optic chiasma

optic

of or pertaining to the eye.

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optic chiasma

see optic chiasm.

optic cortex

see visual cortex.

optic cup activity

see intraretinal space.

optic disk

the disk in the fundus of the eye marking the point at which the optic nerve enters; it is accompanied by blood vessels, is oval, light in color and the blind spot of the retina.

optic nerve

the second cranial nerve; it is purely sensory and is concerned with carrying impulses for the sense of sight. The rods and cones of the retina are connected with the optic nerve which leaves the eye slightly to the nasal side of the center of the retina. The point at which the optic nerve leaves the eye is called the blind spot because there are no rods and cones in this area. The optic nerve passes through the optic foramen of the skull and into the cranial cavity. It then passes backward and undergoes a division; those nerve fibers leading from the nasal side of the retina cross to the opposite side in the optic chiasma while those from the temporal side continue to the thalamus uncrossed. The nerve tracts proceeding backward from the optic chiasm, pass around the cerebral peduncle, and dividing into a lateral and medial root, which end in the superior colliculus and lateral geniculate body, respectively. After synapsing in the thalamus the neurons convey visual impulses to the occipital lobe of the brain.

Injury to the nerve leads to partial or complete loss of sight on the opposite side. Commonly bilateral.

optic nerve aplasia

an uncommon congenital anomaly, most frequently seen in Collie dogs; affected animals are blind from birth. Hypovitaminosis A and prenatal infection with bovine virus diarrhea are possible causes.

optic nerve inflammation

optic neuritis.

optic primordia

the eyes begin in the embryo as a pair of shallow optic grooves on each side of the developing forebrain. The grooves form optic vesicles which invaginate to form a double-walled optic cup.

optic radiation

fibers from the lateral geniculate body entering the occipital cortex.

optic stalk

the evagination from the neural tube of the developing embryo which develops the optic cup at its extremity; the stalk persists as the optic nerve.

optic sulcus

see optic groove.

optic vesicle

the initial evagination from the neural tube which gives rise to the optic cup and the optic stalk.

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chiasma, optic 

A structure located above the pituitary gland and formed by the junction and partial decussation (crossing-over) of the optic nerves. The fibres from the nasal half of the retina of the left eye cross over to join the fibres from the temporal half of the right retina to make up the right optic tract and vice versa. About 53% of the axons of the optic nerves cross to the opposite tract (Fig. C9). A lesion of the chiasma produces a typical field defect (heteronymous hemianopia). Note: also spelt chiasm. See circle of Willis; decussation; visual pathway; stereo-blindness; optic tracts.

Fig. C9 An illustration of the decussation of the optic nerve fibres occurring in the optic chiasma