Functional analysis of the facial skeleton

8/11/2019 Functional analysis of the facial skeleton

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A F U N C T IO N A L A N A LY S I S O F THE FA C I A LSKELETON W I T H SPLIT-IJXE

T E C H N I Q U E

N. C. TA P P E N

Graduate School of M e d i c i n e and Evans Dental Inst i tute ,Uvt ,hwxi ty of ennsylvunh

FIVE F I G U R E S

Th e biological meaning of many of the characters used byaiitliropologists in problems of taxonomy and group historyrcniaiiis vi rt ud ly unknown or in dispute. Recently Hooton( '46) admitted that little is known of the adaptive or non-adaptive value of most hereditary variations used in anthro-pological classification. This is in contrast to Hootoil's '31)earlier exteiisivc list of cliaractcrs he then regarded as non-adaptive arid therefore suitable for taxonomy. Similarly, the

extent to which niorphology is genetically or functioiially de-t ermined f requeii tly remains undecided in problems of fossilr t i a i i (Howell , '51) and modern races (Coon, Garn and Bird-sell, '50). All the above autho rs offer stimulating hypotheses011 th e iiiterpretation of f or m differences but give little directdemonstratioii of their validity. It seems clear that moretechniques of analysis a re grea tl y needed by aiitliropologistsif tlic science is t o progres s satisfactorily.

E'unctioiial analysis of morphological traits is still in its

infaiicy (Schacfler, X), but its raniifications are alreadyclarifying many problems of anatomical form. I n the facialskeleton, one promising aid to analysis is the Bcriningliofsplit-line technique. It demonstrates the orientation of theminute architecture of compact bone by staining methods.Hcnninghof ( 2 5 ) was able t o show th at some areas of theskull were directionally organized in their split-line patterns,particularly in the face , while o t h e r areas, notably the skull-

503


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504 N. C. TAPPEN

cap, showed no such organization. Benniiigliof ('25, '27) and

Seipcl ('48) have given evidence that tlie pa tt er ns of organiza-tion are related to the mechanical forces tictiiig on tlie facialskeleton.

The method lias not been used extensively siiicc its intro-duction. Beniiiiigliof ( '23, '27) was principally coiicerneclwith its introduction and the demonstration of its relation-ship to cancellous bone organization aiicl rncchanical stressesin the compact bone. Henckel ( 31) processed skulls of chini-paiizee, baboon, S e w m o p i f k o r u s ,orang aiid man. Hi s descrip-

tions are very brief, however, emphasizing the siniilnrity ofpa tt ern among these primates. Th e dift'ercnces a rc not ade-quately described. I-Iis generalized drawings clo riot indicatesatisfactorily the direction aiict meeting-points of some ofthe split-line systems. Bruliiike ('29) confined his study tomammals other than primates. Seipel ( '48) concerned hirn-self chiefly with the s t ress systerris in tlie face mid lower jawand their application to orthodontic problcnis. H e outlinesa scheme of trajectories of the face which arc related to theforce of the chewing niuscles as well a s the prcssurc from thetooth row. It is frequently not clear which direction lie be-lieves these stresscs take, liowevei-, arid the nature of severalof his trajcctorial structures is not explained.

Nonc of these iiivestigators gives adequate attcntion to thevariability to be found in the split-line patterns. There hasbeen little cai .eful cornpaiativc work. Sorile are as of tlie faceliavc ncver had their split-line patterns reported, such asthe nasal surface of the maxilla and tlie temporal surface ofthe zygomatic bone. Some patt erned ai*eas liavc nevei- beeninterpreted. Finally, none of the previous wor1~ei.s has beeiiprimarily interes ted in aiit1iropolog'y. There has consequciitlyhceii little discussion of anthropological problems, and the rc-lationsliip of the form of various regions of the facial skele-ton to their split-line pattcims has never been discussed andtested adequately. M'itli these limitations to present knowl-edge in mind, further investigations using the split-line tech-nique were undertnken.


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SPLIT-LINE AKALYSIS OF T H E FACE 505

This study was riiacle possible by a grant from the Wenner-

G r e n Found' ' 011.

MTEKT.2T,R A l l ) METHODS

Split-line prepa rati ons were made on the adult facial skele-tons of 6 human beings and one chimpanzee. Special pr ep ar a-tions 'were rnadc 011 two d o g mid one cat specimens f o r asuppleniciitary problem. Huriiaii material was in two casescorriniei~cinlly p i~p t i i*c t l peciniens of uiikiiown provenanceaiid in 4 case s iiiale dissecting room material. 1)entition wasgood or excellent in all specimens, a masiiiium of oiie toothbeing missing on any side.

Th e split-line tccliiiique has beeii clescribccl most thoi~ouglilyby Seipel ( 48). It coiisists csseritially of decalcifying tliccompact boiie in weak acid (in this case 10 HCl) suf-ficiently to allow easy pcnetration by a sliarpcnccl teasing nee-dle. These pei.pcndicular punc ture s of the surface bone usu-ally form short fissures rather than rouiid holes. India ink

is the n inti~ocluced nto the splits, usnally less tlian4

long,to clari fy tlieir direction. Scipel has shown histologically th atthese fissun.es c o r i ~ ~ s p o n d o the directioii of oi*ientc?tion ofa majority of tlic Ihvcrsiaii systems in tlic particular areaof the puncture. Th e split-line tecliiiique th us samples theminute organization of compact bone.

It was found th at a liypodei~mic ieeclle dipped into the iiikand inserted aloiigsicle the teasing needle while i t was stillholding tlic split opcii introduced the color satisfactorily. Ex-cess iiik a t the surface w a s wiped away with a damp cloth.Fre que nt re-wetting aided puiictuw, distributed the color moreevenly a nd niade f o r easier closure of the fissures. The mate-rial was a l lowcd to dry out wlicii not being processed, siiicet h e bone teiicls to niold and deteriorate if it remains moistfor long pcriocls of time. A brief soaking i n water was adc-quate to rcsofteii tlie specimen.

I n t he more liiglily organized areas of bone it was usuallypossible to join tlie fissures together in coiitinuous lilies by


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506 N. C. TAPPEN

making intermediate puiictures, in the same manlier as Sei-

pel ( '48). This made demonstration much clearer, and gaveclear indications of the meeting points of divergent systemsof split-lines. Coiitinuous pa tt er ns are apparent in figures

Fig. 1 Split-line prrpnration of fncc and brow region of lioiiinii malc. Whitesquares of paper are 011 xvgomntieo-iilnxillnry suture.

a i d 2. Tlic rricetiiig points of continuous systems may be par-ticulai*ly notctl in thc zygoitititic region of the specimen infigure 1.

In areas where the splits gave 110 indication of uniformityof direction there w n s n o attcnipt to joiii them. In tlie morehighly organized ar ea s tlie directioii of the split-line pattcriis

could iiot be a11 art ifact or a creation of the investigator.. O n


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niore tliaii oiic occasion prcconceptioiis a s to tlie direction tlie

pat te rns would take were tlisproved by tlic actual orieiitatioiiof the splits.

Scipel ( '48) found that tlie split-line pattcim in two regionsof the maiidiblc was dif'f'erent, in somc specimoiis, in deepcrlayers f r o m what it was a t the surface. Accorclingly, the sur-face bone of two of the hunian specinlens was whittled downand split-line preparat ions were made, to test the uniformityof direction of split-lines at different depths.

To help assess the cxteiit of corresporidence betwcen bonysti-uctures of different thickness and split-line patterns scc-tioiiing was performed on one sidc of the facc of two undc-calcified spcciniens, aiid scctioiiiiig an d disarticulation of boneson th ree decalcified specimens. Measurmieiits and obscrva-tions not otherwise possible could tlius be takcn. From thismaterial an attempt was also made to evaluate the coi'rc-spondeiice between split-line pat te rn s and obseiwiblc pat ter nsof trabecular boiic in the face.

The procedures outlined above give the most extensive pic-ture to date of thc variability in split-linc patterns of t hchuman face, aiid allow discussion of the comparative analy-sis of similaritics aiicl differences of pattern between man,aiiothcr primate, and other mammals. Regions of t he facepreviously unreported, t2ic interior surface of the frontal re-gion, thc temporal surface of tlie zygomntic boric, and tlicnasal surface of thc maxilla, help greatly i n clarifying thctotal functioiial picture in the split-lines of the face.

OBSERVATIONS

Unless otliei*wise stated, al l subsequent desci~ipt ions eferto humaii specimens. Ex-ccpt for special preparations, figures1 and 2 illustrate all tlic points madc. Descriptions of thesplit-line patterns of sepa rate regions of the human f acc aregiven, along with var ia tions rioted in different specimens. Thesituation i n each corrcsponcliiig region of tlie face of the cliiiii-panme specimeii is described immediately after each region


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N. C. TAPPEN508

i i i nian. Preparations WCI-C iiot iriade in any region of the

cliimpaiizec specimen othei. than in the external face, how-

Alvcolur iuargiuz Imniediatcly above the tooth row in thealvcolar process of the maxilla, a split-line pat te rn can in somespecinleiis be observed quite clearly, running parallel to tlie

ever.

Fig. 2 Split-line p r e p a r a t i o n on f a c e aiicl brow reg ion of adult mrilc (aliini-pnnzce.

inferior b o r d e r of tlic alvcolar pioccss. Its failure to ap-pear in the otliei-s may be due t o excess dectilcification of thevery thin bone. This pattern ma:- bc 3 or 4 mni iii lieight inthe molar region, where it is coiitinuous, arid where tlierc isoften slight but coiitinuous ridging of bone which corre-spoiids to the split-line pat te rn. The ridging and the con-tinuous lines become less well-defined o r disappear. above tlicpremolar teeth. The height of tlie pa tt ern becomes reduced.In the caniiie mid incisor regions the pattern may disappearcompletely. Where i t remains, it is found at tlie base of t hetooth socket itself, but the pattern may be missing above the


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SPLIT-LIXE ANALYSIS OF T H E FA C E 509

interalveolar septa. The pa tt ern ju st described is closely

paralleled on the liiigual side of the alveolar process, buthere i t courses parallel to t he split-line pat te rn of the palatedescribed by Seipel (48) .

The chimpanzee is ail exception in that the highest extentof the pattern is fouiid above the canine tooth rather thanabove the molars. No ridging is observable here.

The two specimens which did not reveal the pattern haveextrcniely thin alveolar bone in the canine and incisor rc-gions. In one case where a missing tooth had been followedby resorption, tlierc was 110 trace of tlic pattern. In the abovecases i t w a s usually difficult to obtain any clear split-lines.

A l r e o l a r p r o c e s s of the mnxiZZa. Above the immediate alveo-lar border region, the alveolar process of the maxilla showsa generally vertically oriented split-line pattern. Thi s maybegin immediately at riglit angles to the lines running paral-lel to the inferior alveolar border, may begin as lines curv-ing sharply away from thc latter region (as sccn in fig. l),01 may begin in sonic parts of the f a c e only at a height oftwo or more centimeters above the alveolar border. I n thosespecimens showing such a gap the vertical split-lines were lessregular and well markcd. These were the same specirrieiis inwhich the alveolar border pattern was not detectable.

The ascending split-lines a re clearest i n the region of thexpgomatico-alveolar crest. They a re also usually well markedabove the canine tooth, coursing beside the lateral border ofthe pirifo rm aperture. Between these two ar eas there is moretendency toward irregularity of the split-lines. Thi s is notobservable in the chimpanzee.

From the incisor teeth the split-lines ascend toward thenasal aper ture , veering somewhat laterally as they approachits inferior border to become part of the system of ascend-ing lines originating from the canine tooth area. Even themost medial lines lead into this system in those skulls inwhich the border of the nasal opening remains sharply de-fined throughout. I n the two specimens whose nas al floor is

continuous with the surface bone the ascending lines enter


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510 X . C. TAPPEEN

the nasal cavity, though there is sonic late ral divergence. Al-

tliougli tlie nasal situation in the chimpanzee resembles thelast-naniecl pattern, tlic asceiiding split-lines never reach theinferior border of the nasal aperture. lnimediately below theinferior margin of tlic nares the lines closely parallel tlic bor-der arid meet at the midline.

The ascending split-lines above the canine and incisor teethdo not extend very far. The more medial ones parallel thesuperior margin of tlie nares and end on the infcrior por-tion of tlic nasal bow. They may IOSC their organization be-

fore extending this far, as is the case with the specimen sliouwin figure 1. This individual h a d suffered a fracture of thelower par t of the nasa l bones, however. Those ascendingsplit-lines wliicli begin more laterally above the canine toothar e in te rrupted by tlic system of split-lines which roughlyparallels the inferior orbital margin.

The ascending split-lincs in the region of the zygoniatico-alveolar crest are also interrupted completely by systems ofsplit-lines coursing at approximate riglit angles in the zygo-rnatic region. I n some specimens, as i n figure 1, some of theascending lines may abrup tly change course, tu rning laterallyand downward to coiiform to thi s predominating pat tern .

L o w e r orbitn.2 wzargiiz Thus all ascending split-line pat-teriis in the alveolar process of the maxilla are conipletelyinterrupted by generally continuous lines beginning in thefro ntal process of ihe maxilla, traveling along the in fraor -bital region, and contiriuiiig downward, laterally and back-ward to the region of origin of the anterior portion of tlieexternal part of the masseter muscle. In figure 1, the whitesquares of paper mark the xygomatico-maxillary su t~i re , l-lus t i~ i ti ng ow the coiitinuous organization of split-lines isiiidepeiident of tlie bones in which tlic lines course. Continu-ous split-lines cu t straight across the suture line immediatelybelow the orbit, and course roughly parallel to the suture o nboth maxillary and zygomatic sides in the mala r region. Thelower limit of this organization pattern is coincident with thelom~er border of the tendinous origin of the masseter mus-


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S P L I T- I J X E AXALYSIS OF T H E FACE 511

clc. The lriuscle bouiiclary is below the sutu re line on the zygo-

iiiatic proccss of the maxilla Sicher, '49).Tlie coritiiiuous iiitcrnal architecture revealed by the split-

l ine p a t t c r x jubt described has a general structural co r r c -spoiideiicc in tlic gross form of the bone. The lower orbi talmargin is u s u d l y sufficiently thickened t o form w h a t Wein-iiiarin and Sicher ( '47) term the infruorbifal but t ress . The

Fig. 3 Split-line prcpnration 011 human adult, indicating interinittent continua-tion of spli t - l ine pi t tern of l a te ra l brow region i n t o central brow rcgion. Infra-orbital split-line pat te rn inte rru ptin g ascending lilies in iiasnl region can also beobserved.


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SILlV-LlX ASALPS18 O E T H E PA C E 513

ally into the xygoiiiatic region or are cut off by a short pat-

tern of lines bomiding the inferior margi n of the orbit. Thesedo not continue in tlte zygoniatic region. Tliey are cut off byliiics associated with tlic lateral boundary of the orbit, de-scending to incct tlie horizontal lines in tlic zygomatic region.Tliiies ascellcling froni the molar region antel-ior to and in thezlv~omatico-alveolar crest tur n back downu7ai-d to stop at thelowermost p i n t of origin of the niasseter muscle. No caniiicfossa is ohserval)lc. It sliould bc noted that the zygomatic~ i i d i i id massetcr niuscle a rc both lower in relation to theiiifcrior niai.gin of the orbit iii tlie chimpanzee.

Zygorucrtic irc7~. In tlic liumaii, thc split-lines in tlic zygo-iitatic region p~ ev io us ly cscrihcd, wliicli ar e associated withtlic origin of tlic itinsscter muscle, become directed more andmore hacku~ai-(1 s tlic zpgomntic arch is approached, finallyl)cc*oiiiing ontinuous wi th lines which course i n the directionof tlic axis of t h e zygomatic arch.

o r b i f a l ro qilion. Ilatei*al o the orbits organized split-lines c o u m c tipprosinintely vertically, parallel to the orbitalniniyin. Tlicse usuallv begin i i i the frontal bone, coursingthrough its zygomatic process aiid across tlie zygoniatico-frontal suture t o clcscend a s far as tlie body of thc zygomaticbone below tlie l w e l of the inferior orbital margin. I n twospecimens this pa tt ern is clearer at deeper levels than at thesurface, where minor irregularit ies occur.

The more posterior lines t ur n a t the levcl of tlic zygomaticarch to bccomc continuous with the lines iwiiniiig in the di-

rectioii of the axis of the arch. This transition does not oc-cui in tlic specirneii shown in figure 1, where the lines in theantei.ior part of the xygomatic arch parallel its inferior bor-der rath er tliaii the superior margin. Thi s is tlic only speci-inen cliscovered with this deviant pattern. More anteriorly,the tlcscending lines p ai dl el in g the lateral border of the or-bit a1-c either cut o f f by the lilies continuing in the zygomaticregion from thc infmorbital area, or die out in an area inwhich the split-lincs bccorne random or do not appear at all


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X C . TAIPBN14

wlicii tlicl clecalcifictl boiic is punctured. Close t o tlic orbital

iiiargin the lilies contiiiuc to parallel t he orbital mar gin ar oundits la tc ra l inferi or fle~xure. Tliese lines join tlic i iifra orbitalsystem of split-lines.

Rzipruorbital region. The region above tlie orbits is markedbj7 coiisiderable variability in split-line patterns. Th e lineslireviously mcntioiietl which tir continuous witli tlic lines run-ning lateral to tlic orbit i n the f1,oiitosphcnoiclal process ofthe zygomatic bone are usually confinecl to the a w a Cun-ningliani ( 08) 11;~s lescribcd a s t he t r igoiwilz szipraorhitcileof i l ic frontal bone. Tliis triangular area is lateral to tlicsu pr ao rb it a l forai-ncii. I n oiic specimen, however, sonic ofthese lines continue, with occasional interruptions, t i s f a r ast h o niiclline, secii in figure 3. Another specimen shows onlyrantlorn split-lines i l l tlic trigonum on one side, tliougli thetypical pattern appears on the other side. The superciliaryridgc wgion usually shows only random split-lines or rouiidholtls and diffuse spots w l i c i ~ lic ink cnteretl. One speci-men sliows a b r i e f , i i a i ~ o w T C A of organization along theridge. Three specimens shorn- N tcndency to continue linescircumorbitally f iwir i the fr*oiitnl proccss of the maxilla intot l ic fi-ontnl. These lines a r c nevcr strong or extmsivc, liow-ever. K O elationship to frontal siiiuscs wtis ohscrvable. T h esinuses I-anged f rom iibsellt to very estc3iisive in the individu-a 1s 11 1.0 ccs s ed .

T n contrast t o tlic h u m a n s ~ ) c c i n i ~ m , lic cliinipanzee showsvery definite split-line organization in the bi*ow region. Theliiics close t o the oibits continuously ptirallel thein supe rio rlyand iiieclidly a s well a s 1atci.ally. Tlic frontal sinus of thisspecinien is extensive, reaching approximately to the mid-point of each orbit.

I i i f rn tewporci l svr f i ce of t h e 111~1~i110 Posterior to t hexpgoiiiatico-alveolar ci-cst nhovc t h c first molar the split-lines course vertically in the alveolar proccss of the maxilla.Fur the r hick on tlic iiif ratcniporal surface of the maxilla tliehonc becomes very thin, the iriasilliii-y sinus being greatly- ex-

tended. In some spcciniens tlic split-line pa tt er n becomes ran-


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dom iii tliis region, while in others the vertical oricntatioii of

tlie split-lincs is nitiinttiiiiccl. T h e cliirviparizcc pattcrii is ran-dom liere.

Nustrl 0011cs . Split-lines i n tlic nasal boiics run verticallyalong tlic asis of tlie bones until the region above tlic iiasalopeiiiiig is reticlied. At tliis point the p t i t t c ~ i m nay becoiilcraiidoni, or tlicl lirics may bc directcd ltitcrally, continuon :

Fig . 4 Split-liiie 1 iq i : i i : i t i ( i i i of i i i : i i i l l : i aiid tciiiporal sui face of riglit zygo-iii:ctic I)oiic, \ i c \ w d froiii I d i i i i ( 1 . JZo1:ir tcvtli l i e t o the riglit , incisors to tlicleft. A l J O V C the iiicisois is tlw riglit 1i:ilf of tlir i i n sa l opeiiiiig. M o r e niitcrior a s -cending lilies froin iii:isilla arc deflected t o tlir r igh t along tlic. inferior niargiiiof the q g o n i n t i c I)oiic. 3101 posttcrioi 17, 1iiic.s ascciiding :iloiig tlic \\:ill of tlicbraincnse re\ else their cooisc : i i i c l cl~wx~iicl it the 1):ick of the 1:iter:il orbitalrcgioii.


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516 N. C. TAPPEX

with lines in the maxilla wliicli parallel the nasal aperture.

The lines in the chimpniizee a r c continuous with those f r o mtlic maxilla.

Fig. 5 Split-line preparation on the nasal surfnre of the l e f t iiinsilln. Leftceiitr:il iiicisor tooth is Xisible below rigli t , premolars n:1(1 1iio1:ii s t o the lef t .Sasnl concliae h a l e bceii remo\c'd. A l o \ c t l ic i nc i so r is tlic ou t l ine of the nasalopening with split -lines coursing p:~r:iIlel t o tlie iii:ii gin, Split liilcs also alleltho iiasal bridge anteriorly, nseeiiding t o tllc frontal regiolL. A h e pos t e r i o i l~split-lines nre cont inuous f r o m floor of nasnl cavity to the frontal bone.

I i i tcr ior s u r f u w of f r o n f u l r c g i o u i So sy)lit-liiic patternwas observed in the area posterior to the frontal sinus ant1brow region. Thi s app ear s to be part of the generrtl innertable of compact boiie of tlic sknll. Tlie bone is laid clow11in a series of thin, extensive layers which can be reaclilyteased apart in the decalcified conclitioii. S o pat tc~m f split-lines

is apparent,and

the iiitlivitiual splits t i re

iiiclcyenclc.i~t


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518 N. C. TAPPEN

Deepe r l ayers . In 110 part of the face region is the split-

line pat ter n in deeper laye rs of compact bone significantlydifferent froni tlic surface pattern. Tliis w a s t e s t d both bywhittling down after the split-lines liad bceii niade and bywhittling away the bonc on the opposite side of tlie face ofone specimen before at tem pt ing split-line preparations. How-ever, in regions where there is cancellous bone underlying, thesplit-lines do not appear a t all in the transit ion d levcls. Hei-pel ('48) also found this to be the case.

Skull s e c t i o n s . There is coiisidcrable c o i ~ e s p o n i l c n c cbe-tween highly organized systems of split-lines ai id thickeningof the bone in the face, rcvealcd by sections of uiidecalcificdaiid decalcified skulls, Tlie irifraorbital region is coiitiiiuouslythickened in the direction of tlic split-lilies. Tlic zygoniaticregion, zygoniatic arch and la te ra l border of the orbit arc allthickened and have definite split-line pnttcims. Tliis is alsotrue of the cn iz ine p i l l w of Keinmarin ai ic l Sicher ( 4i) , itscoiitiiiuous split-liiics bciiig obscrvablc on tlic r i ~ s a l urfaceof the niaxilla.

A strong exception is tlic b r o w rcgioii, wliei~c liicltcncdboric is not accompanied by highly organized split-line pat-terns. On the other hand, well oi*ganized pat tcms arc f o u n din some regions in which thcre is very little tliickciiiiig ofbone. Tliis is tlic case in tlic zygorrititico-alvcolai~ c i w t aiidthe region of tlic niaxilla borclcriiig tlie superior iiiai.giii oftlie nai*ial opcniiig.

111 general, caiiccllous boiic is scanty o r ttbscrit in tlie face.

It appears only in thicltrnecl bony areas, hut usually makesup a minor part of tlic a r c ~ i of m y cross-section of thesethickenings. Trabecnlar orgaiiization is usually weak or ab-sent, as Kciiningliof ( ' 25 ) observes for most flat bones.

The dog aiid cat do not liave a complete post-orbital bar.Instead, there are b o n y processcs origiiiating f r o m thc zpgo-rnatic arch, connected in the living state by a tough band ofconnective tissue. At these origin points thc split-lines de-viate from their antero-posterior courscs in the frontal bone


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520 K. C. TAPPEN

split-line patterns, and this study verifies his finding that

there is no coiisistent split-line organization in the adjacenthuman superciliary ridge. I n these cases there is either littleor no mechanical stress , or the bone is so thick that its minutearchitecture is uriaffected by stresses. In either case the thick-ness of the bone must result from something other than a func-tional response to pressures or tensions on it.

The definite split-line patterns observed in this study areinterpreted as a response to pressures and tensions set upin the face by chewing. I t is assumed that Haversian sys-

tems can be oriented in the direction of either tension or pres-sure, a lthough Benninghof ( 27) and Murray ( 36) ar e dividedon thi s question on theoretical grounds.

It should be possible in the future to determine instrumen-tal ly the direction of st resses in the bone of living, movinganimals. Gurdjian and Lissner ( 44) used an electric straingage to record tension and pressure areas resulting fromblows on the skulls of anesthetized dogs. The instrument issensitive enough to record much lighter stresses. If it canbe adapted to problems of chewing st resses in the living, nioii-keys with split-line patterns similar to those of humans(Henckel, '31) could be used to determine the direction ofstres ses coinciding with major split-linc patte rns. I n the ab-sence of such work tentative interpretations are presented onthe basis of the probable st ress systems indicated by the split -lines.

The split-line pattern close to ancl paralleling the inferiorborder of the alveolar process of the maxilla, perpendicularto the lines immediately above, was noted by other workersbut not explained. Since there a re no appa ren t forces act-ing in different directions at these adjaoents levels, the rea-son fo r the str iking contrast i s not immediately evident. How-ever, the research of Wetzel ( 22) gives a clue to the probableresponse involved. I n the alveolar region the periodontalmembrane is attached to the cortical alveolar bone. Whenchewing pressure is applied to a tooth it is driven into thealveolus. This s tretches the periodontal membrane and pulls


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SPLIT-LINE ANALYSIS O F THE FACE 521

the w7alls of the alveolus together on the tooth. At the most

inferior portion of the alveolar region the bone is a relativelythin compact layer. I n view of the split-line pat tern here, itis iiidicated that the bone is sufficiently deformed so that theprincipal stress is a bending one, with the axis of the bend-ing paralleling the alveolar margin and the split-lines. Thearea immediately above is removed from the pull of theperiodontal membrane, and so unaffected by it.

If this hypothesis is correct, the organization of the split-lines would make the bending of the alveolar compact boneeasier, but would cause less deformation of the Haversian ca-nals. If so, the Haversian systeni orientation is indicated tobe a response t o stresses rather than being primarily astrengthening mechanism, as has been suggested by Ben-ninghof ( '25). A partial experimental test of the suggestedexplanation may be possible by obtaining split-line patternsof alveolar margins of teeth that have never been in occlu-sion, o r in which the opposing tee th have been removed. Thisshould give one critical test of the assumption tha t the split-line patterns are in response to mechanical stresses.

The ascending lines in the alveolar process of the maxillamust be in response to the compressive forces from the toothrow, and thus probably represent areas of pressure. Mediallythese lines are continuous along the nasal surface and fron-ta l process of the maxilla, as shown in figure 5. This indi-cates that pressures are transmitted as far as the frontalbone without interruption, although the surface ascending

lines conform to the superior border of the piriform aper-ture and terminate above this opening.

On the other hand, the more lateral ascending lines con-forming to the zygomatico-alveolar crest of the maxilla arecompletely inter rupted by the infraorb ital system of lines andtheir continuation through the zygomatic region to the areaof origin of the masseter muscle. This indicates th at simul-taneous tensile forces overcome any upward forces from thetooth row. The tensile force is provided by the temporal and


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522 PI . C. TAPPER

niabscter inuscles, with the masseter probably contributing

inore liearily.If siiiiultaneous teiision overcomes ascending pressures in

t he iiifraorbital and zygomatic regions, it follows that no up-ward prc~ssurcs rom the tooth row ever reach the lateral o r -bital region. Tlie iviasseter muscle and some of the anter iorfibers of tlie teniporal muscle pull downward upon this region,and the teniporal fascia may also contribute. This are a isthus probably under tension i n chewing, also. This is con-tradictory t o the generally expressed view of anatomists andanthropologists (TVcinmann and Sicher, 4 i ; Hooton, 46 ;Ashley Xontagu, '51) that upward pressures from che\vingreach the brow region by way of the lateral orbital region.

1\7iile the split-lines indicate that the above regions areunder tension, the reasons for this conclusion require somediscussion. The tooth row receives the combined elevation ofiviasseter, temporal and inte rnal pterygoid muscles, while onlythe iiiasseter acts with full force to counteract the effects ofthis ap va rd pressure on the face. However, it is not certainju st how efficient the transmission of these muscles forces tothe tooth row is (Robinson, 46). The forces are distributedover a fairly wide area by the dentition, though the greatestforce is placed upon the first molar (Friel, 24). Some oft h e pressure goes up through the canine pillar. The curva-ture of the zygomatico-alveolar process may also help in dis-tributing the upward forces laterally, Finally, the lateral or-bital region is lateral to the tooth row, so that forces up toit cannot be transmitted directly. By contrast, the masseterpulls downward directly upon the zygomatic region over avery small area through a tendinous arrangement for the ori-gin of its fibers (Sicher, 49). Temporal muscle fibers alsopull directly upon the lateral orbital region as far down asthey extend. These factors appear to combine to cause thepreponderance of downward forces seen in the lateral orbitaland zygomatic regions.

A small portion of the lateral orbital region is below thefibers of origin of the tempora l muscle (Sicher, 49). This re-


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SPLIT-LISE ANALYSIS OF THE FA C E 523

gion would be under pressure but for the much greater force

probably exerted here by the pull of the masseter muscle.Benninghof 's 25) split-line preparations indicate that thetemporal muscle exe rts it s force along the enti re length of thetemporal lines and within the temporal fossa. There fore onlya relatively small amount of the force of the muscle is e s -crted directly upon the late ral orbital region. The greate rforce exerted by the masseter would explain the continuoussplit-line pat te rn coursing in tha t p ar t of the lat eral orbitalregion below the fibers of origin of the temporal muscle.

The dog and cat split-line preparations give the same indi-cations as to the stresses in the region comparable t o the lat-eral orbital, the post-orbital. The split-line pattern deviatesinto the bony processes giving origin to the tendon-like bandof connective tissue which takes the place of a post-orbitalbar of bone. Such bands can only res ist tension (TT eiss, 39),which is provided by the masseter muscle attached below tothe zygomatic arch. The split-lines in the post-orbital bonyprocesses must therefore also be responding to tension. Thehuman and chimpanzee stress patterns in this region there-fore seem t o correspond closely to that indicated for othermammals.

The zygomatic arch shows a longitudinal arrangement ofthe split-lines. Since the principal force acting upon it is themasseter muscle, it resembles a uniformly loaded horizontalbeam supported at both ends, The longitudinal arrangementof the Haversian systems approximates the more flattened oft h e theoretical tra jec tor ies of such a st ructure and gives con-siderable resistance to shearing forces exerted by the muscle.

The random split-line pa tt er n of the thin pla te of boneforming the posterior wall of the fronta l sinus is similar t othe lack of orientation in the medial brow region. The supe-rior portion of the canine pillar immediately adjacent ishighly organized, the suture line generally marking the breakin patt ern quite sharply. Here observations on sectioned skullsoffer some evidence of what must be taking place. The bonein the canine pillar thickens greatly above the piriforni aper-


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524 N. C . TAPPEN

tur c and loses most or all of i ts cancellation. The frontal

process of the maxilla and the adjoining superior portion ofthe nasal bone have the thickest compact bone observed in theface. The two bones haft to the fronta l over a wide area. In4 pecimens sectioned the fr on ta l sinus was immediately abovethis haft ing region, separated f rom the maxilla by a thin plateof the fron tal bone. I n the other specimen there was no fron-tal sinus, the region it usually occupies containing much can-cellous bone giving no evidence of trabecular organization.

The above data indicate that the upward pressures in the

canine pillar have very little effect on the organization ofbone in the brow region. Whatever pressure reaches the fron-tal is apparently spread over a wide area.

The chimpanzee pattern offers more evidence on the gen-eral problem of the relat ionship of the brow region to thest ress systems of the face. The continuous split-lines indi-cate that the whole area is under tension in chewing. It isthus probably a more extended version of the continuoushuman system of lines in the la ter al brow ridge and la ter alorbital region. Since the braincase in this animal is set wellbeliiiid the orbits, the brow ridges have t o take up all thetension from the masseter and temporal muscle without thesupport of a forehead above. The tensile forces ar e thus notspread out over a wide area. It may be noted from figure 3that some human specimens show faint signs of a similar ex-tension of split-lines above the orbits . It seems likely thatpressure f rom the canine pilla r of the chimpanzee also haslittle or no effect upon the organization of the supraorbitaltorus. Henckel ( 31) also observed the sudden breakdown ofthe ascending pattern at the fronto-nasal suture in man andthe chimpanzee.

The split-line patt erns of the lower face of the chimpanzeeindicate that, as in man, pressures from the dentition neverreach the late ral orbital region. Inste ad of being cut off bya strong system of lines running from the inferior orbitalregion through the zygonia, however, the chimpanzee ascend-ing lines ar e gradually bent late rally toward the region

ofori-


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SPLIT-LINE ANALYSIS O F T H E FACE 525

gin of the masseter. These differences can probably be ex-

plained mainly as a function of the different spacial relationsbetween the orbit and the masseter muscle. I n the chimpan-zee the origin of the anterior tendinous portion of this muscleis relatively much lower and somewhat less lateral to the or-bit, corresponding partly with the longer face. As a conse-quence, the ascending lines come directly into the tensile fieldcaused by the pull of the niasseter priniari ly and the temporalmuscle secondarily. On the other hand, the inf raorbital linesare cut off from entering this system by being too far above

the origin area, and by the lines descending from the lateralorbital region into the zygomatic region.

The progressive looped pattern of the split-lines across therelatively muscle-free zone of flexure between the internalface of the fronto-sphenoidal process and the postero-lateralsurface of the orbita l process of the xygomatic bone, seen infigure 4, s quite puzzling. Possibly this indicates a regionbetween two areas of downward tension. The former wouldbe from the pull of the temporal muscle in the temporal fossa,the latter mainly from the pull of the masseter. The temporalfascia may also affect this region somewhat.

The preceding discussion of the form and functioning ofthe human and chimpanzee facial skeletons can be applied toother anthropological problems. For example, the argumentover the use of non-adaptive or adaptive characters in pri-mate taxonomy exemplified by Wood Jones ('29a, '29b, '48)and Gregory ('30) is put in clearer perspective if referred tothe discussion of Weiss ('49). He shows that the nature ofadaptation must be analyzed in every case, because of thebroad applicat ion of the term. One of the types of adapta-tion he emphasizes is that of different pa r ts o r systems ofparts t o each other within the individual. Gregory ('30) es-sentially suggests such internal adaptations for the traitsKo od Jones ('29b) puts forth a s non-adaptive. The split-lines probably can be used to show when structures of thefacial skeleton are built up in response to muscle action, al-though this remains to be tested experimentally. The absence


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526 X . C. TAPPEN

of pattern in thickened bone such as the cranial vault, con-

versely, indicates where the bone development is not a stressresponse. It cannot, however, deiuoiistrate absence of inter-nal adaptation to other bodily processes.

One anatomical area which has received considerable an-thropological attention, the canine fossa, is clarified throughsplit-line analysis of some of these adaptive adjustn~entswithin the face. The three divergent systems of split-linesin the human face below the orbits, shown in figure 1, forina rough triangle, surrounding an area which is often rela-

tively unorganized in its split-lines. When this area is re-cessed it corresponds to the canine fossa. The chinipanzeepattern is one of ascending lines which ar e diverted late rally,with no relatively unorganized ar ea s in the face. This ani-mal does not have a canine fossa, which is apparently niadepossible in man because the facial bone may be organizedinto definite structures associated with the direction of thedivergent stress systems indicated by the split-line patterns.The area bounded may or may not have the split-line patternwell organized, and may or may not have thinner bone whicliis retracted into a fossa, but the essential feature seems tobe that the stressed bounding structures must remain rela-tively thickened while the intermediate region need not. Thevariability in position or appearance of the fossa may be part lyexplained when the angles and dimensions of the three stre sssystems va ry in different individuals or groups.

The canine fossa in man thus seeins to be a product of thereduction of the face from the probable ancestral condition,with consequently altered muscle relationships and the fre-quent appearmice of the infraorbital bar. It is suggested thatthe forniation of the elevated nasa l bridge may also ha re con-tributed to this development, creating a medially directedstress system above the nasal opening. N o such divergentsplit-line system is seen in the chimpanzee specimen, wherethere is no suggestion of elevation of the nasal bones. Theelevated nasal bridge itself may be related to the shorteningof the face in humans, as Hooton ( 4 6 ) suggests. Thus a num-


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SPLIT-LINE ANALYSIS O F T H E FA C E 527

ber of recognizable characters may actually result from adap-

tation to a single biological process.The lack of consistent split-line patterns medial to the tri-

gonum supraorbitale in man throws doubt upon the mechani-cal stress interpretations of large brow ridges, typified byHooton ( 46) and Ashley Montagu ( 51). Such doubt is sup-ported by evidence from those cases of acromegaly (Wein-mann and Sicher, 47) in which the brow ridges reach for-midable size while accompanying overgrowth of the lowerjaw prevents chewing altogether. The split-line pat terns t hus

contribute negative evidence on one internal adaptation hy-pothesis. On the other hand, the strong split-line pattern inthe chimpanzee brow region suggests that its form may beinfluenced by the st resses on the area. Cunningham ('08) ob-serves that the young chimpanzee has a distinct superciliaryeminence and a trigonum supraorbitale, but that these be-come fused and indistinguishable as the animal grows older.As the brow area becomes more separate from the braincaseduring the growth of the animal, the greater tensile stressesapplied more directly to the brow ridges could be enough t orequire this eventual functional unity. The split-line pat ternsin the brow region are already beginning to develop in Sei-pel's ( '48) preparation of a three-year-old chimpanzee skull.

I t is possible tha t problems of fossil man may be clarifiedby comparative split-line data such as that given above fortwo regions of the face. Neanderthal man may be used a san illustration. Howell ( '51) notes that classic Neanderthals

contrast with early Neanderthals in having lower cranialvaults, larger facial skeletons, semicircular supraorbital toriof a continuous structure with no distinguishable trigonurn,flattened mandibular angle region, shelving of the maxillainto the zygomatic bone with no clear demarcation betweenthe two, and absence of a canine fossa. Howell relates thesefea tures to differences i n the masticatory muscles in develop-ing his theory of differentiation of classic Neanderthals inwestern Europe during the Fourt h Glacial period. This study


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528 N. C. TAPPEN

lends support to his suggestion that the features mentioned

ar e interrelated, and suggests possible analysis of other fos-sil forms.

The classic Neanderthals may represent an intermediatestress condition in the brow region between the chimpanzeeand modern man. The low forehead would by this conjecturemake necessary a strong reinforcement above the orbits towithstand the pull of the muscles, obliterating the separationbetween the trigonum area and the superciliary arch but notrequir ing a continuous torus. The early Xeanderthals have

a higher vault than these later forms, and show the browridge divisions found in modern man, This differs from theinterpretation of Howell, who postulates the differences inbrow ridges as being related to the differences in relativestrength of the masseter muscle, deduced from differences inthe malar region.

Howell indicates that a relatively weaker masseter muscleis implied in the formation of the mandibular angle and ma-lar regions of classic Neanderthals. Seipels ( 48) split-linepreparations on the angle of the mandible indicate th at theeffect of the muscle upon the in te rnal architecture and su-perficial accumulation of the bone is considerable, lendingsuppor t to Howells hypothesis. The large face and relativelyweak niasseter muscle suggest stress relationships similar tothose indicated by the split-line pattern of the chimpanzeedemonstrated in this study. I n both forms there is backwardshelving of the malar region and an absence of canine fossa.I t is possible t ha t the high, rounded orbits of classic Neander-thals noted by Cameron (20) are also associated with thismasseteric relationship, since there would be less downwardpull exerted upon their lateral borders by the muscles. Com-parative analysis of form and split-line patterns in contrast-ing living forms thus tend to support Howells view tha t thesefeatures are part of a functional complex rather than unre-lated structures.

As a cautionary note, however, it niust be noted that thepresence or absence of a canine fossa may be related to more


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SPLIT-LINE ABALYSIS O F THE FACE 529

than a single cause in different forms. Thus Martin ( 28) and

Hootoii ( '46) observe that a canine fossa is ra re in Mongoloidskulls. Here the other extreme from the Neanderthal situa-tion is present in the malar region, which is heavily devel-oped and projected forward. If the shelving of the malarsis related to the lack of a canine fossa in Neanderthals, dif-ferent processes are operating to make for its rare appear-ance in Mongoloids.

The relevance of split-line data to additional anthropologi-cal problems has been discussed elsewhere (Tappen, 52).Such discussions will be much more firmly based when thebiological meaning of the pa tterns is established. As has beenindicated here, the technique lends itself to combinationwith comparative studies, devglopmental analysis, instru-mental testing and experimental demonstration on problemsof the nature of the Havers ian systems and their relation-ship to observable characters. When such work has beendone, the limitations as well as the uses of split-line analysiscan be evaluated properly.

S U M M A RY

1. Split-line technique was applied to a series of humanskulls, a chimpanzee skull, and dog and cat skulls in attempt-ing to clari fy anthropological problems of facial form.

2. The split-line patterns of the human face show varia-bility in extent and degree of organization in different re-gions, but the genera l outlines a re clear. The majority of the

pa tterns can probably be explained in terms of response tomechanical stresses on the bone. They are therefore an in-dex to one kind of internal adaptation and consequent inter-pre tation of the adaptive na ture of some of the charactersused in classification.

3. While many split-line patte rns probably tend to strength-en the bone, their organization is indicated to be a func-tional response t o stress rather than an adaptive strengthen-ing mechanism, f rom the evidence of split-lines in the inferior

border of the maxilla.


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530 N. C . TAPPEN

4. Split-line patt erns of the face indicate t ha t upward chew-

ing pressure in the maxilla is interrupted by tension systemscoursing along the inferior border of the orbit a nd throughthe zygomatic region, associated mainly with the downwardpull of the masseter muscle. The lat era l orbita l border andla ter al portion of the brow region a re also regarded as be-ing under downward tension. Comparison with the dog andcat stress situation in the equivalent area tends to verify this.This conclusion is in direct contradiction to the usual ana-tomical and anthropological interpretations of thi s region.

>lore centrally, maxillary split-line patterns end along thesuperior border of the nasal opening, but the deeper systemcontinues in the canine pillar to reach the brow region. Thestress situation in the more central brow region remainsunclear, since no consistent split-line patterns are observ-able. The variable situation in the canine fossa region indi-cates that it is a relatively unstressed area between three di-vcrgent stress systems, the infraorbital, zygomatico-alveolarCwst, and ascending circumnasal.

5. The chimpanzee differs from man in split-line patternsin several regions. The brow ar ea has continuous patt ernsindicating that tension from muscle pull operates over thewhole torus. This is probably because the braincase does notoverlie the brow region to assist in taking up the stresses.Since there is no elevated nasal bridge, the split-line pat ternsconform to the deeper portions of the canine pillar. In theniaxilla, ascending split-lines are uniformly deviated later-ally by the pull of the chewing muscles. There is thus no re-gion between strongly stressed systems in the face to corre-spond to the canine fossa region of man. The chimpanzeeinfraorbita l split-lines ar e not continuous with the zygomaticorigin of the masseter muscle. They are cut off by verticallines associated with the late ral orbital boundary. The dif-ferences from the human pattern are probably a functionof the relatively lower origin of the masseter muscle. The ab-sence of a canine fossa in the chimpanzee is probably asso-ciated with this st ress patte rn.


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SPLIT-LINE ANALYSIS O F T H E FACE 531

6 . Sections of human skulls show that most thickened areas

of bone are accompanied by strong split-line systems, indi-cating adaptation of the thickened structures to mechanicalstresses . notable exception is the centra l brow region, wherethickened bone shows no consistent split-line patterns. Herethe f o r m probably does not represent a response to mechani-cal stresses. This indicates that s tress resistance is not a pri-mary fuiiction of human brow ridges although in many pri-mate forms they undoubtedly become involved in fac ial s tre sssystems.

7 . The interpretations of split-line patterns made in thisstudy, and their anthropological applications, can i n generalbe evaluated through inst rumental tests, developmental analy-ses, comparative studies and experimental demonstrations.Because of the ready combination of split-line technique withthese other methods it should contribute much more to solu-t ion of anthropological problems.

I J T E R AT U R E CITED

A S H L E Y MONTAGU, M . F. 1951 An in t roduct ion to phys ica l an t l i ropology.Char les C Thomas, Springfield, I l l .

BENNINGHOF, . 1925 Spal t l in ien am Knochen, c ine Methodezur E r m i t t l u n gder Archi tek tur p la t te r Knochen. Verhandl .anat. Gesellsch., 34 89-206.

1927 e b e r d ie Anpassung de r Knochenkompak ta an ge lnde r t eReanspruchungen. Annt . Anzeiger, 9 : 289-299.

E R U H N K E , . 1929 Ein B e i t r a g zur Struktur der Knochenkompncta be i Quad-rupeden. Morph. Jahrb . , 6 555-588.

CAXEBON,. 1920 Contour of orb i ta l ape r ture in representa t ivesof modern andfossi l man. Am. J. Phys. Anthrop. , 3: 4i6-488.

COON,C. S. S. M. GARNAND J . B. BIRDSELL 950 Races. A s tudy of th e prob-lems of race format ion in man. Char lesC Thomas, Springfield, 111.

C U X N I N G H A M ,. J . 1908 The evolution of th e eyebrow region of t he fo re -head, with s pecial reference t o the excessive supraorbital developmentof the Neander tha l race . Trans . Roy. SOC. Edinb. , 4 6 : 283-311.

F R I E L , . 1924 Muscle test ing a nd muscle training. De nt. Rec., 4 4 : 187-204.GREGORY, K. A cri t ique of Professor Fred er ic Wood Joness paper :

Som e landma rks in the phylogeny of the pr imates . Hu ma n Biol.,

Mechanism of head in j ury a s s tud-

1930

: 99-108.GURDJIbN, E. S., AND H. R LISSNER 1944

ied by the cathode ray osci l loscope. J. Neurosurg., : 393-399.


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N. C . TAPI ER

HENCKEL,

Functional analysis of the facial skeleton

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