Injections into the vitreous cavity through the pars plana should be given 3.5âˆ“4.0 mm from the limbus in the phakic eye and 3âˆ“3.5 mm from the limbus in the pseudophakic or aphakic eye.
The pars plicata, which is the target for cyclodestructive procedures in the treatment of intractable glaucoma, occupies the 2âˆ“3 mm directly posterior to the limbus.
consists of about 1 million axons that arise from the ganglion cells of the retina (nerve fiber layer).
The orbital segment of the nerve is 25âˆ“30 mm long; it travels within the optic muscle cone, via the bony optic canal, and thus gains access to the cranial cavity.
The intracanalicular portion measures 4âˆ“9 mm. After a 10-mm intracranial course, the nerve joins the opposite optic nerve to form the optic chiasm.
Eighty percent of the optic nerve consists of visual fibers that synapse in the lateral geniculate body on neurons whose axons terminate in the primary visual cortex of the occipital lobes. Twenty percent of the fibers are pupillary and bypass the geniculate body en route to the pretectal area.
The surface layer of the optic disk receives blood from branches of the retinal arterioles.
In the region of the lamina cribrosa, comprising the prelaminar, laminar, and retrolaminar segments of the optic nerve, the arterial supply is from the short posterior ciliary arteries.
The anterior intraorbital optic nerve receives some blood from branches of the central retinal artery.
The remainder of the intraorbital nerve, as well as the intracanalicular and intracranial portions, are supplied by a pial network of vessels derived from the various branches of the ophthalmic artery and other branches of the internal carotids
near the top of the diaphragm of the sella turcica.
The chiasm is made up of the junction of the two optic nerves and provides for crossing of the nasal fibers to the opposite optic tract and passage of temporal fibers to the ipsilateral optic tract.
The macular fibers are arranged similarly to the rest of the fibers except that their decussation is farther posteriorly and superiorly.
The chiasm receives many small blood vessels from the neighboring circle of Willis.