Optic Nerve, Optic Chiasm, And The Retrochiasmatic Visual Pathways
Forensic science encompasses a broad field. To be a good forensic scientist, one must have the knowledge of the human anatomy and pathophysiology in order to distinguish a deviation from the normal. One of the tools of the trade is a microscope which could include forensic comparison microscopes. Forensic comparison microscopes are a type of microscope that is able to compare two samples side by side at the same time. Tissue samples, hair samples and fiber samples are just few of the many things that could be viewed under forensic comparison microscopes.
A forensic scientist should also be knowledgeable about the anatomy and physiology of the eye.
THE OPTIC NERVE
The trunk of the optic nerve consists of about 1 million axons that arise from the ganglion cells of the retina (nerve fiber layer), as seen in a microscope. The optic nerve emerges from the posterior surface of the globe through the posterior scle¬ral foramen, a short, circular opening in the sclera about 1 mm below and 3 mm nasal to the posterior pole of the eye. The nerve fibers, when examined under a microscope, become myelinated on leaving the eye, increasing the diameter from 1.5 mm (within the sclera) to 3 mm (within the orbit). The orbital segment of the nerve is 25-30 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 4-9 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. Since the ganglion cells of the retina and their axons are part of the central ner¬vous system, they will not regenerate if severed.
Sheaths of the Optic Nerve
The fibrous wrappings that ensheathe the optic nerve are continuous with the meninges. The pia mater is loosely attached about the nerve near the chiasm and only for a short distance within the cranium, but it is closely attached around most of the intracanalicular and all of the intraorbital portions. The pia, as seen under a microscope, consists of some fibrous tissue with numerous small blood vessels. It divides the nerve fibers into bundles by sending numerous septa into the nerve substance. The pia continues to the sclera, with a few fibers run¬ning into the choroid and lamina cribrosa.
The arachnoid comes in contact with the optic nerve at the intracranial end of the optic canal and ac¬companies the nerve to the globe, where it ends in the sclera and overlying dura. This sheath, when examined under a microscope, is a diaphanous connective tissue membrane with many septate connec¬tions with the pia mater, which it closely resembles. It is more intimately associated with pia than with dura.
The dura mater lining the inner surface of the cra¬nial vault comes in contact with the optic nerve as it leaves the optic canal. As the nerve enters the orbit from the optic canal, the dura splits, one layer (the perior¬bita) lining the orbital cavity and the other forming the outer dural covering of the optic nerve. The dura be¬comes continuous with the outer two-thirds of the sclera. The dura consists of tough, fibrous, relatively avascular tissue lined by endothelium on the inner sur¬face.
The subdural space is between the dura and the arachnoid; the subarachnoid space is between the pia and the arachnoid. Both are more potential than actual spaces under normal conditions but are direct continu¬ations of their corresponding intracranial spaces. Sub¬arachnoid or subdural fluid under sufficient pressure will fill these potential spaces about the optic nerve. The meningeal layers are adherent to each other and to the optic nerve and the surrounding bone within the optic foramen, making the optic nerve resistant to trac¬tion from either end.
Blood Supply
The surface layer of the optic disk, when viewed using an eye microscope, receives blood from branches of the retinal arterioles. Branches from the peripapillary choroidal vessels supply the rest of the nerve in front of the lamina cribrosa. In the region of the lamina cribrosa, the arterial supply is from the short posterior ciliary arteries. The retrolaminar 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.
THE OPTIC CHIASM
The optic chiasm is variably situated near the top of the diaphragm of the sella turcica, most often posteriorly projecting 1 cm above it and at a 45-degree angle up¬ward from the optic nerves as they emerge from the optic canals, when viewed using an eye microscope. The lamina terminalis forms the anterior wall of the third ventricle. The internal carotid arteries lie just laterally, adjacent to the cavernous sinuses. 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 o: temporal fibers to the ipsilateral optic tract. The macular fibers are arranged similarly to the rest of the fiber; except that their decussation is farther posteriorly and superiorly. The chiasm receives many small blood vessels from the neighboring circle of Willis.
THE RETROCHIASMATIC VISUAL PATHWAYS
Each optic tract begins at the posterolateral angle of the chiasm and sweeps around the upper part of the cerebral peduncle to end in the lateral geniculate nucleus Afferent pupillary fibers leave the tract just anterior to the nucleus and pass via the brachium of the superior colliculus to the midbrain. Afferent visual fibers terminate on cells in the lateral geniculate nucleus that give rise to the geniculocalcarine tract. This tract traverses the posterior limb of the internal capsule and then fans out into a broad bundle called the optic radiation. The fibers in this bundle curve backward around the anterior aspect of the temporal horn of the lateral ventricle and then medially to reach the calcarine cortex of the occipital lobe, where they terminate. The mos inferior fibers, which carry projections from the superior aspect of the contralateral half of the visual field course anteriorly into the temporal lobe in a configuration known as Meyer’s loop. Lesions of the temporal lobe that extend 5 cm back from the anterior tip involve these fibers and can produce superior quandrantanopic field defects.
The primary visual cortex (area VI) occupies the, upper and lower lips and the depths of the calcarine fissure on the medial aspect of the occipital lobe. Each lobe, when viewed using a microscope, receives input from the two ipsilateral half-retinas representing the contralateral half of the binocular vi¬sual field. Projection of the visual field onto the visual cortex occurs in a precise and orderly retinotopic pat¬tern. The macula is represented at the medial posterior pole, and the peripheral parts of the retina project to the most anterior part of the calcarine cortex. On either side of area VI, lies area V2 and then area V3. V2 ap¬pears to function in a manner very similar to VI. Area V4, situated on the medial surface of the cerebral hemi¬sphere but more anterior and inferior to V1 in the re¬gion of the fusiform gyrus, seems to be primarily con¬cerned with color processing. Motion detection localizes to an area at the junction of the occipital and temporal lobes, lateral to area VI and known as area V5.
