Winter is upon us! In East Tennessee we are fortunate to have a “front row seat” to nature at its finest, with the Smoky Mountains as the perfect backdrop. The majesty of the season begs us to contemplate the intricate gift of sight. While the visual pathway is a complex system involving millions of parts working in harmony, a basic overview is helpful in understanding just how wonderful and marvelous our eyes truly are. We see the world around us through light that is reflected off of the objects that we view and enter through our eyes. This light is first met by the front surface of our eyes, the cornea. The cornea is responsible for providing roughly 2/3rds of the refractive ability of the eye and bends the light so that it can properly focus onto the retina. Next, the light passes through our crystalline lens which is able to automatically focus up to a certain age, and fine-tune the light depending on how far you are from the object you are viewing. Lastly, the light is brought to a singular focal point on the macula of the retina, which is densely packed with cone photoreceptor cells responsible for sharp, central color vision. These photoreceptor cells are able to translate the varying wavelengths of light into separate messages that are carried off to the brain and interpreted as an image. It is almost too intricate to contemplate! There are also many other parts of the eye that aide in this process, each responsible for its own part of the visual system. Some awe-inspiring thoughts to contemplate:
- The tear film covers the anterior of our eyes, and must provide a smooth, consistent surface for us to be able to focus light properly. This is largely accomplished through blinking.
- The cornea is made of the same fibrous material as the sclera, which is the white of our eyes; however, the collagen sheets of the cornea lay parallel while the collagen of the sclera lays perpendicular and becomes opaque. If the sclera were clear, like the cornea, our eyes would allow too much light to enter. It would be overwhelming, and we would not be able to see anything!
- The iris is comprised of two main muscles, a dilator and sphincter. As we go about our day, these muscles are working tirelessly to regulate the diameter of the pupil to allow just the right amount of light to pass though for image formation.
- Since light refracts through both the cornea and crystalline lens, this is called a double refraction. This actually causes us to form images upside down, but our brain intelligently knows to flip the image for us. Similarly, where the optic nerve enters the back of the eye there are no photoreceptor cells (which receive light and transmit information to the brain) – but our brain is able to auto-fill this blind spot, so we see a complete image.
The next time you are caught up in surveying the beauty around you take a moment to contemplate just how awesome and precious the gift of sight truly is!