Читать книгу Essentials of Veterinary Ophthalmology - Kirk N. Gelatt - Страница 128

As the optic nerve approaches the optic chiasm, the location of fibers within the nerve gradually shifts in preparation for decussation at the optic chiasm. Generally, fibers from the temporal retina remain in the ipsilateral hemisphere, and fibers from the nasal retina cross over to the contralateral side. The amount of decussation varies between species, perhaps representing a broad evolutionary scale. Birds, as well as many amphibian and reptilian species, have complete crossover of fibers to the contralateral side. A greater proportion of fibers remain on the ipsilateral side in mammals that have developed binocular vision. In the horse, 15% of the fibers stay on the ipsilateral side, as do 25% in the dog and 33% in the cat. In humans, only half of the fibers cross over.

Figure 2.14 (a) The discs of the outer segments (facing the retinal pigment epithelium) of the photoreceptors contain the photopigment required for vision. The photoreceptors' inner segments contain the mitochondria, and together with the outer segments constitute the photoreceptor layer. The rod spherule and cone pedicle are synaptic expansions where their axons synapse with dendrites of bipolar and horizontal cells in the OPL. Portions of Müller's cells (dotted lines) are shown adjoining the rods and cones. (b) Rod and cone bipolar cells show extensive contacts. Horizontal cells also make synapses with both rods and cones. Interconnections are shown between spherules and pedicles.

The optic tract runs from the optic chiasm to the lateral geniculate nucleus (LGN). Because of decussation at the chiasm, fibers of the optic tract conduct information from the opposite visual field of both eyes. In humans, where roughly 50% of the axons decussate in the chiasm, the left optic tract relays the right visual hemifield of both eyes, and the right optic tract relays both left visual hemifields. In animals, where a greater percentage of fibers cross over, the left optic tract will relay a greater proportion of the right visual field from the right eye and a smaller proportion of the right visual field from the left eye.