THE ORBIT (figs. 316-318)

General features.-The orbits form two roomy cavities which contain the eyeballs, their associated muscles, vessels, nerves, etc., and most of the lacrimal apparatus, together with a variable amount of soft fat. The orbital cavity is some what pyramidal in shape ; its base is formed by the orbital opening on the face, and its long axis is directed backwards and medially. Each orbit presents a roof, a floor, medial and lateral walls, a base or orbital opening and an apex.

The roof is a thin, gently concave plate of bone, which intervenes, throughout most of its extent, between the orbit and the part of the brain in the anterior cranial fossa. In its anteromedial part it is separated into two lamina; by the frontal sinus, which is an air-space in the bone communicating with the nasal cavity. In its anterolateral part it is deeply hollowed out to contain the orbital part of the lacrimal gland, and this depression is consequently called the lacrimal fossa. At the posterior end of the junction of the roof with the medial wall the optic foramen establishes communication between the orbit and the anterior cranial fossa. It transmits the optic nerve from the eyeball and the ophthalmic artery. Close to the superior, medial and lower margins of the foramen the common tendinous ring (fig. 956) is attached to the orbital walls.

Figure 315
Orbit frontal view - Figure 315
The medial wall (fig. 317) is exceedingly thin, except at its most posterior part, and slopes gently downwards and laterally into the floor. Anteriorly, it presents the deep lacrimal groove, which contains the lacrimal sac in the fresh state. In the bony skull the groove communicates below with the nasal cavity through the nasolacrimal canal, which is little more than, 1 cm. long and transmits the nasolacrimal duct. The floor of the groove separates the orbital from the nasal cavity, but behind the groove the air-containing ethmoidal sinuses intervene between the two cavities. Posteriorly the medial wall is related to the anterior part of the sphenoidal sinus and forms its lateral wall.

The floor of the orbit (fig. 316) is relatively thin and constitutes, in most of its extent, the roof of the maxillary sinus (fig. 317). It is not quite horizontal, but faces upwards and slightly laterally. In front it is directly continuous with the lateral wall, from which it is separated behind by the inferior orbital fissure. This fissure leads into the orbit from the pterygopalatine fossa posteriorly, and from the infratemporal fossa anteriorly. The maxillary nerve is the most important structure which it transmits. The lower lip of the fissure is notched by the infra-orbital groove, which passes forwards in the floor, sinking into it anteriorly and becoming converted into the infra-orbital canal. The anterior opening of the canal forms the infra-orbital foramen. The groove, canal and foramen transmit the infra orbital nerve, which is the large, terminal branch of the maxillary nerve. Through the anterior part of the inferior orbital fissure a vein passes to connect the inferior ophthalmic vein with the veins of the pterygoid plexus in the infratemporal fossa.

The lateral wall (fig. 318) is the thickest of the orbital walls, especially behind, where it separates the orbit from the middle cranial fossa. In front it is interposed between the orbit and the temporal fossa. The lateral wall and the roof are continuous anteriorly, but they are separated posteriorly by the superior orbital fissure. This important fissure is noticeably widened at its medial end (fig. 315), and its long axis is directed medially, backwards and slightly downwards. It communicates with the middle cranial fossa and transmits the oculomotor, trochlear and abducent nerves and the terminal branches of the ophthalmic nerve, together with the ophthalmic veins. Where the fissure widens its lower border is marked by a bony projection, often sharp in character, which gives attachment to the lateral part of the common tendinous ring for the origin of certain muscles of the eyeball.

Figure 316
A horizontal section through the nasal and orbital cavities superior view - Figure 316
The apex of the orbit lies in the medial end of the superior orbital fissure, but some anatomists prefer to place it on the bar of bone which separates that fissure from the optic foramen, i.e. the lower root of the lesser wing of the sphenoid bone.

Particular features.–The roof of the orbit is formed almost entirely by the orbital pIate of the frontal bone, but the under surface of the lesser wing of the sphenoid also makes up the posterior part. The suture between the two bones is almost horizontal. The optic foramen lies between the two roots of the lesser wing and is bounded medially by the body of the sphenoid bone. Near the junction of the roof and the medial wall, and close to the orbital opening, the small trochlear fossa (occasionally replaced by a trochlear spine) gives attachment to the fibrous pulley through which the tendon of the superior oblique muscle of the eyeball passes.

The medial wall (fig. 317) is limited in front by the lacrimal crest of the frontal process of the maxilla, which gives attachment to the orbicularis oculi muscle and to the lacrimal fascia. Behind this crest, the maxilla and the lacrimal bone participate in the formation of the lacrimal groove, and the suture between them can be seen in its floor. The upper opening of the nasolacrirnal carnal lies at the lower end of the groove, and its lateral boundary is formed by the tiny hamulus of the lacrimal bone, which curves forwards and medially to meet the lower part of the lacrimal crest of the frontal process of the maxilla. The posterior border of the groove is formed by the crest of the lacrimal bone, which gives origin to the lacrimal part of the orbicularis oculi muscle and to the lacrimal fascia; which bridges over the groove. The posterior part of the orbital aspect of the lacrimal bone is flattened, and articulates behind, by an almost vertical suture, with the orbital plate of the labyrinth of the ethmoid bone. The frontolacrimal suture and the lacrimomaxillary suture indicate the other limits of the orbital aspect of the lacrimal bone.

Figure 317
Orbit medial view - Figure 317
The orbital plate of the ethmoid makes the largest contribution to the medial wall. Somewhat rectangular in outline it is formed of very thin bone, which forms the lateral walls of the ethmoidal sinuses. Above it articulates with the medial edge of the orbital plate of the frontal bone, and the line of this suture is interrupted by the anterior and posterior ethmoidal foramina, which lead into minute bony canals. These canals transmit vessels and nerves of the same names the posterior ethmoidal nerve, however, is often absent-and lead into the anterior cranial fossa, where they open at the lateral edge of the cribriform plate. Below, the orbital plate articulates with medial edge of the orbital surface of the maxilla and, at its most posterior part, with the orbital process of the palatine bone. Posteriorly the orbital plate of the ethmoid articulates with the body of the sphenoid, which forms the most posterior part of the medial wall of the orbit and is separated from the roof by the optic foramen.

The floor of the orbit (fig. 316) is formed for the most part by the orbital surface of the maxilla, and, in its anterolateral part, by the zygomatic bone. At its posteromedial corner, where the floor meets the medial wall, a small triangular area is formed by the orbital process of the palatine bone. In addition to the maxillary nerve the inferior orbital fissure transmits the infra-orbital vessels, the zygomatic nerve, and a few minute twigs from the sphenopalatine ganglion to the orbital periosteum. The fissure is bounded above by the greater wing of the sphenoid, below by the maxilla, and the orbital process of the palatine bone, and laterally by the zygomatic bone or the zygomaticomaxillary suture. * In the anteromedial hart of the floor, just lateral to the.hamulus of the lacrimal bone. a small depression nay mark the origin of the inferior oblique muscle from the maxilla.

The lateral wall of the orbit (fig. 318) is formed by the orbital surface of the greater wing of the spbenoid behind, and by the orbital surface of the frontal process of the zygomatic bone in front. These two bones meet at the sphenozygomatic suture. This aspect of the zygomatic bone is perforated by minute canals for the zygomaticofacial and the zygomaticotemporal nerves. The former lies just within the orbital opening at the junction of the floor of the orbit with its lateral wall ; the latter lies at a slightly higher level, and is close to the sphenozygomatic suture.

Figure 318
Orbit lateral view - Figure 318
The superior orbital fissure is bounded above by the lesser and below by the greater tying of the sphenoid and medially by the body of that bone. The lacrima.l and frontal nerves traverse the narrow lateral part of the fissure, which also transmits the meningeal branch of the lacrimal artery and the occasional orbital branch of the middle meningeal artery. The trochlear nerve is situated more medially and lies just outside the common tendinous ring. The two divisions of the oculomotor nerve, the nasociliary and the abducent nerves pass within the common tendinous ring, and therefore traverse the wider medial part of the fissure. They may be accompanied by the superior and inferior ophthalmic veins, but the former may accompany the trochlear nerve and the latter may pass through the medial end of the fissure below the common tendinous ring.

 


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