The Exterior of the Skull
Henry Gray (1821–1865). Anatomy of the Human Body. 1918.
The skull as a whole may be viewed from different points, and the views so obtained are termed the normæ of the skull; thus, it may be examined from above (norma verticalis), from below (norma basalis), from the side (norma lateralis), from behind (norma occipitalis), or from the front (norma frontalis).
Norma Verticalis—When viewed from above the outline presented varies greatly in different skulls; in some it is more or less oval, in others more nearly circular. The surface is traversed by three sutures, viz.: (1) the coronal sutures nearly transverse is direction, between the frontal and parietals; (2) the sagittal sutures medially placed, between the parietal bones, and deeply serrated in its anterior two-thirds; and (3) the upper part of the lambdoidal suture between the parietals and the occipital. The point of junction of the sagittal and coronal suture is named the bregma that of the sagittal and lambdoid sutures, the lambda they indicate respectively the positions of the anterior and posterior fontanelles in the fetal skull. On either side of the sagittal suture are the parietal eminence and parietal foramen—the latter, however, is frequently absent on one or both sides. The skull is often somewhat flattened in the neighborhood of the parietal foramina, and the term obelion is applied to that point of the sagittal suture which is on a level with the foramina. In front is the glabella and on its lateral aspects are the superciliary arches and above these the frontal eminences Immediately above the glabella may be seen the remains of the frontal suture in a small percentage of skulls this suture persists and extends along the middle line to the bregma. Passing backward and upward from the zygomatic processes of the frontal bone are the temporal lines which mark the upper limits of the temporal fossæ. The zygomatic arches may or may not be seen projecting beyond the anterior portions of these lines.
FIG. 187– Base of skull. Inferior surface. (Picture From the Classic Gray's Anatomy)
Norma Basalis (Fig. 187)—The inferior surface of the base of the skull, exclusive of the mandible, is bounded in front by the incisor teeth in the maxillæ; behind, by the superior nuchal lines of the occipital; and laterally by the alveolar arch, the lower border of the zygomatic bone, the zygomatic arch and an imaginary line extending from it to the mastoid process and extremity of the superior nuchal line of the occipital. It is formed by the palatine processes of the maxillæ and palatine bones, the vomer, the pterygoid processes, the under surfaces of the great wings, spinous processes, and part of the body of the sphenoid, the under surfaces of the squamæ and mastoid and petrous portions of the temporals, and the under surface of the occipital bone. The anterior part or hard palate projects below the level of the rest of the surface, and is bounded in front and laterally by the alveolar arch containing the sixteen teeth of the maxillæ. Immediately behind the incisor teeth is the incisive foramen In this foramen are two lateral apertures, the openings of the incisive canals (foramina of Stenson) which transmit the anterior branches of the descending palatine vessels, and the nasopalatine nerves. Occasionally two additional canals are present in the incisive foramen; they are termed the foramina of Scarpa and are situated in the middle line; when present they transmit the nasopalatine nerves. The vault of the hard palate is concave, uneven, perforated by numerous foramina, marked by depressions for the palatine glands, and traversed by a crucial suture formed by the junction of the four bones of which it is composed. In the young skull a suture may be seen extending on either side from the incisive foramen to the interval between the lateral incisor and canine teeth, and marking off the os incisivum or premaxillary bone. At either posterior angle of the hard palate is the greater palatine foramen for the transmission of the descending palatine vessels and anterior palatine nerve; and running forward and medialward from it a groove, for the same vessels and nerve. Behind the posterior palatine foramen is the pyramidal process of the palatine bone perforated by one or more lesser palatine foramina and marked by the commencement of a transverse ridge, for the attachment of the tendinous expansion of the Tensor veli palatini. Projecting backward from the center of the posterior border of the hard palate is the posterior nasal spine for the attachment of the Musculus uvulæ. Behind and above the hard palate are the choanæ measuring about 2.5 cm. in their vertical and 1.25 cm. in their transverse diameters. They are separated from one another by the vomer, and each is bounded above by the body of the sphenoid, below by the horizontal part of the palatine bone, and laterally by the medial pterygoid plate of the sphenoid. At the superior border of the vomer may be seen the expanded alæ of this bone, receiving between them the rostrum of the sphenoid. Near the lateral margins of the alæ of the vomer, at the roots of the pterygoid processes, are the pharyngeal canals The pterygoid process presents near its base the pterygoid canal for the transmission of a nerve and artery. The medial pterygoid plate is long and narrow; on the lateral side of its base is the scaphoid fossa for the origin of the Tensor veli palatini, and at its lower extremity the hamulus around which the tendon of this muscle turns. The lateral pterygoid plate is broad; its lateral surface forms the medial boundary of the infratemporal fossa, and affords attachment to the Pterygoideus externus. Behind the nasal cavities is the basilar portion of the occipital bone, presenting near its center the pharyngeal tubercle for the attachment of the fibrous raphé of the pharynx, with depressions on either side for the insertions of the Rectus capitis anterior and Longus capitis. At the base of the lateral pterygoid plate is the foramen ovale for the transmission of the mandibular nerve, the accessory meningeal artery, and sometimes the lesser superficial petrosal nerve; behind this are the foramen spinosum which transmits the middle meningeal vessels, and the prominent spina angularis (sphenoidal spine), which gives attachment to the sphenomandibular ligament and the Tensor veli palatini. Lateral to the spina angularis is the mandibular fossa divided into two parts by the petrotympanic fissure the anterior portion, concave, smooth bounded in front by the articular tubercle serves for the articulation of the condyle of the mandible; the posterior portion, rough and bounded behind by the tympanic part of the temporal, is sometimes occupied by a part of the parotid gland. Emerging from between the laminæ of the vaginal process of the tympanic part is the styloid process and at the base of this process is the stylomastoid foramen for the exit of the facial nerve, and entrance of the stylomastoid artery. Lateral to the stylomastoid foramen, between the tympanic part and the mastoid process, is the tympanomastoid fissure for the auricular branch of the vagus. Upon the medial side of the mastoid process is the mastoid notch for the posterior belly of the Digastricus, and medial to the notch, the occipital groove for the occipital artery. At the base of the medial pterygoid plate is a large and somewhat triangular aperture, the foramen lacerum bounded in front by the great wing of the sphenoid, behind by the apex of the petrous portion of the temporal bone, and medially by the body of the sphenoid and basilar portion of the occipital bone; it presents in front the posterior orifice of the pterygoid canal behind, the aperture of the carotid canal The lower part of this opening is filled up in the fresh state by a fibrocartilaginous plate, across the upper or cerebral surface of which the internal carotid artery passes. Lateral to this aperture is a groove, the sulcus tubæ auditivæ between the petrous part of the temporal and the great wing of the sphenoid. This sulcus is directed lateralward and backward from the root of the medial pterygoid plate and lodges the cartilaginous part of the auditory tube; it is continuous behind with the canal in the temporal bone which forms the bony part of the same tube. At the bottom of this sulcus is a narrow cleft, the petrosphenoidal fissure which is occupied, in the fresh condition, by a plate of cartilage. Behind this fissure is the under surface of the petrous portion of the temporal bone, presenting, near its apex, the quadrilateral rough surface, part of which affords attachment to the Levator veli palatini; lateral to this surface is the orifice of the carotid canal and medial to it, the depression leading to the aquæductus cochleæ the former transmitting the internal carotid artery and the carotid plexus of the sympathetic, the latter serving for the passage of a vein from the cochlea. Behind the carotid canal is the jugular foramen a large aperture, formed in front by the petrous portion of the temporal, and behind by the occipital; it is generally larger on the right than on the left side, and may be subdivided into three compartments. The anterior compartment transmits the inferior petrosal sinus; the intermediate, the glossopharyngeal, vagus, and accessory nerves; the posterior, the transverse sinus and some meningeal branches from the occipital and ascending pharyngeal arteries. On the ridge of bone dividing the carotid canal from the jugular foramen is the inferior tympanic canaliculus for the transmission of the tympanic branch of the glossopharyngeal nerve; and on the wall of the jugular foramen, near the root of the styloid process, is the mastoid canaliculus for the passage of the auricular branch of the vagus nerve. Extending forward from the jugular foramen to the foramen lacerum is the petroöccipital fissure occupied, in the fresh state, by a plate of cartilage. Behind the basilar portion of the occipital bone is the foramen magnum bounded laterally by the occipital condyles, the medial sides of which are rough for the attachment of the alar ligaments. Lateral to each condyle is the jugular process which gives attachment to the Rectus capitis lateralis muscle and the lateral atlantoöccipital ligament. The foramen magnum transmits the medulla oblongata and its membranes, the accessory nerves, the vertebral arteries, the anterior and posterior spinal arteries, and the ligaments connecting the occipital bone with the axis. The mid-points on the anterior and posterior margins of the foramen magnum are respectively termed the basion and the opisthion In front of each condyle is the canal for the passage of the hypoglossal nerve and a meningeal artery. Behind each condyle is the condyloid fossa perforated on one or both sides by the condyloid canal, for the transmission of a vein from the transverse sinus. Behind the foramen magnum is the median nuchal line ending above at the external occipital protuberance while on either side are the superior and inferior nuchal lines these, as well as the surfaces of bone between them, are rough for the attachment of the muscles which are enumerated on pages 129 and 130.
FIG. 188– Side view of the skull. (Picture From the Classic Gray's Anatomy)
Norma Lateralis (Fig. 188)—When viewed from the side the skull is seen to consist of the cranium above and behind, and of the face below and in front. The cranium is somewhat ovoid in shape, but its contour varies in different cases and depends largely on the length and height of the skull and on the degree of prominence of the superciliary arches and frontal eminences. Entering into its formation are the frontal, the parietal, the occipital, the temporal, and the great wing of the sphenoid. These bones are joined to one another and to the zygomatic by the following sutures: the zygomaticotemporal between the zygomatic process of the temporal and the temporal process of the zygomatic; the zygomaticofrontal uniting the zygomatic bone with the zygomatic process of the frontal; the sutures surrounding the great wing of the sphenoid, viz., the sphenozygomatic in front, the sphenofrontal and sphenoparietal above, and the sphenosquamosal behind. The sphenoparietal suture varies in length in different skulls, and is absent in those cases where the frontal articulates with the temporal squama. The point corresponding with the posterior end of the sphenoparietal suture is named the pterion it is situated about 3 cm. behind, and a little above the level of the zygomatic process of the frontal bone. The squamosal suture arches backward from the pterion and connects the temporal squama with the lower border of the parietal: this suture is continuous behind with the short, nearly horizontal parietomastoid suture which unites the mastoid process of the temporal with the region of the mastoid angle of the parietal. Extending from above downward and forward across the cranium are the coronal and lambdoidal sutures the former connects the parietals with the frontal, the latter, the parietals with the occipital. The lambdoidal suture is continuous below with the occipitomastoid suture between the occipital and the mastoid portion of the temporal. In or near the last suture is the mastoid foramen for the transmission of an emissary vein. The point of meeting of the parietomastoid, occipitomastoid, and lambdoidal sutures is known as the asterion Immediately above the orbital margin is the superciliary arch and, at a higher level, the frontal eminence Near the center of the parietal bone is the parietal eminence Posteriorly is the external occipital protuberance from which the superior nuchal line may be followed forward to the mastoid process. Arching across the side of the cranium are the temporal lines which mark the upper limit of the temporal fossa.
[[The Temporal Fossa (fossa temporalis)]]—The temporal fossa is bounded above and behind by the temporal lines, which extend from the zygomatic process of the frontal bone upward and backward across the frontal and parietal bones, and then curve downward and forward to become continuous with the supramastoid crest and the posterior root of the zygomatic arch. The point where the upper temporal line cuts the coronal suture is named the stephanion The temporal fossa is bounded in front by the frontal and zygomatic bones, and opening on the back of the latter is the zygomaticotemporal foramen Laterally the fossa is limited by the zygomatic arch, formed by the zygomatic and temporal bones; below it is separated from the infratemporal fossa by the infratemporal crest on the great wing of the sphenoid, and by a ridge, continuous with this crest, which is carried backward across the temporal squama to the anterior root of the zygomatic process. In front and below, the fossa communicates with the orbital cavity through the inferior orbital or sphenomaxillary fissure The floor of the fossa is deeply concave in front and convex behind, and is formed by the zygomatic, frontal, parietal, sphenoid, and temporal bones. It is traversed by vascular furrows; one, usually well-marked, runs upward above and in front of the external acoustic meatus, and lodges the middle temporal artery. Two others, frequently indistinct, may be observed on the anterior part of the floor, and are for the anterior and posterior deep temporal arteries. The temporal fossa contains the Temporalis muscle and its vessels and nerves, together with the zygomaticotemporal nerve. The zygomatic arch is formed by the zygomatic process of the temporal and the temporal process of the zygomatic, the two being united by an oblique suture; the tendon of the Temporalis passes medial to the arch to gain insertion into the coronoid process of the mandible. The zygomatic process of the temporal arises by two roots, an anterior, directed inward in front of the mandibular fossa, where it expands to form the articular tubercle, and a posterior, which runs backward above the external acoustic meatus and is continuous with the supramastoid crest. The upper border of the arch gives attachment to the temporal fascia; the lower border and medial surface give origin to the Masseter. Below the posterior root of the zygomatic arch is the elliptical orifice of the external acoustic meatus bounded in front, below, and behind by the tympanic part of the temporal bone; to its outer margin the cartilaginous segment of the external acoustic meatus is attached. The small triangular area between the posterior root of the zygomatic arch and the postero-superior part of the orifice is termed the suprameatal triangle on the anterior border of which a small spinous process, the suprameatal spine is sometimes seen. Between the tympanic part and the articular tubercle is the mandibular fossa divided into two parts by the petrotympanic fissure. The anterior and larger part of the fossa articulates with the condyle of the mandible and is limited behind by the external acoustic meatus: the posterior part sometimes lodges a portion of the parotid gland. The styloid process extends downward and forward for a variable distance from the lower part of the tympanic part, and gives attachment to the Styloglossus, Stylohyoideus, and Stylopharyngeus, and to the stylohyoid and stylomandibular ligaments. Projecting downward behind the external acoustic meatus is the mastoid process, to the outer surface of which the Sternocleidomastoideus, Splenius capitis, and Longissimus capitis are attached.
FIG. 189– Left infratemporal fossa. (Picture From the Classic Gray's Anatomy)
[[The Infratemporal Fossa (fossa infratemporalis; zygomatic fossa) (Fig. 189)]]—The infratemporal fossa is an irregularly shaped cavity, situated below and medial to the zygomatic arch It is bounded, in front by the infratemporal surface of the maxilla and the ridge which descends from its zygomatic process; behind by the articular tubercle of the temporal and the spinal angularis of the sphenoid; above by the great wing of the sphenoid below the infratemporal crest, and by the under surface of the temporal squama; below by the alveolar border of the maxilla; medially by the lateral pterygoid plate. It contains the lower part of the Temporalis, the Pterygoidei internus and externus, the internal maxillary vessels, and the mandibular and maxillary nerves. The foramen ovale and foramen spinosum open on its roof, and the alveolar canals on its anterior wall. At its upper and medial part are two fissures, which together form a T-shaped fissure, the horizontal limb being named the inferior orbital, and the vertical one the pterygomaxillary. The inferior orbital fissure (fissura orbitalis inferior; sphenomaxillary fissure), horizontal in direction, opens into the lateral and back part of the orbit. It is bounded above by the lower border of the orbital surface of the great wing of the sphenoid; below by the lateral border of the orbital surface of the maxilla and the orbital process of the palatine bone; laterally by a small part of the zygomatic bone: 48 medially it joins at right angles with the pterygomaxillary fissure. Through the inferior orbital fissure the orbit communicates with the temporal, infratemporal, and pterygopalatine fossæ; the fissure transmits the maxillary nerve and its zygomatic branch, the infraorbital vessels, the ascending branches from the sphenopalatine ganglion, and a vein which connects the inferior ophthalmic vein with the pterygoid venous plexus. The pterygomaxillary fissure is vertical, and descends at right angles from the medial end of the preceding; it is a triangular interval, formed by the divergence of the maxilla from the pterygoid process of the sphenoid. It connects the infratemporal with the pterygopalatine fossa, and transmits the terminal part of the internal maxillary artery.
[[The Pterygopalatine Fossa (fossa pterygopalatina; sphenomaxillary fossa)]]—The pterygopalatine fossa is a small, triangular space at the angle of junction of the inferior orbital and pterygomaxillary fissures, and placed beneath the apex of the orbit. It is bounded above by the under surface of the body of the sphenoid and by the orbital process of the palatine bone; in front by the infratemporal surface of the maxilla; behind by the base of the pterygoid process and lower part of the anterior surface of the great wing of the sphenoid; medially by the vertical part of the palatine bone with its orbital and sphenoidal processes. This fossa communicates with the orbit by the inferior orbital fissure, with the nasal cavity by the sphenopalatine foramen, and with the infratemporal fossa by the pterygomaxillary fissure. Five foramina open into it. Of these, three are on the posterior wall, viz., the foramen rotundum the pterygoid canal and the pharyngeal canal in this order downward and medialward. On the medial wall is the sphenopalatine foramen and below is the superior orifice of the pterygopalatine canal The fossa contains the maxillary nerve, the sphenopalatine ganglion, and the terminal part of the internal maxillary artery. 13
Norma Occipitalis—When viewed from behind the cranium presents a more or less circular outline. In the middle line is the posterior part of the sagittal suture connecting the parietal bones; extending downward and lateralward from the hinder end of the sagittal suture is the deeply serrated lambdoidal suture joining the parietals to the occipital and continuous below with the parietomastoid and occipitomastoid sutures it frequently contains one or more sutural bones. Near the middle of the occipital squama is the external occipital protuberance or inion and extending lateralward from it on either side is the superior nuchal line, and above this the faintly marked highest nuchal line. The part of the squama above the inion and highest lines is named the planum occipitale and is covered by the Occipitalis muscle; the part below is termed the planum nuchale and is divided by the median nuchal line which runs downward and forward from the inion to the foramen magnum; this ridge gives attachment to the ligamentum nuchæ. The muscles attached to the planum nuchale are enumerated on p. 130. Below and in front are the mastoid processes, convex laterally and grooved medially by the mastoid notches. In or near the occipitomastoid suture is the mastoid foramen for the passage of the mastoid emissary vein. 14
Norma Frontalis (Fig. 190)—When viewed from the front the skull exhibits a somewhat oval outline, limited above by the frontal bone, below by the body of the mandible, and laterally by the zygomatic bones and the mandibular rami. The upper part, formed by the frontal squama, is smooth and convex. The lower part, made up of the bones of the face, is irregular; it is excavated laterally by the orbital cavities, and presents in the middle line the anterior nasal aperture leading to the nasal cavities, and below this the transverse slit between the upper and lower dental arcades. Above the frontal eminences stand out more or less prominently, and beneath these are the superciliary arches joined to one another in the middle by the glabella On and above the glabella a trace of the frontal suture sometimes persists; beneath it is the frontonasal suture, the mid-point of which is termed the nasion Behind and below the frontonasal suture the frontal articulates with the frontal process of the maxilla and with the lacrimal. Arching transversely below the superciliary arches is the upper part of the margin of the orbit, thin and prominent in its lateral two-thirds, rounded in its medial third, and presenting, at the junction of these two portions, the supraorbital notch or foramen for the supraorbital nerve and vessels. The supraorbital margin ends laterally in the zygomatic process which articulates with the zygomatic bone, and from it the temporal line extends upward and backward. Below the frontonasal suture is the bridge of the nose, convex from side to side, concavo-convex from above downward, and formed by the two nasal bones supported in the middle line by the perpendicular plate of the ethmoid, and laterally by the frontal processes of the maxillæ which are prolonged upward between the nasal and lacrimal bones and form the lower and medial part of the circumference of each orbit. Below the nasal bones and between the maxillæ is the anterior aperture of the nose, pyriform in shape, with the narrow end directed upward. Laterally this opening is bounded by sharp margins, to which the lateral and alar cartilages of the nose are attached; below the margins are thicker and curve medialward and forward to end in the anterior nasal spine On looking into the nasal cavity, the bony septum which separates the nasal cavities presents, in front, a large triangular deficiency; this, in the fresh state, is filled up by the cartilage of the nasal septum; on the lateral wall of each nasal cavity the anterior part of the inferior nasal concha is visible. Below and lateral to the anterior nasal aperture are the anterior surfaces of the maxillæ, each perforated, near the lower margin of the orbit, by the infraorbital foramen for the passage of the infraorbital nerve and vessels. Below and medial to this foramen is the canine eminence separating the incisive from the canine fossa. Beneath these fossæ are the alveolar processes of the maxillæ containing the upper teeth, which overlap the teeth of the mandible in front. The zygomatic bone on either side forms the prominence of the cheek, the lower and lateral portion of the orbital cavity, and the anterior part of the zygomatic arch. It articulates medially with the maxilla, behind with the zygomatic process of the temporal, and above with the great wing of the sphenoid and the zygomatic process of the frontal; it is perforated by the zygomaticofacial foramen for the passage of the zygomaticofacial nerve. On the body of the mandible is a median ridge, indicating the position of the symphysis; this ridge divides below to enclose the mental protuberance, the lateral angles of which constitute the mental tubercles. Below the incisor teeth is the incisive fossa and beneath the second premolar tooth the mental foramen which transmits the mental nerve and vessels. The oblique line runs upward from the mental tubercle and is continuous behind with the anterior border of the ramus. The posterior border of the ramus runs downward and forward from the condyle to the angle, which is frequently more or less everted. 15
FIG. 190– The skull from the front. (Picture From the Classic Gray's Anatomy)
FIG. 191– Horizontal section of nasal and orbital cavities. (Picture From the Classic Gray's Anatomy)
[[The Orbits (orbitæ) (Fig. 190)]]—The orbits are two quadrilateral pyramidal cavities, situated at the upper and anterior part of the face, their bases being directed forward and lateralward, and their apices backward and medialward, so that their long axes, if continued backward, would meet over the body of the sphenoid. Each presents for examination a roof a floor a medial and a lateral wall a base and an apex 16
FIG. 192– Medial wall of left orbit. (Picture From the Classic Gray's Anatomy)
The roof is concave, directed downward, and slightly forward, and formed in front by the orbital plate of the frontal; behind by the small wing of the sphenoid. It presents medially the trochlear fovea for the attachment of the cartilaginous pulley of the Obliquus oculi superior; laterally the lacrimal fossa for the lacrimal gland; and posteriorly the suture between the frontal bone and the small wing of the sphenoid. 17 The floor is directed upward and lateralward, and is of less extent than the roof; it is formed chiefly by the orbital surface of the maxilla; in front and laterally by the orbital process of the zygomatic bone, and behind and medially to a small extent, by the orbital process of the palatine. At its medial angle is the upper opening of the nasolacrimal canal, immediately to the lateral side of which is a depression for the origin of the Obliquus oculi inferior. On its lateral part is the suture between the maxilla and zygomatic bone, and at its posterior part that between the maxilla and the orbital process of the palatine. Running forward near the middle of the floor is the infraorbital groove ending in front in the infraorbital canal and transmitting the infraorbital nerve and vessels. 18 The medial wall (Fig. 192) is nearly vertical, and is formed from before backward by the frontal process of the maxilla, the lacrimal, the lamina papyracea of the ethmoid, and a small part of the body of the sphenoid in front of the optic foramen. Sometimes the sphenoidal concha forms a small part of this wall (see page 152). It exhibits three vertical sutures, viz., the lacrimomaxillary, lacrimoethmoidal, and sphenoethmoidal. In front is seen the lacrimal groove which lodges the lacrimal sac, and behind the groove is the posterior lacrimal crest from which the lacrimal part of the Orbicularis oculi arises. At the junction of the medial wall and the roof are the frontomaxillary, frontolacrimal, frontoethmoidal, and sphenofrontal sutures. The point of junction of the anterior border of the lacrimal with the frontal is named the dacryon In the frontoethmoidal suture are the anterior and posterior ethmoidal foramina the former transmitting the nasociliary nerve and anterior ethmoidal vessels, the latter the posterior ethmoidal nerve and vessels. 19 The lateral wall directed medialward and forward, is formed by the orbital process of the zygomatic and the orbital surface of the great wing of the sphenoid; these are united by the sphenozygomatic suture which terminates below at the front end of the inferior orbital fissure. On the orbital process of the zygomatic bone are the orbital tubercle (Whitnall) and the orifices of one or two canals which transmit the branches of the zygomatic nerve. Between the roof and the lateral wall, near the apex of the orbit, is the superior orbital fissure Through this fissure the oculomotor, the trochlear, the ophthalmic division of the trigeminal, and the abducent nerves enter the orbital cavity, also some filaments from the cavernous plexus of the sympathetic and the orbital branches of the middle meningeal artery. Passing backward through the fissure are the ophthalmic vein and the recurrent branch from the lacrimal artery to the dura mater. The lateral wall and the floor are separated posteriorly by the inferior orbital fissure which transmits the maxillary nerve and its zygomatic branch, the infraorbital vessels, and the ascending branches from the sphenopalatine ganglion. 20 The base of the orbit, quadrilateral in shape, is formed above by the supraorbital arch of the frontal bone, in which is the supraorbital notch or foramen for the passage of the supraorbital vessels and nerve; below by the zygomatic bone and maxilla, united by the zygomaticomaxillary suture; laterally by the zygomatic bone and the zygomatic process of the frontal joined by the zygomaticofrontal suture; medially by the frontal bone and the frontal process of the maxilla united by the frontomaxillary suture. 21 The apex situated at the back of the orbit, corresponds to the optic foramen 49 a short, cylindrical canal, which transmits the optic nerve and ophthalmic artery. 22 It will thus be seen that there are nine openings communicating with each orbit, viz., the optic foramen, superior and inferior orbital fissures, supraorbital foramen, infraorbital canal, anterior and posterior ethmoidal foramina, zygomatic foramen, and the canal for the nasolacrimal duct. 23 Note 48 Occasionally the maxilla and the sphenoid articulate with each other at the anterior extremity of this fissure; the zygomatic is then excluded from it. Note 49 Some anatomists describe the apex of the orbit as corresponding with the medial end of the superior orbital fissure. It seems better, however, to adopt the statement in the text, since the ocular muscles take origin around the optic foramen, and diverge from it to the bulb of the eye.