Eyesight is often described as the most precious of the five senses. Without eyesight, we lose the ability to effectively process and interpret much of the physical world, as well as successfully navigate and interact with the environment. In fact, up to 80% of the impressions that emerge perceptually are directed from our sense of vision, and even the slightest movements-- as well as actions-- are contingent on visual perception. Thus, it is beyond important that our vision is strong and clear overall, as well as accurate in terms of color perception, depth perception, and motion perception. 

Furthermore, it is likely that you or someone you know wears eyeglasses or contact lenses, which are prescribed as external lenses to assist the visual system in properly refracting light onto the light-sensitive tissue known as the retina. Thus, you should be familiar with the comprehensive nature of optometric examinations and how much is medically examined beyond simple refraction, such as the health of blood vessels and ocular tissue. You are probably even aware of the extent by which eyesight is taken for granted when complications are not present, and how tremendously quality of life is improved with clear eyesight. For all these reasons, it is crucial to understand how vision operates on an anatomical level, as well as the mechanics of one of the most common and dangerous vision-inhibiting ocular diseases known as glaucoma. 

It is important to note the visual system relies on both the anatomy of the eye as well as the brain and its perceptual processes. In fact, while vision seems instantaneous when compared to cognitive processes, such as memory and thinking, this is actually due to the tremendous neural organization that is appropriated for vision in the brain. The journey from a physical world to a mental representation of the physical world begins with a physical mapping of light onto the retina in the eye, and then neural signals to the brain where further processing takes place. To begin with this process, light must first enter the eye through a small hole known as the pupil, which is surrounded by the iris, a circular ring-shaped structure that is responsible for eye color. When the circular muscles of the iris contract, the opening of the pupil is made smaller, and when the radial muscles of the iris contract, the opening of the pupil is made larger. 

As the light passes, it is refracted first and foremost by the cornea, which is a transparent dome-shaped covering of the eyeball that is responsible for overall 80% of the focusing power of the eye. The transparency of the cornea is important to note, as the cornea receives nutrients through an alternative system independent of blood vessels. The anterior chamber is the space between the cornea and the lens, and is filled with a watery fluid known as the aqueous humor. The lens is a biconvex protein structure that facilitates the remaining 20% focusing power of the eye. Unlike the cornea, the lens is surrounded by ciliary bodies that expand or contract in order to change the refracting power of the lens, which is necessary for accommodation for vision to  nearby objects. Light should then converge at the retina, which houses sensory cells known as cones and rods. The fovea is the part of the retina that is responsible for central vision and consists of only cones, while the periphery consists of both cones and rods. Furthermore, both nearsightedness and farsightedness are associated with either inappropriate eyeball length or inability to focus. The posterior chamber is the space between the lens and the retina, and is filled with a once-again watery fluid known as the vitreous humor. From the retina, the optic nerve, which is a bundle of axons of retinal ganglion cells, transmits neural signals to the brain.

Given the context of this anatomy, glaucoma is a condition by which there is a buildup of intraocular pressure within the eye. Since the cornea is transparent, there are canals in place to supply nutrients to the cornea in place of blood vessels, and if the canals are blocked, pressure begins to build up. Ultimately, this causes peripheral vision to suffer, as well as the optic nerve to undergo extreme damage. There are two types of glaucoma, open-angle and closed-angle glaucoma.  For open-angle glaucoma, the canal begins to clog, but there is no immediate damage to the optic nerve or the eye more generally. Thus, open-angle glaucoma can often be detected early to prevent further complications, since it is generally painless and gradual in its building of pressure. If detected and treated early, eye drops or a laser technique (SLT) can be used to lessen the pressures within the eye. Surgery is necessary in some cases. For closed-angle glaucoma, the drainage canals are themselves very narrow, meaning there is very little space for the fluid to travel through. Thus, with a closure attack at any moment, there can be a very immediate and painful increase in ocular pressure, which can ultimately threaten vision. Still, closed-angle glaucoma can be detected and treated early just as open-angle glaucoma, and with an effective treatment procedure available known as laser peripheral iridotomy (LPI). 

Overall, prevent vision-threatening complications due to glaucoma by scheduling regular eye exams with your optometrist. Your most important sense is on the line. 

References:

https://www.brainline.org/article/vision-our-dominant-sense 

https://heffingtons.com/how-do-glasses-work/

https://www.frieze.com/article/perception-vision 

https://www.nei.nih.gov/learn-about-eye-health/healthy-vision/how-eyes-work 

https://www.visioninitiative.org.au/common-eye-conditions/how-your-eyes-work 

https://www.mayoclinic.org/diseases-conditions/glaucoma/symptoms-causes/syc-20372839