Structure of the eye
The intricacies and operations of the eyes are intricate. Each eye constantly adapts to varying light levels, focuses on objects at different distances, and continuously generates images that are promptly sent to the brain.
The orbit, housing the eyeball, muscles, nerves, blood vessels, tear-producing, and draining structures, is a pear-shaped cavity formed by several bones.
The eye's external layer, the sclera, is a tough, white covering. Near the front, the sclera is overlaid by a thin, transparent membrane, the conjunctiva, extending to the cornea's edge and covering the inner eyelid and eyeball surfaces.
The cornea, a transparent, dome-shaped covering located at the forefront of each eye, acts as the eye's equivalent of a windshield, shielding it from debris, germs, and other external elements. Its precise curvature significantly influences visual acuity and selectively filters ultraviolet (UV) rays. Situated just ahead of the fluid-filled anterior chamber of the eye, containing aqueous humor, the corneas are positioned in front of the iris and pupil, followed by the lens. Light penetrates the eye through the cornea, enters the pupil, and aided by the cornea the light is focused on the retina. Enveloping the cornea is the sclera, the white outer layer of the eye. Due to their role as the primary protective barrier for the eye's surface, corneas are susceptible to injuries and damage. Fortunately, they possess rapid and efficient self-repair mechanisms.
The iris, which gives the eye its coloration, regulates the pupil, the tiny dark aperture responsible for admitting light into the eye. Much like the nose of a canine or the fingerprint of a human, the coloration pattern of the iris is distinctive and individualized. Each person or dog possesses a unique iris, with no two beings having precisely identical iris patterns. The iris, encircling the pupil, regulates light entry, enlarging in darkness (dilating) and shrinking in brightness (constricting), akin to a camera's aperture. Pupil size is governed by the pupillary sphincter and dilator muscles.
The lens, behind the iris, adjusts its shape to focus light onto the retina, thickening for near objects and thinning for distant ones, controlled by ciliary muscles.
The retina houses light-sensing cells (photoreceptors) and nourishing blood vessels, with the macula being the most sensitive part, providing detailed vision like a high-resolution camera.
Photoreceptors transmit signals via nerve fibers, forming the optic nerve, starting at the optic disk.
Cones, concentrated in the macula, facilitate detailed and color vision, while rods, more numerous and sensitive to low light, aid night and peripheral vision.
The eyeball is divided into fluid-filled sections, maintaining its shape.
The anterior segment (chamber), from cornea to lens, holds aqueous humour, nourishing internal structures and draining via outflow channels. The posterior segment (chamber), from lens to retina, contains vitreous humour, a jellylike fluid.
Muscles, Nerves, and Blood Vessels of the Dog's Eyes
Multiple muscles collaborate to control eye movement, enabling it to shift the dog's gaze without the need to move its head. Each of these eye muscles receives stimulation from a specific cranial nerve.
The optic nerve, a type of cranial nerve responsible for transmitting signals from the retina to the brain, along with other cranial nerves, relay signals to individual eye muscles, traverse through the orbit, the bony socket encasing the eyeball.
Blood supply to each eye is facilitated by the ophthalmic artery and a branch known as the central retinal artery, originating from the former. Likewise, drainage of blood from the eye is managed by ophthalmic veins, also referred to as vortex veins, and the central retinal vein. These blood vessels ingress and egress from the rear portion of the eye.
The Optic Nerve - Central Nerve II (CN II)
The optic nerve isn't exactly a typical nerve; it's more like an extension of the brain. It's a crucial part of how we see things, and it helps with sensory visual perception. This nerve also plays a role in how our eyes react to light, like when we blink to protect them. The nerve fibers start from the back of the eye, pass through the skull bones, and connect to the brain. They then continue on a pathway through different parts of the brain, eventually reaching a structure called the lateral geniculate nucleus.
The visual pathway
The visual pathway comprises three consecutive neurons:
The initial neuron represents the bipolar cells found in the retina and receives visual input from the neuroepithelial cells of the retina (such as rods and cones).
The second neuron corresponds to the ganglion cell of the retina. Its axons are situated within the optic nerve and proceed through the optic chiasm and the proximal segment of the optic tract on the opposite side (with 55% decussation in humans, 66% in cats, and 75% in dogs). Decussation involves the crossing over, particularly of nerve fibers, often forming an X shape.
The third and final neuron has its cell body located in the lateral geniculate nucleus within the diencephalon. Its axons extend to the visual cortex, primarily the contralateral occipital cortex, forming a bundle of fibers known as the optic radiation.
How does Vision Loss Develop?
Blindness in pets can be caused by various factors, and the development of vision loss can vary depending on the underlying cause. The onset of vision impairment in animals may manifest gradually (chronic) or abruptly (acute), contingent upon the root cause. Certain conditions exhibit a slow progression, enabling pets to acclimate to evolving vision changes, whereas others lead to a swift development of blindness. Consistent veterinary examinations, timely addressing of ocular problems, and the promotion of a healthy lifestyle are crucial factors in preserving optimal eye health for pets.
References
Maggs, D.J., Miller, P.E., & Ofri, R. (eds.) (2018). Slatter's Fundamentals of Veterinary Ophthalmology (6th ed.). Elsevier.
Kern, T.J., & Gelatt, K.N. (2013). Ophthalmic Diseases in Small Animals: An Atlas and Guide. John Wiley & Sons.
Gelatt, K. N., & Gilger, B. C. (2013). Veterinary Ophthalmology (5th ed.). Wiley-Blackwell.
Gelatt, K. N. (Ed.). (2013). Essentials of Veterinary Ophthalmology. John Wiley & Sons.
Kern, T. J. (Ed.). (2017). Veterinary Ophthalmology: Two Volume Set. John Wiley & Sons.
Helps, C. R., Lait, P., Damhuis, A., Bjornehammar, U., & Bolta, D. (2005). Detection and prevalence of feline immunodeficiency virus proviral DNA. Journal of Virological Methods, 124(1-2), 137-143.
Gelatt, K. N. (2007). Veterinary Ophthalmology (4th ed.). Wiley-Blackwell.