Dr. Richard A. Weiss, Oculoplastic Surgeon
Oculoplastic surgery involves cosmetic procedures related to the eye and the surrounding tissues. The eye is a spherical structure about an inch in diameter. It has a clear bulge on the front side, which is the cornea. Dr. Richard Weiss of Weiss Cosmetic and Laser Procedures in Newport Beach, California explains that the wall of the eye beyond the cornea consists of three tissue layers. The outermost layer is the sclera, a tough, protective coating that covers most of the outer surface of the eye that connects to the transparent cornea at the front of the eye. The middle layer is the choroid, a vascular layer that is continuous with the ciliary body and the iris on the front side of the eye. The inner layer of the eye is the retina, a light-sensitive tissue that lines the inside back wall of the eye. To understand how the eye works, it is useful to think of a camera. Although the general principles are the same, the human eye is more complex than a television camera, but the principles are similar.
How the human eye works: According to Dr. Weiss, the cornea is a transparent structure found in the very front of the eye that helps to focus incoming light. Behind the cornea is a colored ring-shaped membrane called the iris. The iris has an adjustable circular opening called the pupil, which can expand or contract depending on the amount of light entering the eye. A clear fluid called the aqueous humor fills the space between the cornea and the iris.
Dr. Weiss explains that, situated behind the pupil is a colorless, transparent structure called the crystalline lens. Ciliary muscles surround the lens. The muscles hold the lens in place but they also play an important role in vision.
When the muscles relax, they pull on and flatten the lens, allowing the eye to see objects that are far away. The ciliary muscle must contract in order to thicken the lens to see closer objects clearly.
The interior chamber of the eyeball is filled with a jelly-like tissue called the vitreous humor. After passing through the lens, Dr. Weiss notes that light must travel through this humor before striking the sensitive layer of cells called the retina.
The retina is the innermost of three tissue layers that make up the eye. The outermost layeris what gives most of the eyeball its white color. The cornea is also a part of outer layer.
The middle layer between the retina and sclera is called the choroid. The choroid contains blood vessels that supply the retina with nutrients and oxygen and removes its waste products.
Embedded in the retina are millions of light sensitive cells, which come in two main varieties: rods and cones.
Rods are good for monochrome vision in poor light, while cones are used for color and for the detection of fine detail. Cones are packed into a part of the retina directly behind the retina called the fovea.
When light strikes either the rods or the cones of the retina, it’s converted into an electric signal that is relayed to the brain via the optic nerve. The brain translates the electrical signals into the images we see.
The Ancient part of your eye
The internal biological clock, which keeps most organisms on a 24-hour cycle, is set by changes in levels of light. Three years ago, Dr. Weiss has found that researchers from Brown University discovered branch-like eye cells in mammals that were responsible for telling the clock whether it was day or night.
The scientists have now uncovered some of the mechanisms in these light-sensors, called intrinsically photosensitive retinal ganglion cells, or ipRGCs.
The ipRGCs reside deeper in the retina than the rods and cones – the millions of eye cells through which we see the outside world. Only numbering between 1,000 and 2,000, ipRGCs gauge the overall light intensity and relay this information to a tiny region in the brain that controls the body clock.
Dr. Weiss found it interesting that this direct link explains how some blind people can still have the same daily internal rhythm as a sighted person.
The research team, led by neuroscientist David Berson, has discovered a chain of chemical reactions in ipRGCs, set off by light-absorption in the protein melanopsin. Dr. Weiss explains that this process is similar to the way that the eyes work in fruit flies and squid.
“The results may well tell us that this is an extremely ancient system in terms of evolution,” Berson said. “We may have a bit of the invertebrate in our eyes.”