May 3, 2019
By Dr. Michael Trapani
This month we continue our exploration of Nature’s great wonder, the eye. As we move past the anterior chamber and beyond the deep surface of the lens, we leave all familiar structures behind. Welcome to the Posterior Chamber!
This is a strange and dark place. Gone is the sparkling clear aqueous humor that inflates and nourishes the front of the eye. Instead, the posterior chamber is filled with a gelatinous tissue, the vitreous humor, which fills the space behind the lens and in front of the retina. This material not only helps to maintain the spherical shape of the eye, but also serves to maintain the location and flatness of the retina by gently compressing the retina against the fibrous globe. The posterior chamber is dark - exactly like the back of a camera. The only light that enters this space does so through the pupil, and that light is focused by the lens, which creates a coherent image on the retina that is required for creation of clear vision.
Maintaining the retina’s location is critical. Light is projected on to the retinal surface at a specific focal distance, so that the image is in focus over the entire retina, but the most de-tailed visual image is generated in a relatively tiny section of retinal tissue, the macula.
Even within an organ as complex and compact as the retina, the macula stands out. Here, the high density of light receptor neurons is even higher than the surrounding retina, and the population of receptors is altered to favor detail and color vision. Dogs and cats do not have nearly the level of macular development that is seen in humans, but the structure is present and plays an important role in fine detail resolution and binocular vision. The ability of both eyes to see an object simultaneously is crucial for depth perception. Without stereo-scopic vision, it would be impossible to judge the distance of an object, especially the ever-changing distance of a moving object, and some tasks, like a dog catching a frisbee, would be almost impossible.
Not far from the macula is the bright, shining optic disk where the optic nerve fibers and retinal blood vessels enter the globe. The optic nerve’s shiny white color is due to the insu-lating material the surrounds the individual nerve fibers. A group of blood vessels follow the nerve into the eye, then the nerve fibers and blood vessels spread out over the globe. While spectacular in appearance, the optic disk does not contain light receptors and is re-sponsible for the blind spot present in all mammalian eyes. Instead, the disk serves to con-vey retinal signals back through the optic nerve, where nerve fibers from both eyes are mixed and distributed to multiple visual information processing centers located on both sides of the brain.
We tend to think the sense of vision as the big movie screen that we watch, but some visual senses are far removed from the visual cortex sensations we observe. One such sense is the ability to startle when the eye is menaced by a rapidly approaching object. The menace re-sponse takes place whether visual perception is present or not. Another such sense is the eye’s ability to adjust pupillary size based on the intensity of incoming light. Both of these visual senses are managed by portions of the brain far removed from areas handling con-scious visual perception and may remain even after brain injury prevents an individual from "seeing."
But the distribution of visual function takes place at even more fundamental levels. The ret-ina itself is much more than just a carpet of light sensitive cells. The retina is constructed with multiple layers of nerve cells that pre-process light information before any signal leaves the eye and identify signals that indicate movement, the presence of lines or edges, and other characteristics. This is a natural form of data compression that speeds signal transmission and optimizes information transmission along the optic nerve.
No matter where we look, the eye and its many parts can only be described as marvelous. Differently so are the eyes of birds, whose structure and function varies immensely from the eyes of mammals. The eyes of cephalopods, including octopi and squid, are arranged in a way that is completely different from either birds or mammals, and yet all of these species demonstrate powerfully detailed vision. Mother Nature, it seems, will never fail to amaze us with her ingenuity.
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