Illustrated Encyclopedia of Human Anatomic Variation: Opus V: Skeletal Systems: Cranium

Sphenoid Bone

Ronald A. Bergman, PhD
Adel K. Afifi, MD, MS
Ryosuke Miyauchi, MD
Peer Review Status: Internally Peer Reviewed

The superior part of the dorsum sellae may be a separate bone or it may be joined to the petrous portion of the temporal bone. According to Bruneton, et al. the floor of the sellae may be concave (58% of patients), flat (32-35%), or convex (9.5%). A bridge of bone joining the anterior and posterior clinoid processes is reported to occur in 5% of individuals studied. In another study of 157 skulls, the frequency was 0.64%.

Vesalius was the first to describe and illustrate the foramen that bears his name (foramen vesalii). The foramen is also known as sphenoid emissary foramen, foramen of Vesalius, foramen venosum (of Vesalius) and canaliculus sphenoidalis. When present, it is located between the foramen rotundum and the foramen ovale on its medial side. It is traversed by a vein (vein of Vesalius), a small emissary, from the cavernous sinus to the pterygoid plexus. The presence of this foramen is not uncommon. In 157 skulls it was found 26 times bilaterally (17%) and 20 times unilaterally (13%). It is also variable in size , shape and postion. There is a report that the frequency may be as high as 40% (unilaterally or bilaterally).

Lang (1983) reported that the Vesalian foramen is present in about 40% of his material; the foramen transmits transbasil veins from the cavernous sinus to the pterygoid plexus of veins. The Vesalian foramen (foramen venosum [of vesalius]) was found on the right side in 49% of cases and on the left side in 36%. Lang also reported that a small nerve may also pass through the Vesalian foramen into the cavernous sinus (nervoulus sphenoidalis lateralis).

Occasionally, foramina occur through bridges of bone between the posterior margin of the lateral pterygoid lamina and the spine medial to the foramen ovale; these transmit the nerves and vessels for the medial pterygoid muscle. Pterygospinous foramina occur in about 5% of cases. In another study, the pterygospinous foramen has been found in 6.28% of 1,544 skulls and in 5.46% in the second series of 2,745 skulls in American whites and Negroes; it was more frequent in the whites and in males.

A bridge of bone between the lateral surface and base of the lateral pterygoid plate and the greater wing of the sphenoid (lateral to the foramen ovale) provides passage for some or most of the motor fibers of the trigeminal nerve. A bony bridge may connect the anterior and posterior clinoid processes. The incidence is about 5% in the general population; a hereditary tendency was also noted. The bridge may be of clinical significance in its relation to the hypophysis (Snyder and Blank).

This pterygoalar foramen (Chouké, 1946) (or porus crotaphytico-buccinatorius, Hyrtl, 1862) was reported in about 1% of skulls by Hyrtl and in 8% of specimens in another study.

The pterygoalar foramen was found in 10.3% of the first series and in 6.11% of the second series, with greater frequence in the Negro. It is also more frequent in males. (Chouké, 1946, 1947).

The foramen spinosum may be absent (4.57 or 0.64%), in which case the middle meningeal artery enters the cranial cavity via the foramen ovale.

The foramina ovale and spinosum may be continuous. Wood-Jones (1931) found the foramen spinosum to be more or less incomplete in approx. 44% and in 16% the foramina on the right side was unclosed, 84% were open. The foramen spinosum may be duplicated. The foramen spinosum was small or altogether absent in 0.4% of Lindblom's (1936) cases. This is especially true when the middle meningeal artery arises from the ophthalmic artery.

The sphenoidal emissary foramen may transmit the accessory meningeal artery (20% of cases).

The usual diameter of the optic canal is 5.5 mm but varies from 3.5 to 6.5 mm.

The foramen rotundum may be doubled or absent (0.64% of 157 skulls).

An extra bony canal may run from the medial angle of the superior orbital fissure through the body of the sphenoid, opening at the processus vaginalis. In 0.3% of adult skulls, a craniopharyngeal canal is present. An embryonic remnant, it opens in the hypophyseal fossa, traversing the sphenoid body.

The middle clinoid process may be elongated enough to join the anterior clinoid process to form a bridge over the carotid sulcus, transforming it into a caroticoclinoid foramen. Bony bridges between the lateral border of the pituitary fossa and the apex of the anterior clinoid process demarcate an orifice known as the caroticoclinoid foramen or carotid foramen, because the internal carotid artery runs forward and medially out of the cavernous sinus through this closed and usually circular ring of bone. An accessory bony bridge may also arise from the middle Clinoid process, which may fail to fuse completely with an extension of the anterior clinoid process, thus producing an almost complete ring. Lang found closed caroticoclinoid foramina in about 10% of children and adults.

The optic canal may be divided into two parts by a bony partition. On occasion, the optic foramen is continuous with the orbital fissure. The foramen rotundum may also be continuous with the orbital fissure.

The angular spine of the greater wing of the sphenoid may participate with its lateral surface in forming the articular fossa for the mandible. The vaginal process may participate in creating a canal along its inferior aspect.

The pterygoid fossa is sometimes subdivided by several partitions.

The posterior aspect of the lateral pterygoid plate may be joined to the angular spine of the greater wing by a pterygopetrosal ligament that may calcify. At the base of the spina angularis, to the medial side of the foramen spinosum, there is occasionally a minute canal (canaliculus innominatus, Arnold) through which the lesser petrosal nerve passes. The medial aspect of the spina angularis is frequently grooved by the chorda tympani nerve.

Sutural (wormian) bones may occupy the sphenoidal fontanelles between the second and fifteenth years, after which they may persist as separate ossicles or unite with the sphenoid, temporal, or frontal bone.

The sphenoidal sinus may be absent or small; it may be very large and extend into the basilar process of the occipital bone, the greater or lesser wings of the sphenoid, or the pterygoid processes.

The bony plate separating the sphenoidal sinus from the optic nerve, maxillary division of the trigeminal nerve, the nerve of the pterygoid canal, the cavernous sinus, the carotid artery, and the hypophysis may be very thin or absent. Hence these structures may become vulnerable in chronic sinus infections. It has been said that "the sphenoidal sinus is the most variable in form of any bilateral cavity or organ in the human body (Congdon, 1920)." The dimensions of the average sinus are: 20 mm (height); 18 mm (width); and 12 mm (length). The capacity of the sphenoidal sinus varies from 0.5 to 30 cc., with an approximate average of about 7.5 cc. (Anson). The spenoidal sinus was first described by Berengaria de Carpi in 1523.

The lesser wing of the sphenoid may project forward onto the roof of the orbit. The lateral part of the superior orbital fissure may be closed because of the fusion of the greater and lesser wings. The ophthalmic artery passes through a separate foramen. The middle meningeal anastomotic branch to the lacrimal (r. anastomoticus cum artery lacrimalis) artery in such cases passes through a separate foramen. Also, one or more small foramina arise allowing passage of recurrent meningeal branches from the lacrimal artery (br. of ophthalmic artery) may join the middle meningeal artery. The ophthalmic artery may have a separate foramen located between the optic foramen and the superior orbital fissure.

The groove for the meningeal branch of the lacrimal artery, located on the lateral inferior border of the superior orbital fissure, may be bridged by bone forming a foramen. A lacrimal foramen was found bilaterally in 42 of 157 skulls (27%) and unilaterally in 54 of 157 skulls (34%).

Another canal (canalis craniopharyngeus lateralis) located at the junction of the basi-, ali-, and presphenoid is present in about 4% of cases.

The temporal aspect of the greater wing may be divided vertically by a fissure. An ossicle may be formed by a horizontal fissure separating off the apex of the greater wing of the sphenoid.

The foramen rotundum is not a circular portal [Luschka, K. v., 1867] (cited by Lang, 1983).

The foramen ovale transmits the mandibular nerve (of trigeminal), the venous plexus of the foramen ovale and a meningeal artery ramus. The venous segment of the foramen ovale may be separated from the remainder of the contents of the foramen by a bony spur; thus resulting in a so-called doubled foramen ovale. Such spurs are located anteriorly and medially according to Radiojevitc and Jovanovic (1956) cited by Lang (1983). They are present in 0.5% of subjects studied. The posterior border of the foramen ovale is absent , more or less, in 8% of skulls, bilateral in 3%, unilateral in 5% (Wood-Jones, 1931).

Ginsberg, Pruett, Chen and Elster provided a detailed study of skull-base Foramina of the middle cranial fossa and a reassessment of Normal Variation with High-Resolution CT. (AJNR(Am. J. Neuroradiol.) 15:283-291, Feb., 1994) The following is the results of this important study (their table 1):

Glimbero, Pruett, Chen and Elster studied normal variation of skull-base foramina of the middle cranial fossa by high-resolution CT. The investigators examined 123 CT studies in patients who presented no evidence of disease that might affect foraminal anatomy. Ginsberg, et al point out that the foramen rotundum originates embryologically as the foramen lacerum anterius, a hiatus between the orbitosphenoid and alisphenoid, which are precursors of the lesser and greater wings of the sphenoid bone respectively. Usually a spur of bone develops from the greater wing of the sphenoid forming the foramen rotundum and superior orbital fissure by crossing the foramen lacerum anterius. Persistence of tile foramen lacerum anterius is rare in humans but common in rodents. Ginsberg and his colleagues found a variant of the foramen rotundurn in the form of a small opening (1-3 mm in the floor of the foramen, which leads to the infratemporal fossa or to the space between the pterygoid plates. Sondheimer found this opening in 5 of 50 skulls and suggested that it transmits emissary veins. Ginsberg, et al found this same opening in about 16% of patients studied in their series by CT. They propose the name Inferior rotundal canal for this variant. In addition they found, in eight of 98 patients (8%) a canal lateral to the foramen rotundum, another variant they propose calling the lateral rotundal canal.

Ginsberg, et al also discussed the foramen spinosum and pointed out the Lindblom reported that in rare cases, early division of the middle meningeal artery into an anterior and posterior division may result in duplication of the foramen spinosum. Sondheimer also reported this variant. Ginsberg and colleagues provide additional information as follows: The foramen spinosum may be hypoplastic or absent in a case of an aberrant middle meningeal artery. Curnow reported hypoplasia hi a case of the ophthalmic artery being the origin of the middle meningeal artery. Both Fisher and Greig described the development of the middle meningeal artery. The stapedial artery originates as a dorsal branch of the second aortic arch and is a part of the carotid artery system. The superior, supraorbital branch becomes the middle mengeal artery. In the l5mm embryo, the infraorbital and mandibular branch of the stapedial artery fuse use with the external carotid artery, and is destined to become the internal maxillary artery. The main trunk of the stapedial artery atrophies and its origin from the internal carotid disappears. The usual distribution of the stapedial artery is then replaced by branches from the external carotid artery. If the connection with the external carotid artery fails to occur, the middle meningeal artery arises from the ophthalmic artery. In this case, the middle meningeal enters the skull through the superior orbital fissure. Lindblom reported this variation in 0.4% of his cases. The stapedial artery may also persist. This vessel has also been associated with an aberrant internal carotid artery. The persistent stapedial artery courses through the tympanic cavity, between the crura of the stapes and enters the facial canal distal to the geniculate ganglion. It enters the middle cranial fossa by the facial hiatus. which is the opening for the greater superficial petrosal nerve, and becomes the middle mengeal artery. In both of these cases, the variants of origin of the middle meningeal artery, the foramen spinosum will be tiny, or absent. Ginsberg, et al found the foramen spinosum absent in four (3.2%) patients. Asymmetry of size was observed in 20 (16%) of their patients. A smaller foramen spinosum did not correlate with the size of the ipsilateral foramen ovale.

Variations in Sphenoidal Sinus.

Medium and Extremes of Development of the Sphenoid Sinus.

The Sphenoid From Above.

Various Shapes of Sella Turcica.

Sphenoid Bone.

Sphenoid Bone.

Porus crotaphitico-buccinatorius of the right wing of the sphenoid bone.

References

Anson, B.J., Ed. (1966) Morris' Human Anatomy, 12th ed. The Blakiston Division, McGraw-Hill Book Co., New York.

Arey, L.B. (1950) The craniopharyngeal canal reviewed and reinterpreted. Anat. Rec. 106:1-16.

Bergland, R., Ray, B. and R. Torack. (1968) Anatomical variations in the pituitary gland and adjacent structures in 225 human autopsy cases. J. Neurosurg. 28:93-99.

Blaszczyk, B., Kaszuba, A. and J. Kochanowski. (1980) Atypical foramina of base of the skull. Folia Morphologica (Warsaw) 39:201-209.

Boyd, G.I. (1930) The emissary foramina of the cranium in man and anthropoids. J. Anat. 65:108-121.

Bruneton, J.N., Drouillard, J.P., Sabatier, J.C., Elie, G.P. and J.F. Travenir. (1979) Normal variants of the sella turcica. Comparison of plain radiographs and tomograms in 200 cases. Radiology 131:99- 104.

Busch, W. (1951) Die morphologie der Sella turcica und ihre Beziehung zur Hypophyse. Virchow's Archiv für Klinische Medizin 320:437-458.

Carstens, M. (1949) Die Sella diagnostik. Fortschr. Röntgenstr. 71:257-272.

Cave, A.J.E. (1930-31) The craniopharyngeal canal in Man and Anthropoids. J. Anat. 65:363-367.

Chouké, K.S. (1946) On the incidence of the foramen of Civinini and the porus crotaphitico-buccinatorius in American Whites and Negroes. I. Observations on 1,544 skulls. Am. J. Phys. Anthropol. 4:203-226.

Chouké, K.S. (1947) On the incidence of the foramen of Civinini and porus crotaphitico-buccinatorius in American Whites and Negroes. II Observations on 2,745 additional skulls. Am. J. Phys. Anthropol. N.S. 5:79-86.

Christensen, J.B., Lachman, E. and A.M. Brues. (1960) A study of the roentgen appearance of cranial vault sutures: Correlation with their anatomy. Am. J. Roentgenol. 83:615-627.

Congdon, E.D. (1920) The distribution and mode of origin of septa and walls of the sphenoid sinus. Anat. Rec. 18:97-123.

Congdon, E.D. (1920) Simultaneous occurrence of very small sphenoid and frontal sinuses. Anat. Rec. 19:153-157.

Cope, V.Z. (1916) The internal structure of the sphenoidal sinus. J. Anat. 51:127-136.

Costen, J.B. (1930) Relief of right-sided headache by resection of left sphenoid (Anatomic variation of sphenoid sinuses): Report of a case. Ann. Otol., Rhin. Laryng. 39:1074-1077.

Currarino, G., Maravilla, K.R. and K.E. Salyer (1985) Transsphenoidal canal (large craniopharyngeal canal) and its pathologic implications. AJNR 6:39-43.

Dixon, F.W. (1937) A comparative study of the sphenoid sinus (a study of 1600 skulls). Ann. Otol. Rhinol. Laryngol. 46:687-698.

Gilse, P.H.G. van. (1927) The development of the sphenoidal sinus in man and its homology in mammals. J. Anat. 61:153-166.

Ginsberg, LE, Pruett, S.W., Chin, M.Y.M. and A.D. Elster. (1994) Skull-base foramina of the middle cranial fossa: Reassessment of normal variation with high-resolution CT AJNR (Am. J. Neuroradiology) 15:2&3-291.

Giuffrida-Ruggeri, V. (1901) Division longitudinale de l'ala magna du sphénoïde. Extension de la squame du temporal en hauteur comme caractère hiérarchique. Situation du squelette nasal. Arch. Ital. Biol. 36:337-338.

Goalwin, H.A. (1924) Die exakte radiographische Darstellung des Canalis opticus. Fortschr. Röntgenstr. 32:218-222.

Goalwin, H.A. (1927) One thousand optic canals: Clinical, anatomic and roentgenologic study. JAMA 89:1745-1748.

Goldhamer, K. and A. Schtfller. (1925) Die variatdten der Sella turcica. Fortschr. Röntgenstr. 33:894- 900.

Goodyear, H.M. (1948) Ophthalmic conditions referable to diseases of the paranasal sinuses. Arch. Otolaryngol. 48:202-1208.

Greig, D.M. (1929) Congenital anomalies of the foramen spinosum. Edinburgh Med. J. 36:363-37l.

Gruber, W. (1877) Die Fossa temporalis im Bereiche des aufsteigenden Theiles des Temporaflügels des Sphenoides als eine enorm tiefe Grube. Arch. Pathol. Anat. Physiol. Klin. Med. 70:136.

Gruber, W. (1877) Ungewöhnliche Grube von enormer Grösse an der Wurzel des Processus pterygoides des Os sphenoides. Arch. Pathol. Anat. Physiol. Klin. Med. 70:137.

Gruber, W. (1888) Uber Duplicität des Foramen rotundum ossis sphenoides. Arch. Pathol. Anat. Physiol. Klin. Med. 113:533.

Hammer, G. and C. Rådberg. (1961) The sphenoidal sinus. An anatomical and roentgenologic study with reference to transsphenoid hypopsectomy. Acta Radiol. 56:401-422.

Harwood-Nash, D.C. and C.R. Fritz. (1976) Neuroradiology in Infants and Children. Mosby, St. Louis.

Henderson, W.R. (1966) A note on the relationship of the human maxillary nerve to the cavernous sinus and to an emissary sinus passing through the foramen ovale. J. Anat. 100:905-908.

Hyrtl, J. (1862) Ueber den Porus crotaphitico-buccinatorius beim Menschen. Sitzber. Kais. Akad. der Wissen, Math.-Natur. Kl., Wien. 64:111-115.

Hyrtl, J. (1863) Über den Porus crotaphitico-buccinatorius beim Menschen. Sitzber. Kais. Akad. der Wissen, Math.-Natur. Kl., Wien 46:111:115.

Ingham, F.L. (1947) Cranio-cleido-dysostosis. Brit. J. Radiol. 20:332-334.

Jackson, W.P.U. (1951) Osteo-dental dysplasia (cleido-cranial dysostosis). Acta Med. Scand. 139:292-307.

Jadhav SD, Ambali MP, Zambare BR. Sphenoidal emissary foramen and its clinical consideration. Int J Res Med Sci. 2016; 4(7): 2926-2929. doi:10.18203/2320-6012.ijrms20161978

Kerner, D. (1956) Andreas Vesalius, 400 Jahre deskriptive Anatomie. Fortschr. Röntgenstr. 85:629-632.

Klinke, K. and H. Pahlke. (1930) Dysostosis cleidocranialis. Bericht über 2 Fälle. Arch. Kinderheilk. 91:46-54.

Klöppner, K. (1939) Die Sella turcica des Neugeborenen im Röntgenbild (Grössa, Kontur, Form und Formvarianten der Sella turcica). Fortschr. Rontgenstr. 60:370-379.

Kullnig, G. (1953) Persistierender offener Ductus craniopharyngicus. Fortschr. Rongentgenstr. 79:127.

Lachi, P. (1914-15) Di una "fossetta del tubercolo pterigoideo" nel cranio umano. Arch. Ital. Anat. Embriol. 13:273-283.

Landzert, T. (1867-68) Ueber den Canalis cranio-pharyngeus am Schadel des Neugeborenen. St. Petersburger Medicinische Zeitschrift 14:133-339.

Lang, J. (1983) Clinical Anatomy of the Head, Neurocranium, Orbit and Craniocervical Region. Springer-Verlag, Berlin.

Lang, J. (1991) Clinical Anatomy of the Posterior Cranial Fossa and its Foramina. Georg Thieme Verlag, Stuttgart.

Lanzieri, C.F., Duchesneau, P.M., Rosenbloom, S.A., Smith, A.S. and E.E. Rosenbloom. (1988) The significance of asymmetry of the foramen of Vesalius. Am. J. Neuroradiol. (AJNR) 9(6):1201-1204.

Lazorthes, G., Gayral, L. and R. Huron. (1953) Etude anatomique de la fente sphénoidale. Assoc. Anatomistes Comptes Rendus 40:165-168.

LeDouble, -. (1895) Soudure des apophyses clinoides du sphenoide et crète sous l'pineuse du scapulum dano l'espèce humaine. Bull. et Mém. de la Soc. d'Anthrop. de Paris 6:639-642.

LeDouble, -. (1902) Sur quelques variations des trous optiques. Assoc. Anatomistes Comptes Rendus. 4:209-212.

LeDouble, A.F. (1903) Traitè des Variations de Os du Crâne de I'Homme, Vol. 1. Paris.

LeDouble, -. (1902) A propos d'un cas de communication de la fente sphenoidale et du trou grand rond de l'alisphénoïde humain. Assoc. Anatomistes Comptes Rendus 4:207-208.

Lindblom, K. (1936) A roentgenographic study of the vascular channels of the skull, with special reference to intracranial tumors and arteriovenous aneurysms. Acta Radiol. (Suppl.) (Stockholm) 30:1-146.

Lowman, R.M., Robinson, F. and W.B. McAlister. (1966) The craniopharyngeal canal. Acta Radiol. [Diagn] (Stockholm) 5:41-54.

Luschka, H. von. (1867) Die Anatomie des Menschen. Der Kopf, Vol. 111/2, H. Laupp, Tübingen.

Lyman, H.W. (1935) The sphenoid sinus and the sphenopalatine ganglion as factors in so-called atypical trigeminal neuralgia. Ann. Otol., Rhin. Laryngol. 44:654-661.

Ottolenghi, S. (1896) Sur un os surnumeraire de la paroi interne de l'orbite dans des crânes de dégénérés. Arch. Ital. Biol. 25:152.

Peele, J.C. (1957) Unusual anatomical variations of the sphenoid sinuses. Laryngoscope. 67:208-237.

Pfister, J. (1890) Über Form und Grösse des Intervaginalraums des Schnerven im Bereich des canalis opticus. von Graefes' Arch Ophthalmol. 36:83-93

Picozzo, T. (1896) La soudure des sutures crâniennes en rapport avec le sexe en l'âge. Arch. Ital. Biol. 25:333-334.

Poornima B., Phaniraj S., Mallikarjun, M. (2015) A Study of Incidence of Emissary Sphenoidal Foramen in Dry Adult Human Skull Bones. Indian Journal of Pharmaceutical Science & Research. 5(4):273-275.

Pruett, B.S. (1928) On the dimensions of the hypophyseal fossa in man. Am. J. Phys. Anthropol. 11:205-222.

Radioévitch, S., Jovanovic, S. and N. Lotric. (1961) Les variations de la selle turcique, du corps du sphénoïde et des sinus sphénoïdaux en fonction des variations de l'angle sphénoïdal. Acta Anat. 44:354-362.

Riggle, P.P. (1932) Ethmosphenoidal cells with agenesis of the sphenoidal sinus. Arch. Otolaryngol. 16:532-537.

Ruppricht, W. (1924) über einige Fälle von Pneumatisation der kleinen Keilbeinflügel, besonders im Bereich des Processus clinoidant. Anat. Anz. 58:480-496.

Schumacher, O. (1929-30) Weitgehende Asymmetrie des Sinus sphenoidalis. Anat. Anz. 68:419-423.

Shapiro, R. and F. Robinson. (1967) The foramina of the middle cranial fossa: a phylogenetic, anatomic and pathologic study. AJR (Am. J. Roentgenol.) 101:779-794.

Sondheimer, F.K. (1971) Basal foramina and canals. In: Newton, T.H. and D.G. Potts, Eds. Radiology of the Skull and Brain: The Skull. Vol. 1, Book 1. Mosby, St Louis.

Snyder, L.H. and F. Blank. (1944) Studies in human inheritance XXIV. Bridged sella as a genetic trait. Ohio Med. J. 40:318-320.

Staurenghi, C. (1896) Sur la topographie de "l'ostium sphenoidale" de l'homme. Arch. Ital. Biol. 25:154-155.

Staurenghi, C. (1901) Rapports topographiques entre les petites ailes du présphénoïde et l'angle sphénoïden du pariëtal humain (suture xiphoparietal, processus orbitaire de la grande aile du sphénoïde, suture fronto-grand-aile). Homologie des centres orbito-sphénoïde de l'homme. Arch. Ital. Biol. 36:333-334.

Sternberg, M. (1888) Ein bisher nicht beschriebener Kanal im Keilbein des Menschen. Anat. Anz. 3:784-786.

Sutton, J.B. (1884) The relation of the orbito-sphenoid to the region pterion in the side wall of the skull. J. Anat. Physiol. 18:219-222.

Tenchini, L. and U. Zimmerl. (1903) Un nouveau processus anormal de l'Os sphenoidale de l'homme. Arch. Ital. Biol. 39:482.

Vail, H.H. (1929) Vidian Neuroglias from diseases of the sphenoid sinus. Arch. Surg. 18:1247-1255.

Van Alyea, O.E. (1941) Sphenoid sinus. Anatomic study, with consideration of the clinical significance of the structural characteristics of the sphenoid sinus. Arch. Otolaryngol. 34:225-253.

Van Alyea, O.E. (1944) Sphenoid sinus drainage. Ann. Otol., Rhin. & Laryngol. 53:493-501.

Warwick, R. (1951) A juvenile skull exhibiting duplication of the optic canals and subdivision of the superior orbital fissure. J. Anat. 85:289-290.

White, L.E. (1924) An anatomic and x-ray study of the optic canal in cases of optic nerve involvement. Ann. Otol. Rhinol. Laryngol. 33:121-140.

Williams, P.L., Warwick, R., Dyson, M. and L.H. Bannister, Eds. Grays Anatomy of the Human Body. 37" ed. Churchill Livingstone, New York and London.

Wood-Jones, F. (1931) The non-metrical morphological characters of the skull as criteria for racial diagnosis. Part 1: General discussion of the morphological characters employed in racial diagnosis. J. Anat. 65:179-495.

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