Seeing Life (Part I)

How do my eyes work and how do they protect themselves?

Life on earth is dependent on the electromagnetic energy (light) that radiates from the sun which moves in waves at a constant speed of about 300,000 Km/sec (186,000 miles/sec).  This energy has a wide range in wavelength and frequency.  Very long radio waves have a wavelength in the millions of meters with a very low frequency in the 10’s to 100’s of waves per second.  At the other end of the spectrum are high energy gamma rays which can have a wavelength in the order of a picometer (10-12) and a frequency in the order of a septillion (1024) waves per second.  In between, in increasing order of frequency and decreasing order of wavelength, are AM and FM radio waves, microwaves, infrared rays, the visible light spectrum, ultraviolet (UV) rays and X-rays. 

The human eye is a very complex sensory organ in which all of its parts work together to let light pass through to the retina.  Although it is in the retina where the nerve impulses for vision begin, the other parts of the eye play important roles that support and protect retinal function.

The five different bones that make up the orbital cavity protect about two-thirds of the eyeball and provide the base for the origin tendons of the six different muscles that allow the eye to move in every direction.  The eyelids and eyelashes protect the eye from exposure to too much light in addition to dust, dirt, bacteria and other foreign objects. 

A film of tears, consisting of oil, water and mucus are produced by the oil glands of the eyelids, the lacrimal gland and the conjunctiva that overlies the sclera, the white outer protective coating of the eyeball.  The tear film lubricates the eye, protects it from infection and injury, nourishes the surrounding tissue and preserves a smooth surface to aid in light transmission. 

The cornea is a convex transparent connective tissue that protects the front of the eye while allowing light to enter.  To remain transparent the cornea doesn’t have blood vessels and gets oxygen, water and nutrients from two sources.  One is the tears that constantly wash across it by the blinking eyelids and the other is the clear fluid (aqueous humor) within the anterior chamber that sits behind the cornea and in front of the lens

The lens is a transparent elastic biconvex structure that is kept in place by suspensory ligaments.  Like the cornea, it doesn’t have blood vessels and obtains its oxygen, water and nutrients from the aqueous humor in the anterior chamber that sits behind the cornea. 

The choroid is the layer of tissue located between the sclera and the retina and provides the circulation to the back of the eye.  The extension of the choroid in the front of the eye is the colored iris which consists of two different muscles that control the amount of light that enters through its opening called the pupil

The thick, transparent and gelatinous substance that forms and shapes the eyeball is called the vitreous.  It is able to be compressed and return to its natural position allowing the eyeball to withstand most physical stresses without serious injury.
The back of the eye contains the retina which consists of photoreceptor cells called rods and cones.  When photons of light strike the retina they interact with certain sensitive molecules in the photoreceptor cells causing an electrical change and the release of a neurotransmitter.  These optical messages are then processed and passed on to the optic nerve which carries them to the brain.  Within the occipital cortex of the brain all of this retinal information is interpreted into what we experience as vision.

It’s important to realize that only about eighty percent of the optic nerve impulses travel to the visual cortex in the brain.  The remaining twenty percent veer off early to provide sensory data to the neurons in the brainstem that service muscles which help the eye to function better and provide protection. 

Enter a dark room and the dilating muscle of the iris immediately contracts, causing the pupil to enlarge, letting more light into the eye to help improve vision.  Conversely, shine a bright light into the eye and the contracting muscle of the iris instantly goes into action, causing the pupil to diminish in size to protect the retina from too much light.  This is called the pupillary light reflex, which is often used by physicians to determine the presence or absence of brainstem function.  

So, that’s how your eyes work and protect themselves from damage.

Three Questions for Mr. Darwin

  1. How did my body anticipate the need for and where did it obtain the information to produce all of the parts of my eyes so they could work properly?

  2. How did my body know that it had to divert twenty percent of the optical information from the retina to the brainstem so it can use help protect my eyes from damage?

  3. How does my brain process the information from my eyes to let me see?

 


Also see Dr. Glicksman's Series on

"Beyond Irreducible Complexity"

"Exercise Your Wonder"


Howard Glicksman M. D. graduated from the University of Toronto in 1978. He practiced primary care medicine for almost 25 yrs in Oakville, Ontario and Spring Hill, Florida. He now practices palliative medicine for a Hospice organization in his community. He has a special interest in how the ethos of our culture has been influenced by modern science’s understanding and promotion of what it means to be a human being.

Comments and questions are welcome.

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