The External Anatomic Landmarks, The Extraocular Muscles, And Blood Vessels And Supplies
The External Anatomic Landmarks
Accurate localization of the position of internal struc¬tures with reference to the external surface of the globe is important in many surgical procedures as well as in forensic science when these landmarks are compared using forensic comparison microscopes. The distance of structures from the limbus as measured externally is less than their actual length. Externally, the ora serrata is situated approximately 5.5 mm from the limbus on the medial side and 7 mm on the temporal side of the globe. This corresponds to the level of insertion of the rectus muscles. Injections into the vitreous cavity through the pars plana should be given 4-5 mm from the limbus in the phakic eye. In the aphakic eye, it is possible to inject 0.5-1 mm more anteriorly. The pars plicata, which is the target for cyclodestructive proce¬dures in the treatment of intractable glaucoma, occu¬pies the 2-3 mm directly posterior to the limbus.
THE EXTRAOCULAR MUSCLES
Six extraocular muscles control the movement of each eye: four rectus and two oblique muscles. Forensic comparison microscopes can detect any abnormality when these muscles are examined and compared.
Rectus Muscles
The four rectus muscles originate at a common ring tendon (annulus of Zinn) surrounding the optic nerve at the posterior apex of the orbit. They are named according to their insertion into the sclera on the medial, lateral, inferior, and superior surfaces of the eye. The principal action of the respective muscles is thus to adduct, abduct, depress, and elevate the globe. The muscles, when viewd using a microscope, are about 40 mm long, becoming tendinous 4-9 mm from the point of inser¬tion, where they are about 10 mm wide. The approxi¬mate distances of the points of insertion from the corneal limbus are as follows: medial rectus, 5.5 mm; inferior rectus, 6.75 mm; lateral rectus, 7 mm; and su¬perior rectus, 7.5 mm. With the eye in the primary position, the vertical rectus muscles make an angle of about 23 degrees with the optic axis. Forensic comparison microscopes can detect even the smallest deviation from the normal angles.
Oblique Muscles
The two oblique muscles control primarily torsional movement and, to a lesser extent, upward and downward movement of the globe.
The superior oblique, when examined under a microscope, is the longest and thinnest of the ocular muscles. It originates above and medial to the optic foramen and partially overlaps the origin of the levator palpebrae superioris muscle. The superior oblique has a thin, fusiform belly (40 mm long) and passes anteriorly in the form of a tendon to its trochlea, or pulley. It is then reflected backward and downward to attach in a fan shape to the sclera beneath the supe¬rior rectus. The trochlea is a cartilaginous structure at¬tached to the frontal bone 3 mm behind the orbital rim. The superior oblique tendon is enclosed in a syno¬vial sheath as it passes through the trochlea.
The inferior oblique muscle originates from the nasal side of the orbital wall just behind the inferior or¬bital rim and lateral to the nasolacrimal duct. It passes beneath the inferior rectus and then under the lateral rectus muscle to insert onto the sclera with a short ten¬don. The insertion is into the posterotemporal segment of the globe and just over the macular area. The muscle is 37 mm long when seen under a microscope.
In the primary position, the muscle plane of the su¬perior and inferior oblique muscles forms an angle of 51-54 degrees with the optic axis.
Fascia
All the extraocular muscles are ensheathed by fascia. Near the points of insertion of these muscles the fascia is continuous with Tenon’s capsule, and fascial conden¬sations to adjacent orbital structures act as the functional origins of the extraocular muscles, as seen under a microscope.
Nerve Supply
The oculomotor nerve (III) innervates the medial, infe¬rior, and superior rectus muscles and the inferior oblique muscle. The abducens nerve (VI) innervates the lateral rectus muscle; the trochlear nerve (IV) innervates the superior oblique muscle.
Blood Supply
The blood supply to the extraocular muscles is derived from the muscular branches of the ophthalmic artery. Branches from the lacrimal artery and the infraorbital artery also supply the lateral rectus and inferior oblique muscles, respectively.
