Talk:Paper - The early embryology of the auditory ossicles in man

TH F. F..-\Rl.Y I~'.MBRY()l.()GY OF THE AUl)lT()RY OSSICLES IN MAN‘

Illustrated in Atlas Series"-’

_IF.R()l\-IF. R. H.-\'.\'SOT\', M.S., BARRY J. AN'SOI\', l’h.D. (Med. Sc.) and THEODORF. H. BAST, l’h.D.3

AI\"I‘lC(‘l*Il)l*II\"l‘ phases of an oto- logical invest igation, carried out at .\'orthwestern I'ni\'ersity and the Uni- versity of Wisconsin dealt with develop- mental stages in which the otic capsule, the auditory ossit [es and the related branchial arches were well formed in cartilage.‘ .As a recent supplement to these more general discussions, three articles have offered detailed information on the morphogenesis of the otic capsule, the stapes and the incus (with atlas ar- rangement of figt11'es)."’ A fourth article, comparably concerned with the malleus, is in preparation.

It now remains to report upon the de-

‘From the l)epartment of Anatomy of the L'ni\'ersity of Wis:-onsin and the Department of Anatomy of North- western l'ni\'ersity Medical School (Contribution No. 640 from the latter department).

A stlld_\' carried out with the continuing support of the (‘entral Bureau of Research of the American Otologicnl .\'ociet_\' and of the _\'ational Institutes of Health of the l'nite<l States Public Health .\'er\'iee (Grant No. B. 2237). The cost of engravings was met by the use of the NIH grant.

Based upon a thesis presented by Jerome R. Hanson for the degree of Master of Science at the University of \\'is-cousin: enlurgedas a collaborativeinvestigation. during his service as Project-Assistmit on the above-named grant.

The drawings were executed by Miss Jean l\Ie.(‘onnell. The plmtomierographs were taken by .Mr. Homer Montague. labelled by Miss Rosamond Howland.

The authors are indebted to Dr. Hhafilt F. Ricliany who provided valuable assistance during the initial phase of the in\'estii.'ation.

Rec:-i\'td for publication Septetnber I8. l!):')9.

'-‘The illustrations w.-rt prepared from the following series in the ( P. |.. Hticeter ('ollection. ('arneizie Laboratory. Baltimore, Figs. 1 and ‘.2. series 6317; Figs. 3 and 4. 6517: Fig. .3. .';.')‘.l; Fig. ti. 652-1; Figs. 8 and ‘J. 1430; and from the T. H. Bast ('ollection. I'ni\‘crsit_\' of Wisconsin. Fig. 7. series 180; l"i«1s. 10 and 11. 10; Fig. 12. 174; l"igs. 13. H. 15 and lti. 138 Fig. 17. 168; Fig. 18. 29; Fig. 19. B 124: Fig. 20:1. I-"I; Fig. 30b, 2!l.\; Fig. 20c. H2: Fig. 2(!d. ‘JR.

.\Iagnification~i. reeonstruetions: Fig. 1. X 32; Figs. 3, 7., ti and 7. X 20; Figs. N and 10. X 18; Fig. 12. X 16. Fig. 13. X 21).

.\I:lL'llllit‘:tiil)ll.'-3. 5rhutontierographsz Fig. 2. X 120; Figs. 4 and ‘J, N 13!); Figs. 11. H and 1.3. X 81; Fig. 16. X 25: Fig. 17 X Figs. 18 and 19, X 33; Figs. ‘_’0a and 20 In. X 20; Fig. 20¢-, X 13; Fig. 20d. X 12.

“lleeeaseal January 23. ltlfiil.

‘I’:-rtinent articles already have been cited in an earlier issue of this journal (vol. 32. 19.38, pp. 157-172).

"Alison. Ii. J. and ll-ust. '1'. H.: llevelopment of the (Mir (‘apsule o. the Human Hair. Quart. Bull. l\'orthwe~itern I'ni\'ersit,\' .\Iedit'al School. 32:]-'37-17L’. 19:38.

.~\nson. I5. J. and Bast. T. lI.: l)e\'elopment of the Slaves of the Human Ear. Quart. Hull. Northwestern l'ni\'ersit_\‘ .\I(-dieal .\'('ltm)I. 332 -I-I--'19. ]95‘.I_

.\nson. ii. .I. aml Mast. 'l'.lI.: l)e\'4-lopment of the Int-us of the Human liar. Quart. Bull. .\lorthwestern l'ni\'crsity ml-Ilieal School, 33: 110-119. l!l.')9.

358

velopmental steps, taken before the stage of 28-mm., in which localized portions of the mesenchyma in the branchial region suddenly become distinguishable from the tissues of which they are primordially a part and, at the same time. assume an appearance and a position predictive of their adult form and topography.

In order to determine accurately the origin of the ossicles it is necessary to examine them prior to the aforementioned stage, beginning with specimens in which the future ossicles and the related branchial arches have not reached pre- cartilaginous constituency. In the past, investigators attempted to explain the origin of the ossi:-les without benefit of early embryos. In such studies. limited as they were to the use of en1bt‘_vos in which the branchial arches and their derivatives were already ('ompos(‘d of cartilage, conjecture inescapably took the place of fact in the attempt to account for the genesis of the auditory ossicles in man.“

Microscopic sections used alone are of limited value in following the rapidly changing architecture of the branchial arches, but reconstructions provide the needed aid in determining what contribu- tion the mesenchymal condensations give the developing structures.7 The follow- ing discussion will be based chiefly upon the use of such “models.”

In the fifth week of fetal life the stapes is not yet a definiti\'e portion of a

"Reviews of the literature on ossicular development have been presented by Fraser (1882). (lradenigo (1887). Dreyfuss (1893). (iaupp (1899). Broman (1899). Kingsley (1900). Fllelts (1905). Reagan (1917). van der Klanuw (1924). and (‘auldwell and Anson I19-I2). Translations of the German literature prepared by .\Irs. Eyeke Strickland have been used as guidance in this present study. A com- plete presentation of the literature on early embr_\'o|og,\' of the ossicles by Mrs. Strickland and the present authors is in preparation.

'-As the authors have commented before. only by the use of reconstructions can the continuity of fluct.uatin;: tissues and interrelationships of shifting structures be

established to the conviction of the writer and be pictured t.o the .“lIII\‘f:l('IlUl] of the reader.

blastemal lobe which is situated at the proximal extremity of the hyoid (Rei- chert’s) bar (fig. I). This lobe is grooved by the facial nerve. which serves to divide it into a stapedial primordium and the laterohyale, both of which temporarily retain h_\-oid continuity (fig. 2). In the Sl.t'i-mm. stage the division is more definitely marked. The portion which connects the above-mentioned structures is called the interhyale (fig. 3) ; it will give rise to the stapedial tendon.

In the earliest stages the two visceral bars are broadly connected by an inter- branchial bridge of mesenchyme (figs. l :m(l 3). the greater part of which is in the territory of the second arch as indicated by its relationship to the pharyngeal g1'oo\'e (figs. 3 and -I). This blastemal mass is located between, and continuous with. the middle segment of the hyoid bar and the proximal portion of the mandibular (.\Ieckel’s) bar. This circumstance indicates that the man- ubrium of the malleus and long crus of the incus are mainly of second arch origin.

In the 8-mm. specimen. which is more advanced in ossicular development than the 9.1";-inm. stage. the interbranehial bridge is separating from the hyoid bar along a sulcns which transmits the chorda tympani (fig. .3). Separation is complete in the 11.7-mm. stage. at which time a secondary connection between the arches is formetl as the incus. now of distinguishable outline. comes to approximate the stapes (fig. Iii).

l)uring the fifth week the stapedial artery attains a central position in the blastemal lobe. thereby seeming to aid in the cot1\'eI'sion of the rounded mass into a ring-like structure with an obtur- ator foramen (figs. 5 to 9). Occasionally a ring fails to form. and an anomalous stapes is the result: the l1ol'sesl1(ie-sllapetl stapes in a l-l-mm. embryo is an example of this defect (fig. 7).

The malleus and ins-us in the l.3..")—nun. embryo. although still precartilaginous. become demarcated as the mannbrium and long crus begin to assume char- acteristic form and as a gl'o0\'t‘ appears between the head of the malleus and body of the incus (fig. 8). At this stage the ring-shaped stapes impinges upon the otic capsule. The laterohyale and short

35'!

crus of the incus end in loose ad\'entitia (fig. 9). In the 17-min. embryo. both the incus and malleus are clearly iden- tifiable as chondrified structures. The malleus temporarily remains in contin- uity with the mandibular (.\leckel’s) bar (fig. 10).

The interhyale passes from the junc- tion of the stapes and incus to the hyoid bar (fig. 11) where the latter makes an acute angle to become the laterohyale (in contact with capsular tissue). The primordium of the stapedial muscle is identifiable in the 21-mm. stage (fig. 12) as an outgrowth from the interhyale. located posterior to the angle of Reichert 's cartilage and medial to the facial nerve (figs. 13 and H). The tendon of the stapedial muscle is derived from the interhyale in the portion between the developing stapes and Reichert’s bar (figs. 15 and 1(3). The stapedial ring now fuses with the otic capsule. the conjoined tissue thus becoming the lamina stapedialis. The periphery of the lamina. through (litferentiation of its tissue. will become the annular ligament. The zone of change is the site of the future vestibular fenestra (fig. 15>.

('ontinuity between the interhyale (seemingly the primordium of the sta- pedial muscle as well as the tendom and the laterohyale is lost in the ll)-mm fetus: at this stage the constituent cells begin to assume the appearance of muscle (fig. 17).

As the muscle (l(‘\‘elops and the incudo- stapedial joint is formed. the tendon usually gains an attachment to the neck stapes. Ilowever. the frequent partial insertion of the stapedial tendon into the incus (figs. IS and 19) supports the belief that the long crus of the incus. as well as the stapes, is a deri\'ati\’e of the second arch.

Both of the cartilaginous arches con- tribute not only to the s-ubstance of the atulitory ossicles, but also to that of related structures in the head and neck.‘

‘Bast. T. H.. .\nson H. J. and lticluan). 5. Ii.‘ The l)e\'elopnn-nt of the .\'et-<-ml Iiranchial .\l‘I‘ll ltn-it-In-‘:‘l‘.~ (‘artila-_vel. Facial ('annl and .\~Isu<'l:th-cl .~‘tru«-turn-_a III .\lan. Quart. Hull. .\'«-rtlmeste-rn l'nm-vsit) .\lI'4i{l'{|i Fclltml, 30: ‘.’1§.’>-‘_’l‘J. l‘.l.'-ti.

l{iclu|Ir\. 5. llast. T. ll. and .\nson. Ii. J ' The l)e\'elopnn~ul of the Fi:-st l{!‘:|ln‘l|l.‘Il \rch ll| .\l;nn an-l the |"ate of .\Ieekel's(':urtilaue. Quart. llull. Xorvlnu-~m-rtt l'ni\'ersity .\lt-tlical Scluml. ::o: Ilfil-'2.';.':. l‘.t'.I‘». 300 QUARTERLY BULLE'1‘ll\', N.U.M.S.

While the inalleus is undergoing os- sification, the zone of continuity with .\/Ieckel’s cartilage narrows rapidly (fig. 20a). This alteration in form is due t.o a retrograde change in the cartilage which precedes differentiation of the tissue into that of the future anterior ligament of the mallcus. As soon as bone is formed in the head and neok of the mallcns the anterior process (an osseous rod before bone-formation begins in the ossicle itself) fuses with the malleus (fig. 20b). Distally the cartilage will be

ll)S()l'l)(’(l in the inaudible.

l{eichert’s cartilage passes through an e\'en more dramatic series of changes in attaining adulthood. At its proximal e.\;tremit_v. that part of the cartilage which is (lcrived from the laterohyale of the

ea1'ly embryo forms :1 transito1'_v lateral boundary for the developing facial canal (fig. 200). By the 215-mm. stage, how- ever, membrane bone is developing internal to the -artilage-—thus interven- ing between cartilage and nerve (and stapedial muscle, derived from the inter- hyale). Concurrently, the otic capsule is ossifying. In early infancy, Reichert’s cartilage is merely a remnant within new bone, and the facial canal has acquired new and permanent walls (fig. 20d). I)istally the tissue of the original arch is converted into the styloid process of the temporal bone, the stylohyoid ligament and the lesser cornu of the hyoid bone.

The 2'llustratz'0ns appear on the succeed- ing pages. I-IANSON FT AL.~—.-XUDITORY OSSICLES Cml

Embryo , 7mm.

_ ‘3’-,__\ b msxcmsms CARTIIAGE gtapedlal 5;’ l \ ( ranc1}1a1 archlt) ”%t1aPedm1 - ' H ’ astema Faclal nerve iatemhyale (Broman)

Chorda tympan v

MI-:CKEI.’S CARTILAGE / _ _ (branchial arch I )

b‘ Artery of branchial arch II I

/ ‘ I ‘3tapedia.‘l

Stapedial a.’ b15'5t"3ma \ Facial nerve

iaterohyale

‘Chorda tympani

SUPERIOR

’"“Facia1 nerve C‘ L'°t‘3"°hY51Q' “\~B1astema1 mass for stapes ANTERIOR POSTERIOR I’ INFERIOR

Chorda tympani

Lateral View

Fig. 1. In the embryo of .41/3 weeks (?'mm.) the precursor tissue for the mulitory 0s.\'tt‘lt‘.s' is n enm- mon blastemal mass which is (listinynishable from the surroumflny mesenehyma only lu'l'IIItse of the greater concentration of its ('()It-S‘l'lllI('Il! cells (compare fig. 3).

A lobe-like portion of this mass is located at the ertm-ial end of the seemul riseeral lmr (precursor of Re'ichert’s cartilage), where it is yroored by the facial nerve. Part of this mass is the prinmrtlium Inf the stapes. However, it is still separate from the uyqregation of cells u'hieh will laeemne the ntie capsule (the latter not included in the reconstruction). The stupetlial urtery ertemls from its snIII‘('t‘- vessel to encroach upon the interiomerlial aspect of the l)l(1steIna ( f ('0. I (1); the (‘lll)l'(l(l t ym pan 1' lII'ltIl('lH‘.\' from tI1e facial nerve to pass metlial to the first riseeral bar, the forerunner of J! eel.‘el's eurtiluye (fig. Ill).

A mesenchymal bridge of cells between the first and second I*ls('('I‘(ll lmrs (’.1‘l('Iuls f/‘um the proximal end ofthe mandibular bar (at I in fig. 1e) In a point on the hyoizl bar (at Il)just distal to the l)l(l.\'lt'IIl(tl lobe of the stapes ( 1c). The prz'mor(l‘ia of the mulleus (tnrl lneus are not yet dist-inet .<.’rnrtures tl'itlIiIt this c0mlens(.ttion of cells (ehich will soon yire rise to them. 362 QUARTERLY BULLETIN, N.U.M.S.

I \\ Pharynéeal

KI , 4 .- ‘ roove I

‘ - ,. _

I « ~ - "' ‘$3

\}.,_Je.;'.-?_:fl1‘::‘.’¢. ~‘ . o ..

9'.‘ '%=.°3:'.5f°..C. \ ..ln. . -

I"i(/. J. .-l.\' is (‘tear/y (lmna/mtmtml in .s-m'tion,s, the rommon blastemal lobe is situated dorsal to the _/irst phary/tyml yramw, ll‘/l(‘I'(’ it is indmtzltt by th0far"iaI )wr1'0 on its d0r.s'al(1t(’ral aspect. I t is through the /n‘t'.s'('u('(' of the .s‘u[(‘I(.s' [I)‘(IIIlt('(‘(/ by the nerve that the followingforeraxt may be made: the metlial amt laryor part of the tube will _r/tm» rise to the .s'I(l[)(’.s'.' the part lateral to the nerve will become the latera- hyal('.' tho L'n.tm'(:m:rwrtiuy /mrtiun, u'h2'('/1 is still rcry broad, will b(I(/mm’ the inlorhyale. The 71.arrou'in_(/ of the intt-r/1yalr' amt .s-ubscquont .x'('])ar(lti(m of [hr stapotlial porlimt from the Iaterohyale is seemingly -inflrtcnt-mt by (hr [)l'(‘.\'.\'IlI'(' of thr facial mrrro, as this ma.s-.- im-:'('a.s-e.x- at a more 1aup1'd rate than does th.(* H(’)‘l‘(‘ 2't.x-1'1].

The p)‘('.s'(.'I1(‘(' of thv .s-tapmlial artvry zirlmtll/its part of tho q0m'raI blastema as the “stapes"; it is still snl1'(l, not having yr‘! aw/uirml thcfarm of an annul(’t. 7'/mfI1tI()'(* p0.s'/ition. of th(' artery in the ('(’II[I'I' of tho ma.»-.s a,ppmr.x- to be ([110 to a f0Id~z'ng around the arty,-y as the blastema enlm-gcs, The /II'()('('.\'.s' s('(’III.\‘ to bv one of ('ru'o[n/nnm/t of (ht! ar'cry by yrmrth of the ma.\-.:«- rather than one of ])erf0rat2'(m I/I/‘ally/I ('/mI_.//atirm of the 1'o.s‘.s-('/.

.1! this .s-tayv tho blastoma may be .s-air/, for the _/irxt (mm, In ;u).s-.sc.s.s- a (list'iII_(/‘rlisllable boumlary. I It th(- h‘7,,/,3-/uni. .-tayv thr» ti.x~.x-Ia‘ fit the ('0r/'('spaIuti-ng branchial arm is a .simpl(', tliflttsc I7I('S(’It(‘hynl('. HANSO;\' ET .-\L.#AUI)ITORY OSSICLES M3

Embryo , 9. bmm.

d. REICI-lIER'I"S CART ILAGE (II)

/ ,, Blastemal nmésczs:

~Stapes ‘:Ma11eus. , mcus

Latevohyale \

Interhyale ~’ T

\1:‘ac1a1 nerve

t§r£r21joSuper'1or View a»

MECKE/3L'S cA1mLAeEl"‘

b. I

Triqeminal nerve

Facial nerve- iaterohyaie

“lncus , malleus

I Postszromfczrior View ‘

SUPERIOR

Triqczminal nerve

\

Pglastema of 1ateroh_\,u1c

PO ST ERIOR \Fac1a1 nerve

Latgyal Vigw | Bleziétema of maueus , incus

INFERIOR

Fig. 3. In the embryo of 51/3 weeks (0.6 nun.) the branehial slrnetures have inereaserl greatly in size through hyperplasia of the znesenehynzal cells amt the outlines of the ossieutar prinmwlia hare become more clearly defined.

The second visceral bar projects farther tou'ar(t the otie labyrinth than /he first anrl has at its pro.ri- mal extremity the blasternal lobe with the ])I"tIlt()l‘(llltIIt of the stapes. 'I'he _,"aeial nerre has groom'Il the lobe more deeply, the su,lcns serving to mark ofl more ctistinetly the stapedial from the laterohyale portion. The primorzti um of the stapes is still separated from the otie capsule (not shown) by a zone of cells which is less dense than the /nesenehyme of either the sttlpes or the capsule. The general location of the stapes (lateral to the otie capsule and medial to the faeial nerve) is one I('h.ieh persists throughout fetal life and into adulthood.

Although a stapedial artery is not yet centrally l0('(Il(‘(l within the blas/ema, an artery is fonml (not shown in the reconstruction) at the periphery of the lobe as it iras in the 1‘-nun. stage.

The malleus and incus are not yet tlifferentiatett from the interhran.ehiat bridge of mesenehynu- which now broadly connects the two visceral bars aeross the first pharyngeal groore (jig. Jr at arrmr). Since the groove is the boundary externally between the arehes (I am! I I ), rlmlble origin of the two ossicles is thereby indicated.

The chorda tympani, branching from the facial nerre, passes from the hyoltl arch (I I ). in_t'eri¢.-r to the blastemal mass, into the mandibular arch ( I L It appears as if the nerve were playing an im- portant role in the separation of the blasterna from the serond I'isceral hm‘. 304 QU.-\R'I‘F.RLY BULI,F.TI‘.\', N.U.M.S.

(V

___4__-— Glosso—

e a].

nerve

Second vxsceral

bar

Triéernindl

I1€I"V€

Fig. 4. The first pharyngeal groove makes a deep impression on the lateral aspect of the blastemal mass zrhieh is established as the primary connection between the first and second visceral arches. The first visceral bar is located closer to the groove than is the second bar, meaning that the major portion. of the cellular mass belongs to the second arch.

Although no longitudinal grooves or diflerences in cellular density exist to distinguish the pri- mordium of the malleus from that of the incus, the relationships to adjacent structures and the course of the chorda tympani (about to branch from the facial nerve) are indications that this mesenchymal mass is the source-tissue of both of these ossicles.

.-is seen in neighboring sections, the chorda tympani, after branching from the facial nerve, passes medial to the pharyngeal groove and into the first visceral arch, where it joins the mandibular branch of the trigeminal nerve. The pathway of this nerve from the lateral to the medial side of the blastema serves to separate the primordium for the malleus and ineus from the second visceral bar. A large portion of this mass is located on the second arch side of the pharyngeal groove, suggesting that most of the primordial tissue which will develop into the manubriusm of the malleus and long crus of the incus comes from mesenehymal cells which are of second arch origin. This contradicts the usual belief that the malleus and incus are derived entirely from the first arch, H.-\.\'SO.\' ET AI..—AUI)lTORY OSSICLES St-5 Embryo, 8 mm. d. RI£ICHER’I"S CARTILAGE

, Hyoid arch

/ \

I Int why 81 Q ll 1§‘ac1a1 nerve \\ J,

V "l '\ ‘blastema of

malleus and lncus

CAR’1‘ILAGB' ' tympani ~«

Mandibular ..\ V _ A nerve,/L Tnqemmal nerve Anterosupenor vlew Facial nerve-. . ' Hyold ‘ '

Chorda tympam -

I

Blastema for :1 malleus and incus

Triqemlna -—- C. nerve I SUPERIOR ~ “ _IriQemina1 Mandibular nerve‘ nerve ‘ J POSTBRIOR

Anmuon ’Stap“3 Ivlalleus and incus :Int"“rhya1"’ iaterohyale Facial nerve Hyéid INFERIOR

Fig. 5. In the 8-mm. (5-week) embryo branchzfal devclagtnzent zs athvtrtcert over that of the -.'I.t2'-mm. stage in respect to the followirztg: deeper entry of the stapcdzal artery veto the .s't(Iped'tal aulage; _t:_1rth‘¢-r separation of the laterohyale from the stapes by the faeml nerve; (l[IIIO.s't complete rupture of contuuuty of the intterbmnchial bridge adjacent to the second ms-coral bar (througlathe lIt.fl'lt(?7lC€ 0} the chnrda tympani). Ifhether the 8-mm. specilnen is adranccrl, 07‘ thc .’t.f>‘-nun. spcczlncrr ‘cs retarded zn I[¢'t‘('[0p- ment, cannot, of course, be deternn'm-rt.

‘m(. QU.~\R'l‘F.RI.Y BULl.F.'I‘I N, N.U.M.S.

Embryo , 11.7 mm.

d. REICHERT '3 CAIIUILAGE

‘ I . ,Int<2vhya1e

V’ ‘ /Laterohyalc

— Stapczs ' {nous xMa11euS

if/\ntc1»osu1)ic2.Jr1_oh1hr vLe_v\r I g MECKEUS CARTILAGE

Blqstema of hyoid cart11aQ<2

b.

Latevohyale

Postcaroinfczlrior View I

lncus, shgnrt crus (related to capsule) POSTERIOR

Ma11c:us\ SUPERIOR ‘ “

Remnant of conmzctmon, arches I ,I[

INFERIOR

I Lateral viéw ;

l"[r/_ (I. I)r’.~‘pi((' !Iwf(1('( (ha! [ha bran(:hi(1[ ol_¢'n1('n.’.s' )‘(‘III(liIl nuns-(’n.('/1.1/n1(1l, .s'em'ra( (l([I‘(lIl(‘(‘.\' have I,”-,, ',,,m1pI;mp(w: (ho .‘).6‘-mm. .\'I(I_()('(lm1 the .s-pccznzm of I 1.? 221.211. ((2'_u'eo/vs), the latter here villustrated. 'I'/ur .-lnpmlial (1I‘[(’7‘_I/ has (rI:a11.{/ml lhc .s-tap('rI*1'r1l anI_ag('.1nIn (L (mg Il‘llIl the formalzbn of an obturator fmwnmz (fig. 6(1); (ho inI.orl1_1/al(' has I‘(’_(]r('.\‘.s'('(I tn. _.s‘zz({, ozrmg [().[)I'('.s:.5‘ll7‘(.' pf llw fac1'al_ nerve,’ the .\'I(I[)(‘(1I.(II m1.I(1.ge (mt? l(1I1‘roh_:/(1142 I)v(:q)n(* nmre (l('_.fI‘IlrI((’ p(1r!s of [has _sl1lI vn1z*n1b'rnm)u.s v'z.s'c:cr(1I l?ar,' 1}“, p,«i,,m,-y (:(m[iu,m'[_:/ b(’!u'(’('I1 (he m..s~cm'(zl b(1I'.s' z.s _Io.s't as a sepuratum of the uc.‘_crbrarz.cI1zal brulgo from the .s'e('.(n1.d vi.s('('2'al bar is (-mnplctcd through (ho 1/1_/I u('_n(-v of the chor(la_ Iynzpafrz (compare fig. 5); (L qrrmw in the III-(‘.S'(‘Il('}I._I/Hill] m(1.s'.s b('(11*v(»7{ (hr: (l{‘C}I(‘.S' (SUN x.‘0IlI”('Il 110118 11:1/)1 Ih(_' firs! 2'-z'st'e1'_(1I bar) has I[(‘l‘(’[0])(.’(I at riglzt angles to (he first bar, mznnlzng scparalzon of the vrmzordzum of the zncus from that of tho maylleus (fig. (ic); 41 scconrlary contvinzz./il_11 b0Iu'cm the firs! two arches is established as the unlugn of the incus np]n‘o.1‘1'7n(1!v.x- the .s-tapulitzl ring, (1 r(*1(IlI'0n.s'In'p zrhit-I1 .\'(‘I'I'('.\' in 27:19:/Iify the rapital par! of (he przTnu'!1'mz s!upo.-. H.-\NSO=.\' ET AL.——AUDITORY OSSICLES 31»,

Embryo , 14- mm.

R1-:1CHERT’S CARTILAGE

d' Hyoid blastczma

Stapes

[Ant.erosupev1or view!’ ,

M13CKEL’s CARTILAGE

b.

—‘Stapr2s

“1ncus,ma11euS

I

SUPERIOR

• Ma11s2US

«stapes

ANTERI OR POS ‘lnterhyale h II . CO1EI:§;I:1aOI;t Caamtrc has I - II I N FE RI OR

Fig. 7'. In. the embryo of 14 mm. (6%; -weeks), although general advancement in development is evidenced, the stapes has not attained the form of a ring; it has been grooved by the artery, but has assumed the form of a horseshoe. This observation would indicate a congenital origin for the exces- sirely thinned and malformed ossir'les frequently seen (luring emlaural surgery.

The interbranchial bridge remains on the right side, but the continuity on the left side is broken: only a remnant of this connection remains (fig. 7c). The interhyale is a rather prominent link between the stapes and the hyoid bar (fig. 7a). A groove has begun to separate the malleus from the incus, but the latter is not yet in contact with the stapes. .‘»!»8 QUARTF.RI.Y BULLETIN, N.U.M.S.

Embryo, 15.5mm.

a. .REICHER’I"S CARTIIIAGE Hypid arch ,’

I

Laterohyqlz

llptczrhyalz ‘ _ Stapes

_/’mt«21rosup<2v1'o1» V1<ZW I

b.

SUDERI OR Incus

-I\/lallcus POSTERIOR \ .__ ~: I 7 ~ Latczrohyalcz . \ Lateral vxew ’ 11 Inmyhyale INFERIOR I"i_q. 5’. I N an. 4‘III’II‘_I/I) of 1.5.5 mm. (I11/3 lzw/rs) the annular .s-[(1 pox ron1(1iu.s in I-unflnuily u*i(h the /:_I/uirl hm‘ lhrmzglr I/In nmnbrtm.nu.x- I',II[(‘)‘/I]/(![(' as a ('()lIII‘(‘('[‘l..II.'(/ liulr. .-ll!/mug/1 r('du('mI SONI€Il’/N1! ‘in. //I'i('/.'m‘.s'.\', I/:1‘ -I'n,lw'lI_I/«(Iv /ms 7'Iu'2'('II.s-ml in ('oIIulm' (l¢+n.\*[l_I/. Only 4: .\‘/1'_(//I! I/n'('/.‘«'Iu'n.g in UN‘ /u/(ml hm‘ r(')n(11?ns I0 so/‘1'(' as u ronn'ml('r of its primary mrztinztily

will: //I(’ mumI2'l;ulm' lmrs, (hr I»I(1.s-(('m(1I IIl(I.\'.s‘ ii.»-(‘If I1(w1'n_(] rI1'fi}'rm1II'((I¢'(I inlo mcscn.c}1_1/mnl “m0dcls" of //I(’ IIHII/(’Il.\‘ mu! ’l'I1('Il.\’. ’I'Iu' pm-ilinn of HM’ c/mrda t1/mpaluf sorlws (0 remlor tI'isl‘in,gz12'sImble the prinumliul ];(II‘l.\' for [he nu1n.ub/"1'Iun of !/Io mallous and (he Iouq crus of [ho 'l'n(‘I1s,' they are not -yet Ir/mlly .s-4'/mrrxlv, lm-(ruse lm).s-oI_1/ urr(m_r/(Id m('.s-cn.cl1_I/mt: brirl_(/('.x- the _(/(lp be./uwcn. the more densely /nu-km] st/'u«:[ur('s. '1'/u’ bmly of tho 'zIn.(-us mu] the /mud of the m(tI[('u.s' are not lzzklologically scparaml, bu! (1 rirvunIf('r¢'I1I'I7(Il .\‘Il[(‘Il.\' is iI:rl1'(:r1(I'z'r- of [/10 s17[(* of I/u*fu[ur(* 1'I1('mIunu1Il('0lur_jm'nl.

A .\'(’.(‘0)1(/(l)‘_I/ ('m1.!im1-I'l_I/ I)('((1'cm I/10 1'1'.s‘('('r(tI burs is m(11'-n!aITno¢l by [he iucmlostapetliul 1111-1701». 'I'/:(' IIl(’.S‘PI(.(‘/I]/lI((l[ (wlls of tlwir (mlu_(/on l;l(*ml in su('/I (1 n1a1nm' as [0 ub.s-1-urc (/10 mm'gin.s, u (.'2'rcu.m- .\'/(lH('(’ 11'/rir/I .s-up/mI'!s (hr conrrpl (ha! (ho long crux of the 'l'I1(‘Il.\' and [he .s-lup('(I7'IIl ring (IeI.v*l0p from a rmnmon. sourrv, mu/u'1_1/, I/Iw .\'(‘('IIII([ I'im'('/‘(II nrcll. HANSON ET .-\L.——AUDITORY OSSICLES 369

Anterior 15-5 rnIn-- cardinal vein.

Auricle \\ (prirnordiuxn) ‘

. '. /. :~°‘:5‘\- Otlc Ca sule

Fig. .9. The precartilage stage -is reached in the embryo of 15.5 mm. (7 weeks). However, only the distal portion of the first and second visceral bars (n'e so adeanced; the pro.cirnal portions of both bars and the prirnordia of the ossicles are still com posed of condensed mesench_I/me (more compact than in the previous stage). The stapes is now definitely annular in form, with a distinct obturator foramen whose margin is separated from the contained stapedial arterg by a less dense zone of Ines- enchyme.

The crura of the stapes are round in cross—section. The stapes itself is annular. The medial part seems to be imbedded in the precursor tissue (precartilage) of the developing otic capsule. It is as if this annular collection of cells, by pressing inward against the otie capsule, were causing capsular cells to condense in the territory of the future lamina stapedialis ( which will contribute to the base of the stapes and the annular ligament).

The facial nerve passes between the laterohyale and the stapedial ring just above the level of the interhyale. The laterohgale and the short crus of the ineus (that is, its Iuesenchymal forerunner) end freely in loose mesenchymal tissue; they are not get in contact with the otic capsule. In the period between this and the preceding stage the i nterh gale has undergone further decrease -in size. 370 QUARTERLY BULLETIN, '.\'.U.M.S.

"(wk (17mm.)

3, Sxte of upper attachment of

Re'1c;hert's cartilaqe (lat. hyale

1’ ~ mqlon) rt/S cartllaqe

Rciche

stapedial muscle ' (interhyale region )

Oslsificatxon center of mandible

Mecke1’s cartil/aqrz . M‘ ‘H V '*' " ‘ L‘ L ~-Ossificatxon center (mandible)

Posterainierior View

thy. I I). I1’ _I/ the 1 7-mm. ()"-week) stage, the mandibular and hyoid bars have developed into cartilage through the q/‘eater fraction. of their length, and hence may be properly termed Meckel’s and Reichert’s carttlagcs. The prozrimal e.rtreImt_I/ of each. bar and the ossteles are still composed of precartilage; /IOII‘(‘l’(‘,I', the malleus and turns are more rtistinctty outlined ~— and less a part of the mesenchyma

from urhich they were deriI'ed.

The taterohg/ale and the short crus of the incus approximate and fuse to the under surface of the rleI'etop't'n_q capsule between. the 15.5 and 17-mm. stages (fig. 10a, at reader's left). The secondary nature of this c0ntin.I1,it_I/ with the (tevelomng capsule means that the capsule makes no contribution to these prz'numI2'a.

C'on.¢~urrentty, oss2'ficatt'on centers for the mandible appear as membrane bone, one on each half,

_t'orrm-:1 at the distal end of the mandzbular arch. HANSON ET AL.—AUDlTORY OSSICLES 371

R1E11<c1HnERJr’s 17 mm’ CARTIIILAGIE

(Branchial arch H) Facial «Va ‘ nerve « T

Fig. 11. Although the interhyale has become considerably narrower in the embryo of 1?’ mm. (7 weeks), it still serves as a definitive connection between the visceral bar (Reichert's cartilage) and the stapes. The second visceral bar may be regarded as bifurcating at its proximal extremity, the diver- gent parts continuing as the laterohyale and the interhyale, with the facial nerve passing over and between. them. The laterohyale is now continuous with the capsule and the interhyale is (as it was from the beginning) continuous with the stapes. The size, shape and relationships of the interhyale have changed markedly since its difierentiatioen from the blastemal lobe of the 7-mm. embryo; its mes- cnchymal composition has not yet been altered, despite the fact that it will soon assume the ‘role of tendon for the stapedial muscle.

The otic capsule, continuing its increase in size, shows transition from mesenehyme to preeartilage to cartilage. The stapedial impression into the capsular wall is thus increased by diflerential growth, but the division between the annulus stapedialis and lamina stapedialis remains apparent. The cells of the stapes, however, blend with those of the incus, there being no articulation as yet at the im-mlo- stapedial junction.

A small stapedial artery (arrow) is seen medial to the stapedial ring; it can be traced through the obturator foramen to its source lateral to the ossicle. 372

QUARTERLY BULLETIN, N.U.M.S. Embmyo, 21 mm. 6‘ /Stapedml muscle - __Laterohya1e Q, _ —Facia1 nerve L l‘ \ lncus Chorda tympani \ MGHQUS

ll/[\nLQ1‘OSUpC V101» vléw \

b Latcro\hya1c2 MECKEVS CART“-‘AGE 6tapeq1a1Lnusc1c

D

jg, H . Maneus . \ H‘)/o’1d cartilage

’‘ \

/

r ..y, ‘ .1_

LPoSte1*omEer1or vxcw

SUPERIOR C’ Malleus ’l.‘DCUS érél Choyda tylnpant :‘ Facial nerve. ANTERIOR ; POSTEMOR .. \ ‘Sta cs 11 / . \Stap<z§1a1mu5c1«2 Latcrohyalc

INFERIOR

I"'i_t/. 1...’. I n the wnlnyo of .31 mm. (71/3 Il‘('(‘/Ls‘) the ()ss'i(‘l('.s' are almost entirely cart‘ilag'tn0us. The m(1mI'ihl(*, fornml -in nm)1ln'mu' bum’, is now I wo-thirds as long ax the br(m('h1'r1l arch wh'tch it will ullimal('[_I/ r('pl(1(‘c.

A (‘omit’)/.s-(1I't0n of cells «lor.s-mrml'i(1l to tho _f(1(:‘£nl ncrrc can. be lI'(l(.'('ll u.pu‘ard to join with the inter- hyalv, u'}m'¢' the latter (toparts from Ifei(-lm'('.s- cartilage; the zmcrhyale, in turn, is connected with the stapm at the point of its future arm-ulrltion. wit}: the stapes (cmnpare figs. 18 and 1.9). This con- dcn.s-atitm. is apparently an outgrowth of me.s'(.'m'l1_I/Inal cells from the interhyale, hence, the primordimn uf tho .s-tupmliul musclt’ (thv 'intr'rII_1/01¢» -it.s'('If tlemllopirzg into the temlmz. of the mus('le). H.-\.\'SO.\' ET AL.—AUDITORY OSSlCI.F.S 373

2 8 mm. Tensor tympam

I"'i_(/. 1-)’. At a ppro.L'iIm1tel_I/ the J8-mm. stage the embr;/o ])Il.\'.\'('.s‘ ‘into the fetal phase of its 4leI'elop- mental histo/'_z/. The ettrtilaginozts os.sieIe.s' hare beeome elearly pretlietiee “mo(lel.s-" of the mlult -S'lI‘ll('- lures, e1‘('t'])l for the retention of two embr_I/orrie feature.»-: the broad eonneetion between. the mull:-u.» amt JIeekeI'.- 4-m'tila_(]e and .\'(’[)(II'(I((‘IH'.s'.\' of the (Interior ]))‘I)(‘(’.\'.\‘.

Stapedial " Otic

muscle - A

Qprirnordiuny)‘

   capsule

.4." '

éu

.u g '.

Laterohyale

l"1'y. 14. By the ,3 -mm. .\'l(l_(](' (81/2 week), the ear(ilaginmt.»- “mo(Iel.s-" of the tI.\‘.\'it‘l(‘.s' I'('.\'eInl:le mlult .s-trueture.s,' formatiee .s-teps in (lerelopment hare been talcen amt prtmitire t;rr1neh1'al relation..-hip.» have been lost - ~ the latter faet .s'ere-inq to em pt1a.-‘ize the importanee of xtmlying very young ('IIIl)I‘_I/(M.

The suleus zehieh will l)e(:ome thefaeial eanal is partially ('l().\‘('tl laterally by the laterohyale (pro.riIIml /)ort'ion of Re'iehert'.s‘ eartilage) where it fll-\‘(‘.s‘ with the otie eapsule. Thi.s- portion of the eartilagI'mm.- har 'I'€lll(tl7t-\' an important parietal element up to the ;!.£-u'eel.' .s'IIl_(/e, when -it hegI'n.s- to l)(‘I‘0Ittt’ {neur- porated in the lower part of the (le_/init2'1'e faeial eanal. El'(‘IIlIlIIll_l/ the entire luterohyale t;eenme.s- ()8-s"ifi(’rl amt loxex ‘its identity in the wall of the adult fueial eanal.

The primordium of the .S'l(l[)(‘(ll(Il mu.s-ele oeeupies a position in the .s-uleux u-hieh l)e('nIIte.\' the faeiul eanal, lying medial to the nerve. The ea/cal is relatiI'ely small and the eontents are elosely ('()It_llIIt'll therein. (see fig. 15 for the oppo.s-ite, or o.s-.s-iertlar e.1'tremitg of the .\'l(I[)(‘Il'l(ll musele).

The tympanie eaeity is e.rpaml1'n_(/, from the lateral end of the (1 u!litor_r/ tube, to imvule the prim itire mesenehymal tissue. The tympanie -mueous nzembrane will lure.»-t the 0.\'.\'l('l('.\' amt eorer the entire contents of the middle ear as the eaeity enlarges.

The ehorda tympani ln'anehe.s- from the fuel"! nerve below the laterohyale amt eou1'.x-es upzrarrl to pass betteeen the man uhrianz of the malleu.- am] the long eras of the ineu.s' teom1;m'e_tig. Id) and .ll:rre. from into lllt mam!/itmlar areh. 4 Ql'.~\R'l‘F.Rl.Y Bl.'I.l.F,'l‘lT\', N.l'..\1.S.

Co chlea

Stap edial .~ ten&on

T (inte rhyale)

I"i_(/. (:3. At a more cranial lerel than that .s-houvn in Fiqure 1.4, the 'iu,terh.t/ale (lemlou of the stapezlial muxele) m.~'ert.~' into the head of th.e stapes (where the articulation. will later be formed). The course of the .s-tapedial muscle can now be traced from -its source in the prim'iti1'e facial canal, to the angle in l.’eielIert'.s' cart,ila_(/e where the laterohyale be.giu.s',' then u pecan] and forward as the lateroh_I/ale, through the loose meseu,ch_I/me of the mirlrlle ear; _/inally to an. iIiserti()u into the posterior aspect. of the heat] of the stapes. At the stage here illustrated the primortlium of the .s-tapedial muscle aml its tendon are still in approxiniation with Iteichertls cartilage.

The stapes is z'mp'ingtug on the cart1'lagiuou.s' otic capsule. This area of compact tissue ('_*) serl'e.s to '1'/lelttif 1/ the developing lam,iu.a .s'l.ape(lia.lis at the site of the future I'e.stibulaI‘fcneslra (oval u'iml0u'). The ea psular t17.s-sue contril)utes to the base of the stapes and to the annular ligament, by which the stapes will be held w27tl1m the capsule in later stages. The vestibular aspect and feuestral surface of the base will retain this ea‘rtilaqin.ou.s' lam iua throughout the 'imli1'i(lual's lifetime, as u‘17ll, lil.'ezrI'.-e, the periphery

of the feuestrtl. H.-\.\'SO'.\' ET AL. ~—.-\UDITORY OSSICl.F.S

$4 ‘Jo

FETUS, 81/2 WEEKS

O Dural venous >...—-—;|--- Sinus

I 4

Meninéeal — » P°5.t°.r'i°r1

tissue ’.,,.»sern1:1lr‘cu ar Can Otic C2aP5ul°' ,_.—‘-—Utri.cl.e cartflage C y _. _L Laterpl ‘lafclurn ‘*5 V1.3’.-' , :‘ 5ernic1r*cular‘ V2st1 u are ._ ; ‘-1. __ ___ duct

9}/’ Periofic tissue ; .‘

(future scala) J‘ l ‘ Cochlear d'L1Ct’;" Auiitory tube '

— -'- Facial nerve

Ectoclerrnal

3 plate for

Intern_a1 eXtQ1",n‘3l Pteryéold —’ . acoustlc meatus

muscle _ ._ I . .- . . Ml1EClKlEIL°5/ 7 l 1 F l . g “‘g;§f§§;°,,_§';§‘j5

CARIIIILAGIE » ’ ‘ ' - center)

(Branclnial \‘\

arch I )

\\ Intecjument

I \ Manclible Masseter muscle I"i_I/. I I-'. llllwr in1pm'tanl ('l1an_qr'.< l7(II‘(’ taken [)lm'(' in [he .38-Inm. _l'¢'lu.~'. Tilt’ 4-H/'lila_a¢ of flu’ nliv‘ (’(l]).\'Hl(' is tlworgrmizing armuul l/14' (‘IulI)l_l]IIIplI(Ill(' .\'[i(l('(‘.\‘ lo lporonzr pvrinlir li.~*.-m-. frnm Il‘lIl('lI ll/4 future I'c.s-lilml(n' aml fympaniv .s-ralar will be fornml. 'l'hi.~' [II‘I)l'(‘.\‘.\' has I"'”!lI'I'-'.~'wl nmrr I'(lI".'”!I in the )'(’{]l0ll of the ('l)('lIl(’(7 lhan. i H the ('IIIlflll('Ill!lI' l'(’{]I‘()II. TIN’ .~'lII]u'Ill(Il I'IIIpI'¢-.-.-inn on tho mp.~'ulm' wall, l)(’lIl‘(’(’.'! capsular arul tympani!‘ .s-I1l>4l1'I*i.sir)I¢.~‘. 'i4l('nli_Ii¢’.~‘ Ihc rl('I'¢'lnpin_(/ lamina .~'lapuliuli.- (4-u,.-- .-ular twnlrilmlimt of the base). 'l'h(' (I(l/lil0)‘_I/ lube is e.rlerulin_q -into (hr region of (In fulurr lllllllllt‘ our mwliul In (hr nirnutlr/‘irlrn of H11’ III(Ill(’ll.\',‘ ils mm'0u.s- IIl(’lIIl)I‘lIl.'(’ (Pill (has ('.l'[)flI.'(l (I) h¢'('0m¢’ (ha! of (he l_I/mpllllil‘ ('m'il_I/. JIo('kr*l's ¢'m‘lila_(]e mm‘ lim nmlial to (ho 0.s.~‘ifyin_(; mrzmliblc Irilh [ho (‘l0.s('l_.'/ u.s.~'m'ialwI nay.‘-.-«hr IlIIl.s'('l(’ and lateral (.0 [he infernal [)l(')'i_(]oi(l mu.~*('1('. The ('m'lil(Igin(m.s hnr pu.-.-r.s- up Imrunl llu mull: u.~'. u-ill: u‘hi¢'h. i.‘ is still broadly ('IHIIl('l'll‘(l (fig. 13). The anlcrior promss (no! seer: (I! this lorel) aml l_I/I)I[)(1Ill(‘ rinq ma/cc llzwir _ti>'.~'l (1 p[u‘(II‘IIIu'( ul lln'.- stage’ in llu’fo1'm of 0.\‘.s'iji('(1li0n ¢‘ml('r.s of mr°n1bram' born‘. The (’('l0tl(’I'I)I(ll platefor (ho (’.l‘l('l’II!Il (uwuslic mcalus apprmu'h¢'.- the lalrml (I.\'p(‘('l of llw nmnuhrimu of the mall¢*u.s', u‘lm'c it will um’! llw mm'(m.s- mmnbrmze of Me’ lympauit‘ ('(II‘ll[j aml flu’ inlarrrniny l‘0IIll(’('llI‘(’ li.-.-Iu’ (4lm'im-cl frmn llw nu-.x-mu-l1_:/M111) In furln the lympm:.i(' IIl(’llll)I‘IlIlf'. 37!, QUARTERLY BULLETIN, N.U.M.S.

FETUS. 9WEEKS MIDDLE EAR

40 l11l1f1- Tympanic cavity {Auditory ossiclczs

Chor1drocre\n1'urn—-—_—‘—‘/

EXTERNAL EAR

~ //’AUPiC1€

.- ,/ /Cartila 2

. ' ///1VI<za’cus%pri1nor<iiurn)

~. \ Dural /

venous sinug

.»_.

INTERNAL EAR //_é_

Ohc 1r:\by1‘inth«€—/’—:'

per-iotuc 1aLbyr1nt}1’ ;

(hltmu veshbule) / Otic capsule/’

• _ RE LATED

STRUCTURES

\:‘\——Condy1g

‘ ‘~- Coronom process

RELATED STRUCTURES / '/

M k 1'5 C / / Z ‘ I ‘ '‘ ‘‘‘—~——Pa1att2 ekfditoilyltgg: ////,’ \-' ‘ _ k » ———— ——Body ogntandible pharynx. (ndfiem —/” /' . «- ““—'*" N<‘15<i1C01’1Cha , _ / [/1 fl I’/// ‘\ Intcguznentum Narwzé Nasal septun1(car't11aQe)

I"i{/. I 2. I u //I('f('lIl.\' of ,5!) mm. (fl :1'z'r'/rs) all -\'[I'IlI‘(I(I'(‘-N‘ u'lI1',r'/I will (m'umo pm‘! of, 0/‘ rc/4114-rl In, .’/Iv rm‘ of [/10 mlu/I, are /m'.s'z'n( in u slulr of /'«:m_(/m'2(1l)I«' 'I.Il(‘I'/)l'(.'Il(']/.

'I'/In ('uI'Iil4I_r/1'/mus 011?‘ ('1:/;.s-rile is u [))'(’().\'.s'(‘0I(.\' (’[(‘lII('II( of [/l(' 1'/mmlrorrrmium; mil/Iii: [1 HM r('lir-ulrtr I/.-wt‘, Irhir/I /'('pIm'('«I /n'('('tIr(i[u_r/(I, /ms in turn Imw 1'('.wrh('rl In pr0rlu('¢' I/11' (‘II/l‘I'(‘l)lIlIIIlIIl‘!'(‘Il/I'II{[ N‘!/.\'(l’IIl of Ila‘ pm‘/'/.1/m p/m./ir (pm'in[i<') /(I’)_I/I"l'Il(/I. Lmlqml wil/n'n (his [(1')]/I't-Ill/I ix ./lml uf('pi!/u'I1'u 1//u'(.~', /II!‘ rm/oI_I/m/;/IrI{1'(' (otir) (r1l)_I//‘/HI/I.

'I'/u’ /mrls nf /In’ mlull (‘.I‘(("I'Il(I[ mu‘ are I‘(‘[II'(’.\‘(’Il.[(’(I by [I'm uuri('I(- (in 11'/lit’/I ¢'m'(z'Iu_(/0 is /zlrcmly «/w'r'In/n'n,(/) uml by //I(’ priINm'4Ii.rIl slul/.‘ of ('rImI¢'rm u‘/I1'('/I, upon I‘('lIl'l‘(lIl{[(‘III(‘I(/ um! ([1.-WI/)[)(’(II'(lIl('(’ of I’('H.\', :ri// I;rm;m' (ln' ('.r(w'u.u( m-uu.s'!ir NIWI/Il.\‘.

I u I//r' m/«Ir//(' war //I(’ umIiIm']/ ns.s'1'('I('.s‘ (lI'(‘ Imly/ml in pI‘im[l1'1'(.' In('.w*Iu'II_I/III!!! I1'.\'.s'1(¢’.' UI¢'_I/ ll"(’ slill u'/m/l_// rur(iluqiImu.x-, am] will ronmin so unlil (hr fetus Ims I'(‘II('/l(’(] [he 117'-mm. -\'IIl_(/I’, (It 1/‘/u'('h limw n.\-.x~I'/it-I1(I'm1. II[I[I(‘(lI‘.\' _/ir.x'/ in (/10 1'm'u.x'.

'I'//w_l'm-iu/ (mm! um! I/11' /n'1'nmr(I1'm/1 Of I/:4‘ .s'((1pml[Iu' mIl.s‘('](' /mm’ (-I1(1n_(/ml lflllo .\'l'IlI‘(‘ HI!’ J8-mm. .-/11.:/v, (Ir/a'r1nr'4'/mm! /:('/'n_(; I /In "(ml (4; .\‘(’[)(lI'(l[I.(II(, of (/10 .-[um'(liu.x‘ from (/10 .\'!)Il)'('(’-N/I'll/‘IIlI‘(’, l/'10 Iu(('r0- I/_r/ulv. Sq//‘('_(/zllinn. ix (‘()III[)((’/(‘ in. [/l(' 4.3-mm. f('(u.s, and ('UI1.I'(')‘.\'i(m of (hr unr[I:[ft'r('n['irI(wl III(*.\‘('II- ('/I_I/IIHII r'('[l.s- of flu‘ [(1/(‘I'0/I]/.’l,(’ Dir/[0 Illll-\‘l‘[(‘ (i.s'.s‘Iu' is umlor (I'(I_I/.

I II .\‘/N'(‘I'III('I( of Nu» .wm1(' (l_(](’, _/ih(*1‘.s'fI‘tmI I/I('_fn('ial )l(’l‘l'(' [)(l.s'.s‘ In I/Io IIII(.\'('[(‘ I)/(I.s'(¢'nm ~— ma nb.s'('1'm1- Hun u-/:{r-// /ml .s-mm’ in.I'('.<I{g(I(.(n‘.w In mm'lurl¢' I/ml 1',nIm'wI[1'rm. by [/1 ix n('r1'(' from NH.‘ sm-I)/ul Ill‘('/I xpollwl m‘/g/in uf I//1' .\‘/ll/I(‘.\‘ from //H’ sumo I;mn.('I11'u.’ .s-uurr-('. H.-XNSON F.T .-\I..——.-\UDlTORY OSSlCI.F.S 377

Cafitiiag I “T , \ ‘

' P2 r 1 chonclral :, ' b O 1' (:,-7'.

_ ‘ _ M‘ ca 5 1 u e . _ I"1'q. 18. In. the ,3]-r1'('('A' -\'[(I_lI(' I/Io n.x--[rle.»~ arr .~'lilI rurIilu_r;innu.»- u.’ I/u’ in.-pr/inn of ,/In’ I: L!/rm mul the in(‘Iu[osII1p<'tIinI joint is .s-[ill lIl(‘.\'(‘Il('/I_I/III(l[. 'I'Iu- n.~'.s-.-'ji4-4Hirn/ (‘I'll/4'I' in [hr .-/u/u-.~' /.\' m/:~um'/nu Iolmrd (ho hcml, /1uI'£ngu[rm(I_1/ ('l)II!‘('I‘/(‘/I (hv rruru mul llrr (_I/mull/iv purl of M4» /mxr in/n pu'i:-Im;ulru/ lmml.

The sltzpxv/2'(1l lwmlrm is 'L.Il-\‘(‘I'/(‘ll fulo Im//1 (hr lrmy ('ru.~' Hf Nu’ inru.- um’ ll/w /mul uf /In .\/rr_:u.\- mu (Irrun_(/enzwzl u‘/tit-/I uws on(-mm!(':'ml in -)’II 0 f 10.8’ .-u'ie~.\-, fvlnl mu.’ mlu//\. 378 QUARTERLY BULLETIN, N.U.M.S.

Incufi - [long proces

Tendon of , stapczdial muscle A ’ i

.4‘ '

l"i,r/. HI. I n I/w m-whorn tlw U.s'.s'[('/(‘N are, in all majorf('atu.r('s, adult 0lonLmt.s- of the hmmm skeleton. 'I'/u» .\'{(l[I('.\', for (*.ram/:11», r('tm'n.<.' a nzvre renm.a.nt of its fetal .s-[rm'lur0, mmwly marrow limited (0 the 'intrrior of the r'orI'-[ml mu] c-upilal ])(II‘l.\' am] to tho imwr ('[r('unzforen.('e of the base.

l£'I‘1'(I('/(I I z'Ir(':I"1'.~'o in (Iris mu! mam] 0!/wr po.s-(natal S])(’("iIII('II.S' (1'm'Imling sonic to the age of 70 _z/ears) is the 0(‘I'IlI'l‘(’II(‘(‘ of rIoul)I(' in.s-vrtftm of (/20 .s‘tapPd1'al tendon. 'I'InT.s-_mcr1n.s that the early association. will: I/u‘ I'II(‘I(.\' (mun/m/'(' _/if/. I8) is /to! lost as the n.5'.s'i(‘[:'.\' mature and ax the inczulostapedial joint is fIn'IIIml.

T/I('.w' nh.\-m‘1wIi(m.s- arr intvr/2/‘('14-rl to mean. that the mrmubrium of the mall('us and long (‘rim of llw .-'m~u.\-I, (Ix uwll (Ix //l(' arm]:-r par! of the slit/ms, arr rIu"1'1'('rI from [hr .x'('('on4[ branrhial arch. H.-\f\'SO.\' Ff!‘ .-\l.. .-\L'l)|TORY OSSlCl.F.S 3"!

‘V '. _' /ryrnpanic c.‘1\v.'17c _ ..’Ext..e.'T :1. acoustic X C V

' ‘rrgafiéatus -—”  2,4 

I"1'g. .30. In [In-frills of Hi] mm. (1!) u‘:'z'A'.s') JIw-l.'('I'.x- rurlilugw is I».«;m’u::[/:_r/ In unr/rrgu u.\-.-v:'_/iv:/(inn near [ho zone in u‘/lirlz rlllorwl If.s-mu will 4Ii_[f('r.~n(irm' inln Hm! uf [hr uni:-riur /i_I/mm'u/ nf I/Ir mu//e rm (fi!l- .3011). 'I'hro¢’ u'o('l.'.s' lu.’m‘, in .'Iu~f('lu.s- of I!!!) mm. (.3...) uwI.'.s'\ //u’ rmIrrim- /I/'n(‘(‘.x'.\' nf flu mu//ru.- has ht-come (1[Im'huI to the n.u'Ix' of (low 0.»-.<;';-Io (jig. ..‘nh, ul urrm:-.-‘._

In the .31.’?-nzm. (3,;-Il‘¢'c!\') .\‘((I_(/t‘, lfm'<-Iu-rl‘.s- mrfilugr, 4-uniiuuuuw u-ill: //Ir ulir ('41/r.~’II’4', _/‘arms purl vfl./10 wall of lhcfrwittl (mm! (fig. Jllr). In curly inftuu-_I/, I/H’ rt:/'l"lug/r (41 ¢l¢'r[1'ulir:- of {hr /ulr:-u- /l_I/(1[(') is (1 more rcnznm-I (fig. 301/). _\'m~[g-_,"nrnu'(I rm-mh/'um' ham and «gr/mruIin_q I1//n/xunir ruri/_/; tat * in both _/i_([.\'. 3()(‘ am! 3011) l'II((‘I'l'('II(' bl‘(II'(‘(‘lI H10 ('m1lm.'.-.- nf /hr mnul unrl /hr r4-.-/rluuf. /III'e‘.~'/4'1/_ rnrl-ilage.