlumbosacrapl lexusandoriginsof neighboringmuscles.

(fernoraln.) and the anterior iliotibial divisiontogetherare equivalentto the sartoriuselement. Whether both are representedby the mammalian sartoriusor whichone,is impossibleto say. The evidenceindicatesthat the anterioriliotibialhassplitofffrom the lateralsheet;hencein birdsthere is no longextensorunit that couldrepresenta part of the vastuslateralis that migratedover the hip joint to becomea two-joint extensor,and accordinglyin this classthe rectusfernotisis undeveloped.

It is characteristic of the ambiens that it arises medial to the iliacus and

accordingliys associatewdith the vastusmedialisf,romwhichit is believed

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