J. Anat. (1987), 154, pp. 187-200 187With 6 figuresPrinted in Great Britain

The development of the orbital region of Caretta caretta(Chelonia, Reptilia)

SHIGERU KURATANI

Department of Anatomy, University of the Ryukyus, 207, Uehara, Nishihara,Okinawa, 903-01, Japan

(Accepted 16 December 1986)

INTRODUCTION

The orbitotemporal region of vertebrates is a highly modified part of the skull andhas been one of the most intriguing subjects of cranial morphology. Such study hasdealt primarily with the basic definition of the components of the neuro- andsplanchnocranium (Gaupp, 1900, 1902; de Beer, 1937; Goodrich, 1930; Portmann,1976; Jollie, 1962; Romer & Persons, 1977; Starck, 1979 a). In particular, the morpho-logical concept of the cavum epiptericum (Gaupp, 1902) was an innovation whichenabled later anatomists to homologise the complicated cranial base elements (Rice,1920; Matthes, 1921; Presley & Steel, 1976). These works were mainly on the hom-ology of the skeletal element itself (de Beer, 1926; Rieppel, 1976; Starck, 1979 a, b), butthe interrelationship with the eye muscles, which has often been implied by someauthors (Terry, 1917; de Beer, 1937), has never been extensively investigated.The present paper is intended to give a precise description of the developing orbital

region in reptiles for the purpose of reconsidering the meaning of the orbital region ofthe developing skull in the light of functional and comparative morphology. TheCaretta caretta embryos were used because it is a reptile from which it is easy to getstaged embryos.

MATERIALS AND METHODS

One hundred and twenty Caretta caretta, (Loggerhead turtle) embryos weregathered in Shirahama, Wakayama, Japan in 1984. The eggs were taken from twosites and were probably laid by two different females. Each embryo was staged fromthe day when the maternal footprints were discovered for the first time. Afterincubation, they were fixed either in Bouin's solution to make paraffin sections or 10%formalin to make whole stained specimens according to the method of Dingerkus &Uhler (1974). The carapace length of each embryo was measured after fixation fordetermining the stage of development (see Table 1). The sections were cut serially infrontal, sagittal and horizontal planes and stained with Weigert's iron haematoxylinand eosin or with Azan.The sections were projected on to a screen and traced to makegraphical reconstruction drawings for which the sagittal sections were mainly used.

OBSERVATIONS

General morphology of the orbital regionFigure 1 shows a lateral view of the orbital region of the Caretta embryo at a stage

where the developing skull is seemingly most appropriate for the general descriptionof the initial architecture of the orbital region of this animal.

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Table 1. List of the sectioned embryos

Carapace lengthSer. no. No. (mm) Stage

A-28e 4 898 IIA-29e 3 9.43 IIA-30e 2 9 67 IIA-32e 1 116 IIA-34e 1 139 IIIA-36e 1 166 IIIA-38e 1 16 76 IIIB-16e 3 675 IB-17e 2 7 05 IB- 18e 1 8 90 IIB-26e 2 131 IIIB-27e 2 14 08 IIIB-32e 4 23 4 IVB-34e 1 26-3 IVB-36e 1 30-1 IV

The orbit of Caretta develops inferolateral to the developing forebrain as an extra-cranial space. The walls of the orbit are composed of six irregularly shaped cartilages,each being a constituent of the neurocranium. The superior wall or roof of the orbitis composed of the taenia marginalis, the anterior wall of the planum supraseptale andthe interorbital septum, the posterior wall of the pila metopica and the postorbitalcartilage or the later pila antotica. An important feature of the orbit at this stage is thatthe pila metoptica protrudes prominently into the space of the orbit, and therebydistinctively delineates the posterior portion, the cavum epiptericum.At this stage, the formation of the inner wall of the orbit by these cartilages is not

yet completed; at the apex of the orbit, the primitive optic foramen remains wide open,transmitting the optic nerve and the ophthalmic artery into the orbit. Near the baseof the orbit, the margin of the cartilage is more or less deeply indented. From thesecartilages originate the extrinsic eye muscles, i.e., the superior, posterior, inferior andinternal rectus muscles, and the superior and inferior oblique muscles.The ophthalmic nerve (V1) traverses the orbit anteriorly, passing inferior to the

superior rectus muscle, crossing over the ophthalmic artery and the optic nerve, andfinally, after having passed between the superior oblique and the inferior rectusmuscles, leaves the orbit to enter the nasal region. The major veins, the supraorbitaland the infraorbital, are also observed to traverse the orbit posteriorly. The supra-orbital vein runs inferior to the superior oblique and superior to the superior rectusmuscle, while the infraorbital passes first across both the superior and inferior surfaceof the inferior oblique muscle, and then inferior to the internal and inferior rectus

Fig. 1 (a-b). (a) A somewhat simplified reconstruction of the Caretta chondrocranium withneighbouring extracranial structures, viewed from the lateral side. Some parts of the visceral skeletonand the posterior part of the chondrocranium, including the otic capsule, are not shown. Theproximal heads of three rectus muscles, superior, inferior, and posterior are situated in this cavum.(b) the chondrocranium of the same embryo. The extracranial space lateral to the pila metoptica andthe postorbital cartilage is called the cavum epiptericum, the posterior portion of the orbit. Theanterior portion of the inner wall of the cavum epiptericum, the pila metoptica forms the posteriormargin of the optic nerve foramen.

Abbreviations for all Figures are on p. 200.

188

Orbital development in Caretta caretta

1(a)

1(b)

189

190 SHIGERU KURATANI

muscles. Both these veins thus enter the cavum epiptericum and unite to give rise tothe anterior cardinal vein. The prominent pila metoptica (the anterior reflection of theflexured cranial wall), and pile antotica (the posterior reflection), are arranged at anacute angle to each other, resulting in the formation of a flexured wall with a deephollow extracranial space called, as mentioned above, the cavum epiptericum (Fig.l,a,b). The flexured wall is pierced by the foramina for the cranial nerves, theophthalmic artery, and for the hypophysial vein. The postorbital cartilage, whichrepresents the posterior moiety of the flexured wall, is marked in its lower half by adeep indentation for the trigeminal ganglion.

Contained in the cavum epiptericum are the proximal parts of the posterior,superior, and inferior rectus muscles, the ophthalmic artery, the anterior cardinal veinwith its tributaries (the supraorbital, infraorbital and the hypophysial veins), thelarge trigeminal ganglion with the proximal part of the ophthalmic nerve, and thecranial nerves (Fig. 1 a).

Development of the orbital region

Stage I, 7 mm carapace length (Fig. 2 a, b)This is the earliest embryo employed in this study. The anterior portion of the orbit

is not yet formed; it is merely bordered posteriorly by the pila metoptica and inferiorlyby a pair of trabecular cartilages which are portions of the cranial base. The taeniamarginalis, planum supraseptale, and interorbital septum are not formed yet, andneither is the primitive optic foramen. Posteriorly the postorbital cartilage is develop-ing. At this stage, the pila metoptica is observed to be a complex which consists of thesupratrabecular cartilage and its four processes, superior, middle, antero-inferior, andpostero-inferior. The anteromedial process is not yet formed (see below). The supra-trabecular portion located superior to the ophthalmic artery is the principal, and infact, the most densely condensed part of the pila metoptica. The superior process uniteswith the postorbital cartilage superior to the foramen for the oculomotor nerve. Thisprocess extends upwards to become continuous with the primordium of the taeniamarginalis and gives rise to the superior rectus muscle. The middle process extendsbetween the oculomotor nerve and the hypophysial vein posteriorly to unite with thepostorbital cartilage. The postero-inferior process lies just medial to the posteriorrectus muscle uniting the supratrabecular cartilage with the polar cartilage. Theantero-inferior process also unites the supratrabecular cartilage with the cranial base,and the inferior rectus muscle originates from this cartilage.The later pila antotica, or the postorbital cartilage, on the other hand, develops as

a thick vertical cartilage wall traversing the cranial cavity. This cartilage is penetratedby the trochlear nerve where the superolateral portion of this cartilage makes thesummit of the flexure. The abducens nerve penetrates the cranial base at the base ofthe postorbital cartilage, or at the junction of the latter and the parachordal cartilage.The nerve runs anteriorly for a short distance to innervate the posterior rectus muscleanlage. The proximal head of this muscle anlage is attached to the inferolateral partof the anterior surface of the postorbital cartilage. The oculomotor nerve passes downalong the anterior surface of the latter cartilage making the groove on it for theproximal part of its course. Having penetrated the cranial wall, it innervates thesuperior rectus, grazing the ophthalmic artery from behind and ending in the belly ofthe inferior rectus muscle.

Orbital development in Caretta caretta 191

2(a)

soy~~~~~i

2(b)

Fig. 2(a-b). (a) Stage I Caretta embryo. Lateral view of the orbitotemporal region of thechondrocranium, along with other structures, seen from the lateral side. The posterior part of thechondrocranium is not shown. Note the low position of the optic nerve. (b) Chondrocranium of thesame embryo. The pila metoptica is composed of the supratrabecular cartilage and neighbouringprocesses.

SHIGERU KURATANI

Stage II, 9 mm carapace length (Fig. 3 a, b)In Stage II, the planum supraseptale, the interorbital septum and the taenia

marginalis develop and constitute the superior and the posterior walls of the orbit;they simultaneously form the superior and the anterior boundaries of the primitiveoptic foramen respectively, thereby completing the formation of the foramen.The superior process of the pila metoptica expands to the front of and above the

oculomotor nerve to unite with the newly formed taenia marginalis, thus forming thesuperior margin of the primitive optic foramen. The superior rectus is attached to theposterior end of this cartilage.The lateral edge of the postorbital cartilage now lies medial to the abducens foramen

and this cartilage itself is not perforated by the latter nerve.The anterior continuation of the taenia marginalis, i.e. the planum supraseptale,

which develops dorsal to the interorbital septum, serves as the roof of the orbit, andsimultaneously bounds the primitive optic foramen superiorly. This planum givesorigin to the superior oblique muscle. In the cavum epiptericum, the anlage of theposterior rectus and retractor bulbi muscles are observed to be separated from eachother. The anlage of the former muscle is attached to the postero-inferior process andthe latter to the basal part of the postorbital cartilage. The inner rectus muscle is foundto develop from the anteromedial portion of the inferior rectus muscle which is not yetattached to any part of the chondrocranium (Fig. 1 a).

Stage III. 15 mm carapace length (Fig. 4a, b)In Stage III, rather drastic morphological changes occur, especially in the region of

the pila metoptica with its processes and in the pila antotica. The most characteristicchange is that the postero-inferior process degenerates. This results in the fusion of theforamina for the hypophysial vein and for the ophthalmic artery, and simultaneously,the posterior rectus muscle loses its central portion of origin, and bridges between theposterior part of the supratrabecular cartilage and the cranial base. The antero-inferiorprocess is also beginning to fade, causing the primitive optic foramen to fuse with theforamen of the ophthalmic artery. Some of the whole stained embryos have lost thisprocess prior to the formation of the anteromedial process (see below), so that theinferior tip of the pila metoptica looks as if it is hanging in the middle of the orbit. Thesupratrabecular portion of the pila metoptica is beginning to develop an anteromedialprocess passing medially towards its counterpart and anteriorly to the posteriorprocess of the interorbital septum, i.e., the cartilago hypochiasmatica.The superior process of the pila metoptica expands further anteriorly, rendering the

cavum epiptericum still shallower. The middle process is also beginning to be reducedto form a large foramen, the fenestra metoptica, through which the oculomotor nerveand the hypophysial vein pass out of the cranial cavity into the cavum epiptericum.The trochlear nerve passes through the foramen in the taenia marginalis. The superiorand inferior oblique muscles and the inner rectus muscle originate from the interorbitalseptum.The medial portion of the postorbital cartilage is beginning to be resorbed, but the

superior and lateral edges of this cartilage remain. The latter is now to be called pilaantotica. Simultaneously, the pila antotica has also moved a little posterolaterallytoward the abducens nerve foramen, thus making the expansion of the cavumepiptericum less distinctive. By this stage, the pilae metoptica and antotica no longermake an acute angle. The separation of both pilae and the obliteration of the cavumepiptericum proceed still further in the following stage.

192

Orbital development in Caretta caretta3(a)

jos

3(b)

Fig. 3 (a-b). (a) Partly reconstructed drawing of Stage II chondrocranium along with other structures.The taenia marginalis is indicated by broken lines. The inferior rectus is attached to the antero-inferior process and the posterior rectus to the postero-inferior process. The taenia marginalis andplanum supraseptale are developing encircling the optic foramen. (b) Chondrocranium of the sameembryo.

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4(a)

4(b)

~~~~~~~~ofVA-

~~~~~amp.:-

Fig. 4(a-b). (a) The orbitotemporal region of a Stage III chondrocranium with the extracranialstructures. Note that the posterior rectus bridges the posterior corner of the supratrabecular cartilageand the cranial base. (b) Chondrocranium of the same embryo. Note that the postero-inferior processhas degenerated and the anteromedial process is beginning to develop. The middle process is alsobeginning to disappear.

194

\. 1-

Orbital development in Caretta carettaStage IV. 35 mm carapace length (Fig. 5 a, b)The chondrocranium of the Stage IV embryo shows the more or less fully developed

features of the Chelonian orbit as is described in the literature (in general: Kamal &Bellairs, 1980; Dermochelys: Nick, 1912; Chelonia: Fuchs, 1915; Emys: Kunkel, 1912;Chelydra: Rieppel, 1976). Specifically, the formation of the subiculum infundibuli andthe cartilago hypochiasmatica are shown in these forms.The anteromedial process of the former stage, the subiculum infundibuli, has united

with its counterpart on the opposite side, and united anteriorly with the cartilagohypochiasmatica which has developed as the projection of the posterior edge of theinterorbital septum. This fusion occurs inferior to the optic chiasma in the midline,causing an elevation of the inferior margin of the secondary optic foramen. As a result,the level at which the optic nerve passes through this foramen has moved superiorlyas compared to the Stage III position. The antero-inferior process has totally dis-appeared (Figs. 4b, 5), and the inferior rectus muscle is now attached to the subiculuminfundibuli but remains orientated as much downwards as before. As a result, theformer ophthalmic artery foramen has become confluent with the inferior portion ofthe primitive optic foramen. In this connection, it should be mentioned that theophthalmic artery, which at earlier stages passed through the ophthalmic foramen, hasdisappeared by this stage. Thus, the newly formed optic foramen is now encircledposterolaterally by new cartilages and comes to be called the secondary optic foramen.The inner rectus, on the other hand, has moved still farther superocaudally up to theanterior edge of the secondary optic foramen. Thus, the proximal heads of the fourrectus muscles, superior, posterior, inferior, and the inner rectus muscles are gatheredaround the secondary optic foramen as close to each other as before.The pila metoptica thus has no original cartilage part that connects directly with the

cranial base. The middle process has completely disappeared, leaving a more enlargedforamen, the fenestra metoptica, which, as mentioned above, is confluent with theformer ophthalmic artery foramen and the space just beneath the cartilago hypo-chiasmatica. On the other hand, the superior process has moved more obliquely andanteriorly to border the superior edge of the secondary optic foramen. This processnow lies in a transverse plane so that it does not serve as the laterally reflected wall ofthe cavum epiptericum but appears to be a part of the lateral wall of the neurocranium.The basal portion of the pila antotica has moved still further laterocaudally to end

close to the abducens nerve foramen. Its superior end, which is fused with the posteriorportion of the taenia marginalis, also bends caudally. The retractor bulbi musclecontinues to be attached solely to the pila metoptica as before.

DISCUSSION

In the above descriptions, it has become clear that the pila metoptica changes itsform during development (Fig. 6). The two main factors or changes which seem to beinvolved in the configuration of this structure are the formation of the expandedinterorbital septum and the rearrangement of the extrinsic eye muscles. These changesalso result in the apparent obliteration of the cavum epiptericum. In the followingdiscussion, the configuration and deformation of the pila metoptica will be illustratedfirst and then the two factors which are correlated with the cartilage development inthe orbitotemporal region will be dealt with.

Configuration and deformation of the pila metopticaThe pila metoptica is comprised of the main part called the supratrabecular cartilage

(de Beer, 1937; the cartilago suprapolaris: Goldschmidt, 1972), and the peripheral

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SHIGERU KURATANI

5(b)

los ch

'.:.- ., '. :'. .'- :'

Fig. 5 (a-b). (a) Graphically reconstructed illustration of a Stage IV Caretta embryo. Lateral view ofthe orbitotemporal region of the chondrocranium along with other structures. The subiculuminfundibuli is fused with the cartilago hypochiasmatica, elevating the optic nerve. The antero-inferiorprocess is lost. The ophthalmic artery has degenerated as well. The middle process has vanished,leaving the fenestra metoptica. (b) The chondrocranium of the same embryo. Note that the cavumepiptericum no longer exists as a separate space.

196

5(a)

Orbital development in Caretta caretta

tm

Fig. 6. Diagramatic illustration of the changing orbital region of the Caretta chondrocranium. Thelines representing the contours of the developing chondrocranium are superimposed, assuming thatthe distance between the optic nerve and the cephalic flexure does not change during development.Note that the position of the optic nerve moves higher with the development of the interorbitalseptum and the pila metoptica shifts anteriorly during development. The pila antotica or postorbitalcartilage also moves posteriorly. Black arrows indicate the movement of the cartilage elements andthe large arrows indicate the shift of the proximal heads of the superior, inferior and the posteriorrectus muscles respectively.

processes around it, i.e. the superior, anteromedial (chondrifies later), antero-inferior,postero-inferior and the middle processes. These processes have had little mention inthe literature and are provisionally named as shown above in the present study. Thecartilages are all chondrifications of the primary cranial wall and the changing pilametoptica is to be illustrated through the chondrification and the degeneration of thesesubdivisions as follows.The supratrabecular cartilage is at first almost independently chondrified and is seen

as the posterior portion of the subiculum infundibuli in the fully formed chondro-cranium (Fig. 6). The postero-inferior process serves as the posterior edge of theophthalmic artery foramen connecting the supratrabecular cartilage and the cranialbase, and is the first to regress. The antero-inferior process is another element whichconnects the supratrabecular cartilage with the cranial base. This forms the anterioredge of the ophthalmic artery foramen. By the regression of this cartilage, the pilametoptica loses its original connections to the skull base. The middle process dis-

197

appears next and the pilae metoptica and antotica become separated completelyleaving the fenestra metoptica between them. Around the time the last two processsesdegenerate, the anteromedial process grows from the supratrabecular cartilage, andfinally fuses with the cartilago hypochiasmatica. This fusion results in the formationof the posterior edge of the secondary optic foramen. In the course of these changes,the superior process also moves gradually forward and the pila metoptica as a wholelooks as if it has shifted anteriorly, moving over the ophthalmic artery.A similar shift of the pila also seems to take place in other reptilian embryos that

have some of the above cartilage elements (Kamal & Bellairs, 1980; Kunkel, 1911,1912). All through the above stages of modification, the pila metoptica is always foundbetween cranial nerves II and III, and it can be so-called wherever it is situated,although it consists of different elements at each stage of development. This seemsnever to have been mentioned in the literature but is important because this changedoes not appear to occur in other vertebrates such as placental mammals which alsohave a cartilage called the pila metoptica (de Beer, 1926, 1937; Presley & Steel, 1976;Starck, 1979 a, b; Jarvik, 1980). The homology of the pila metoptica, especially inrelation to the ophthalmic artery, needs further study.

Arrangement of the eye musclesAs mentioned above, the proximal heads of the rectus muscles in Caretta have a

tendency to gather around the optic chiasma during development (Figs. 5 a, 6), andthese movements seem to be correlated with the anterior shift and the deformation ofthe pila metoptica as well as with the formation of the cartilago hypochiasmatica andthe subiculum infundibuli. The postero-inferior process disappears with the movementof the posterior rectus muscle, and the antero-inferior process disappears with theupward movement of the inferior rectus muscle which finally attaches to theanteromedial process. The rostral movement of the superior process is related to thesuperior rectus muscle which also reaches to the posterior edge of the secondary opticforamen.

This final condition of the eye muscle appears in many lower vertebrates and isregarded as the basic arrangement of the muscles, i.e. the rectus muscles are attachedto the interorbital septum around the optic foramen and the obliquus muscles to theplanum antorbitale or the reptilian planum supraseptale (Corning, 1900; Goodrich,1930). However, in those forms which have an extensively chondrified cranium, themuscle migrations probably do not affect the apparent development of the chondro-cranium, even if such migrations exist.

Growth of the interorbital septum, elevation of the optic chiasma and the modificationof the cavum epiptericum

The reptilian skull is classified as being of the tropibasic type in which the trabecularcartilage narrows into a broad, vertical interorbital septum (de Beer, 1937; Starck,1980). The formation of the latter cartilage results in the inward invasion of theextracranial space (the orbit) and consequently, modification of the inner wall of thecavum epiptericum. The narrowing begins from the anterior tip of the trabecular andcontinues posteriorly whereby the anterior half of the cranial flexure, i.e., the pilametoptica and a part of the taenia marginalis, is obliged to lie longitudinally. In doingso, the superior process of the pila metoptica moves forward with the proximal headof the superior rectus, and in the fully formed chondrocranium, the anterior half of theflexure does not surround the cavum epiptericum any longer but continues craniallyto become a part of the planum supraseptale. The original cephalic flexure is now to

198 SHIGERU KURATANI

Orbital development in Caretta carettabe observed merely as a faint midway curvature of the taenia marginalis (Fig.5a,b).The dwindling of the cavum in reptilian chondrocranium development might be

thus associated with the formation of a broad interorbital septum, which may be anadaptation to the relative growth of the large eyeball. Such a condition is repeated inthe avian chondrocranium which has a similar extensive interorbital septum (de Beer,1937; Starck, 1979a). Curiously enough, the reflection of the flexure is reserved inmammalian skulls as the meningeal membrane covering the trigeminal ganglionanteriorly as a part of the tentorium cerebelli (Presley & Steel, 1976). This may be dueto the smaller size of the eyeball and the low grade of deformation in the orbitalregion.

SUMMARY

In the development of the orbital region of the Caretta chondrocranium, therearrangement of the several eye muscles seems to be correlated with the apparentanterior shift of the neurocranial element, the pila metoptica. The pila consists of themain part, the supratrabecular cartilage, and five processes, the superior, middle,anteromedial, antero-inferior and the postero-inferior. The superior process forms theattachment of the superior rectus muscle and, together with the muscle, movesanteriorly during development. In the course of the anterosuperior shift of the inferiorrectus muscle, the antero-inferior process degenerates and the anteromedial process isnewly formed. The postero-inferior process gives origin to the posterior rectus muscleand regresses as a result of the upward shift of the muscle. All these changes are theresult of the secondary arrangement of the eye muscles gathered around the secondaryoptic foramen which has been newly formed through the pila metoptica deformation.The elevation of the optic nerve which is brought about through the formation of theinterorbital septum, is another factor that brings about the above changes. Because ofthese changes, the anterior part of the cranial flexure, the pila metoptica, lies in alongitudinal plane and consequently it, as well as the cavum epiptericum, is obliteratedand a large antero-inferiorly opening extracranial space, the orbit of the reptile, isproduced.

I am most grateful to Mr. H. Tanase of Seto Marine Biological Laboratory,Kyoto University, for his help during the search for turtle eggs on Shirahama seashore, to Dr. M. Tasumi of the Department of Zoology, Kyoto University for pro-viding me with the ideal environment to carry out this study and to Professor Dr.S. Tanaka for his critical reading of the manuscript.

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GAUPP, E. (1900). Das Chondrocranium von Lacerta agilis. Ein Beitrag zum Verstaendnis des Amnio-tenschaedels. Anatomische Hefte 15, 433-595.

GAUPP, E. (1902). Ueber die Ala temporalis des Saeugerschaedels und die Regio orbitalis einiger andererWirbeltierschaedel. Anatomische Hefte I. Abtheilung 19, 155-230.

GOLDSCHMIDT, A. (1972). Die Entwicklung des Craniums der Mausvoegel (Colidae Coliformes Aves) I.Morphologisches Jahrbuch 118, 105-138.

GOoDRICH, E. S. (1930). Studies on the Structure and Development of Vertebrates. London: Macmillan.JARVIK, E. (1980). Basic Structure and Evolution of Vertebrates. London: Academic Press.JOLLIE, M. (1962). Chordate Morphology. New York: Reinold.KAMAL, A. M. & BELLAIRS, A. D'A. (1980). The chondrocranium and the development of the skull in recent

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Abbreviations for Figuresaip, antero-inferior process of the pila

metoptica;amp, anteromedial process of the pila

metoptica;btr, basitrabecular process;ch, cartilago hypochiasmatica;fin, fenestra metoptica;gc, ganglion ciliaris;gt, ganglion trigemini;hv, hypophysial vein;ic, internal carotid artery;ios, interorbital septum;iov, infraorbital vein;mp, middle process of the pila metoptica;na, nasal capsule;oa, ophthalmic artery;of, primitive optic foramen;oi, m. obliquus inferior;os, m. obliquus superior;pa, pila antotica;pc, parachordal cartilage,

pip, postero-inferior process of the pilametoptica;

PM, pila metoptica;po, postorbital cartilage;pq, palatoquadrate cartilage;pss, planum supraseptale;ret, m. retractor bulbi;rinf, m. rectus inferior;rint, m. rectus interior;rp, m. rectus posterior;rs, m. rectus superior;si, subiculum infundibuli;sof, secondary optic foramen;sov, supraorbital vein;sp, superior process of the pila metoptica;st, supratrabecular cartilage;tr, trabecula cranii;tc, trabecula communis;tm, taenia marginalis;vca, vena cardinalis anterior;II-VII, cranial nerves.

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