October 1, 2004
Seeing is believing, or so they say. Being able to physically see or detect something gives us much more confidence in its existence. Moreover, being able to intellectually see or understand something provides us with a higher level of justification for trusting in our mental ability to know the truth. Yet, the expression seeing is believing itself seems to present a false understanding of what the word believing really means. For if one can physically detect or truly understand something, then one need not believe because one already knows by way of the senses or the intellect. A belief in something generally requires that it be neither fully appreciated by the senses nor completely understood by the intellect. If indeed something is capable of being seen by the senses or fully understood by the intellect, then the only limiting factor for each of us is our trusting that what we see and think are really true.
Having said this, it is interesting to consider that most scientific investigation is to a large extent dependent on our ability to experience the sensation of sight. From constructing the devices for detection, making the necessary observations, and collating the data for analyses and interpretation; having the ability to see is very important in our being able to analyze the world around us.
But how is it that we are capable of experiencing the sensation of sight; of being able to see our loved ones, the grandeur of nature, and the inspiring creations of humankind? This months column and the two to follow will explore this very question. How indeed are we capable of detecting a specific range of electromagnetic energy and converting it into an image for us to see?
From the focusing of light on the retina at the back of the eye, to the creation of nervous impulses that are sent to the brain where it is interpreted as the sensation of vision; we will review the necessary components that make sight a reality for humankind. But I warn you, that despite us having the knowledge of how vision takes place and the many reasons why it can dysfunction, keep in mind that we have absolutely no understanding of how the brain is able to accomplish this feat.
Yes, we know about light refraction and the biomolecular goings-on within the photoreceptor cells of the retina all right. We even understand how these nervous impulses impact other nearby nervous tissue and the secretion of various neuro-transmitters. We also know the various pathways that vision takes within the brain resulting in a mish- mash of neuroexcitatory messages in the visual cortex of the brain.
But even this knowledge cannot tell us how the brain can convert all of this electrical information into a panoramic view of the Grand Canyon, the face of a new born child, or the artistry of a Michaelangelo or the great Leonardo. We only know that it just does. It would be like asking what the biomolecular basis for a given thought might be. At this moment, science does not have the necessary tools to be able to answer this question.
The eye is a complex sensory organ that is capable of taking light rays and focusing them on the light sensitive receptors contained in its retina. There are many parts of the eye all of which play an important role in either directly accomplishing this function or acting in support. (See Figures 1, 2, 3)
Figure 1. Side view of the eye with parts labelled. See text for further description of characteristics, function and effects of dysfunction. Illustration obtained from www.99main.com/~charlief/Blindness.htm.
Figure 2. A view of the outside of the eye with some of its more important parts. Illustration obtained from www.99main.com/~charlief/Blindness.htm.
Figure 3. The tears are produced in the lacrimal gland, and are swept across the surface of the eye by the eyelids, and drain into the nose through the nasolacrimal duct. that's why your nose gets stuffy when you cry a lot. Illustration obtained from www.99main.com/~charlief/Blindness.htm.
The eyelid must be open and the eye muscles must position the eye so that it is in proper alignment with the light rays that are projecting from what is being observed. As the light rays approach the eye they first encounter the cornea, which is necessarily bathed by tears from the lacrimal gland. The curvature and the nature of the cornea allows the photons of light to be bent, or refracted, as they start the process of being focused on our area of central vision; called the macula.
The light then crosses through the anterior chamber that sits behind the cornea and in front of the iris and the lens. The anterior chamber is filled with a watery fluid called the aqueous humor, which is derived from the structures nearby, and allows the light to penetrate further into the eye.
From the anterior chamber, the light continues through the adjustable opening in the iris, called the pupil, which allows the eye to control how much light enters. Then it enters the front (anterior) surface of the lens where further bending occurs. The light then continues through the lens and exits through the back (posterior) surface, again being bent on its way to being focused on the center of central vision; the fovea, which contains a high density of specific photoreceptor cells.
It is at this critical moment that the eye must make any necessary adjustments to allow all the photons of light that have reflected off of the object of interest to focus in on their intended target in the retina. It does this by actively changing the curvature of the lens by way of the action of the ciliary muscle.
The photons of light then travel through the gelatinous vitreous, which is what largely supports the eyeball, and head toward the retina. The photons then spread out over the retinal tissue with the projected rays from what is being focused on hitting in the fovea and the macula that surrounds it. The photoreceptor cells in the retina are then stimulated in a way that allows for nerve impulses to eventually be sent along the optic nerve to the visual cortex of the brain where they are interpreted as vision.
This month well look at some of the parts of the eye and how they perform three fundamental functions; defense and support; transmission of light; and focusing of the image. Well also see what happens when problems occur and vision is compromised, which will lead us to some questions for macroevolution and its step by step mechanics of development.
Next month well look at the photoreceptor cells, their distribution in the retina and how this relates to function, and the biomolecular basis for nervous impulse generation along the optic nerve. In the last column, well review how the visual message is sent to the brain by way of various pathways and well get an overview of the complex nature of what the visual cortex actually sees.
To Serve and Protect
There are many components that are responsible for not only protecting and defending the eye, but also providing nutrients and physical support. Without any of these important factors, we would not be able to see as we are able to at present. Heres a list of some of the more important ones with a brief summary of what they do for the eye.
The orbital cavity: is composed of five different bones which coalesce: they are the frontal bone, the ethmoid bone, the zygomatic bone, the maxillary bone, and the lacrimal bone which provides bony protection of about 2/3 of the eyeball. These bones also provide the stable base for the origin tendons of the muscles that are responsible for moving the eye.
The eyelids: upper and lower, each needing neuromuscular control and reflex activity for protection of the eye; protect the eye from the exposure to light, dust, dirt, bacteria etc. The blink, or corneal reflex, provides for quick protective closure of the eye as soon as the cornea is irritated by a foreign object such as dust or dirt. The dazzle reflex provides for quick closure of the eyelids when the eye is exposed to very bright light thereby cutting off more than 99% of light entering the eye. The menace, or threat reflex, provides for instant-aneous protective closure of the eyelids from any movement that seems to be heading to the eye. The stimulus to initiate these two latter reflexes originates in the retina. In addition to their protective function, by blinking, the eyelids spread a tear film across the front surface of the eye which is necessary for eye function and survival. (see below)
Tear film and production: Composed of three layers consisting of oil, water and mucous; provided for by the oil glands of the of the eyelids, the lacrimal (tear) gland located under the outer orbital rim bone, and the gland cells of the conjunctiva that overlies the sclera (see below). The tear film keeps the eye moist, preserves a smooth surface on the front part of the eye to aid in light transmission, nourishes the front of the eye, and protects it from infection and injury
The sclera: This is also known as the white of the eye. It is an outer protective coating which is covered by the conjunctiva which produces and secretes mucous that moistens and lubricates the eye
The choroid: This layer lies between the sclera and the retina. It provides the circulation to the back of the eye and the retinal pigmented epithelium (RPE), that sits directly behind the retina, which absorbs light. Therefore, when light penetrates through the retina, this layer that sits behind, absorbs it and prevents it from reflecting back, thereby preventing visual distortion
The cornea: This specialized, transparent connective tissue is on the same plane as the sclera to which it abuts at the corneoscleral junction However, it is directly in line with where the light enters the eye on its way to being seen by the retina. The cornea has an absence of blood vessels, i.e. it is avascular. This is one of its more important features that allows it to remain clear for light transmission into the rest of the eye. The cornea obtains its supply of water, oxygen and nutrients from two sources: the tears that are secreted by the lacrimal gland which are evenly spread over the cornea by the action of the eyelids and the aqueous humor present in the anterior chamber (see below). Although the cornea protects the eye, it is the eyelids that protect the cornea. The neuromuscular system in the body has provided the cornea with the highest density of sensory nerve fibers in the body in order that it may be on guard for any miniscule irritation that may end up as an invasion. One of the last reflexes to go before someone dies is the corneal reflex which is tested by touching the cornea of an unconscious persons eye with a wisp of tissue. A positive reflex will yield an immediate attempt to close the eyelids which can be seen by the muscle action around the eye.
The aqueous humor: This is the watery fluid that is produced by the ciliary body and is secreted into the anterior chamber that sits directly behind the cornea and in front of the iris. This fluid nourishes not only the cornea but also the lens and plays a role in the form and shape of the front of the eye by taking up space in this region. The aqueous humor drains out of the anterior chamber through the canals of Schlemm.
The vitreous: This is the thick, transparent and gelatinous substance that fills the eyeball and gives it much of its form and shape. It has the ability to be compressed and then return to its normal shape thereby allowing the eyeball to withstand most traumatic events without serious injury.
Breakdown in Defense
Examples of what can happen in real life to these various components when they dysfunction and how this can affect vision may give you an understanding of how important each one of them is to preserve proper eyesight.
Trauma to the orbit can cause serious eyeball damage resulting in internal eye injury but also entrapment of the nerves and muscles that serve the eye resulting in double vision and difficulty in depth perception
Eyelid dysfunction can result from inflammation or injury of the 7th cranial nerve (facial nerve) in which the ability to properly close the eye is severely compromised. This can result in corneal injury because the eyelid can no longer protect the cornea from the environment and trauma while also interfering with the passage of the tear film over its surface. Often the patient will wear an eye patch and put some lubricating eye ointment in the lower lid sac to keep the cornea moist and to prevent injury
Sjogrens and dry eye syndrome both result in the compromise of tear production which not only is an irritating condition but can also result in blurred vision
Corneal injury such as infection or trauma can result in further damage resulting to the structures behind it, and rarely endophthalmitis, a severe infection of the inside of the eye which often results in surgical removal of the eye.
A full tear through all the layers of the cornea may result in the aqueous humor being released from the anterior chamber which will make the front of the eye flat and then the anterior chamber will only exist in potential resulting in loss of vision.
The vitreous often ages and starts to retract and often can pull the retina off of its moorings resulting in a retinal detachment. Hemorrhaging can occur in the vitreous as well which can result in partial or total loss of vision.
In summary, it is evident that each of the parts of the eye is absolutely necessary for the support and function of vision. It may be that the retina plays the starring role by having the photosensitive cells that can ultimately send messages to the brain for interpretation. But each of the components mentioned above has an important supporting part to play without which our vision would surely suffer or would not exist.
Macroevolution and its step by step mechanics must explain in more detail than what has been done so far, how human vision, as it claims, developed by random mutation from light-sensitive spots on invertebrates, while taking into account the complex structure, the physiological nature, and the interdependence of all of the components mentioned above.
Let The Light Shine Through
In order for the eye to function properly many of its parts must be able to allow light to travel through them without disruption or distortion. In other words, they must be trans-parent. Look around the rest of the body and youll be hard pressed to find any other tissues that have this vital quality that allows for light transmission. Macroevolution needs to be able to explain, not only the genetic mechanisms for how the macromolecules that comprise these parts came into being, but also how they happen to have the unique quality of transparency and be located in the one organ of the body that requires this for proper function.
The cornea protects the eye from the environment but it also allows light to enter the eye on its way to the retina. The corneas transparency is dependent on it not having any blood vessels within it. But the corneal cells still require their own supply of water, oxygen and nutrients to survive, just like any other cells in the body. They obtain these vital necessities from the tears that cover the corneas front surface and from the aqueous humor that bathes the back surface. Clearly, to hypothesize on the development of a transparent cornea without taking into account how it was able to sustain itself and remain transparent during the process is to simplify something that is in fact much more complicated than was first thought. Injury to the cornea by trauma or infection can cause scarring which ultimately can lead to blindness because light cannot readily pass through it to the retina. The commonest cause of blindness in the world is trachoma, an infection that causes corneal scarring.
The anterior chamber, which is bound anteriorly by the cornea, is filled with aqueous humor derived from the ciliary body, that is a clear watery fluid which not only allows light to pass through unhindered but also supports the cornea and the lens. There are many other fluids that are produced in the body. e.g. blood, urine, joint fluid, sputum, and mucous etc. Most of these are not conducive for light transmission to the degree that is necessary to allow for vision. Macroevolution must be able to explain the development of the ciliary body and its ability to produce this clear aqueous humor which fills, forms, and maintains the anterior chamber, an absolute necessity for vision, in light of the fact that it also provides other tissues (the cornea and the lens) with the necessities for survival. Which of these components came on the scene first and how did they function without each other?
The iris is the anterior extension of the pigmented choroid which gives the eye its color. The iris controls the amount of light that enters further on its way to the retina. It consists of two different types of muscles, both of which are under nervous control, for the size of the opening, called the pupil. The sphincter pupillae which is situated along the edge of the iris contracts to close the opening in the pupil. The dilator pupillae runs radially, like the spokes of a wheel, through the iris, and when it contracts this opens the pupil. The iris is very important for controlling the amount of light that enters the eye at the right moment. Anyone who has ever had their eyes dilated for an eye exam and then has had to go out into the sun can certainly appreciate this fact.
Macroevolution must answer how each muscle developed and in what order while still allowing proper pupillary function. Which muscle developed first and what genetic changes were responsible? How did the iris function for an intermediate eye with one of the muscles missing? How and when did nervous reflex control develop?
The lens sits just behind the iris, is contained in the lens sack, and is kept in place by suspensory ligaments, called the zonules, that are attached to the ciliary bodies. The lens consists of proteins that allow it to remain trans-parent for light transmission to the retina. Like the cornea, the lens is avascular (has no blood vessels) and therefore is dependent on the aqueous humor to obtain its water, oxygen and nutrients for survival. Cataract formation can occur due to trauma or degeneration of the lens causing discoloration and stiffening of the lens which interferes with proper eyesight. Like the cornea, the lens consists of a complicated network of tissue made up of different macromolecules which are dependent on the genetic code contained in DNA. Macroevolution must explain the exact nature of the genetic mutations or cellular transformations that must have taken place for more primitive light-sensitive organs to develop such a complex tissue with its unique qualities for light transmission.
The vitreous, as mentioned in the previous section, is the clear gelatinous substance that fills much of the eyeball and gives it its overall form and shape. Once again, the body has been able to produce a material with the right qualities and placed it in an organ that requires it. The same questions for macroevolution that encompass the macromolecular development of the cornea and the lens, as mentioned above, also apply here, keeping in mind that all three of these tissues, having different physical natures, are also located in exactly the right positions that allow for human eyesight.
Focus, Focus, Focus
Id like you now to turn and look out a window or through the door of the room youre in and gaze at an object of interest as far away as possible. How much of what your eyes see do you think that you are truly focusing on? The human eye is capable of high visual acuity. This is expressed as the angular resolving power, i.e. how much of the 360 degrees in the visual field the eye can clearly focus on. The human eye can resolve one minute of arc, which represents 1/60th of a degree. The full moon takes up 30 minutes of arc in the sky, i.e. about ½ a degree of arc. Pretty remarkable isnt it?
Some birds of prey are capable of resolving 20 seconds of arc which gives them a higher visual acuity than our own.
Now go back and look at that faraway object again. But this time take notice that although at first glance you seem to be focusing on a larger portion of the field, when you really concentrate on what youre indeed looking at, youll realize that it represents only a small sliver of the whole image. What youre experiencing is your central vision which is dependent on the fovea, and the macula that surrounds it, in the retina. This region consists largely of the cone photoreceptor cells which work best in bright light and allow us to see sharp images in color. Why and how this happens will be discussed in next months column. More to the point, people who suffer from macular degeneration are intimately aware of what can happen when ones central vision is compromised.
Now, go back again and look at that object thats faraway, but this time take note of how fuzzy and poorly colored everything else outside of your central vision appears. This is your peripheral vision and it is mostly dependent on the rod photoreceptor cells that line the rest of the retina and provide us with our night vision. This too will be discussed in next months column when we look at how the retina is capable of sending a nerve impulse to the brain. But in order for you to be able to appreciate the need for the eye to be able to focus, you first need to have some idea about how the retina actually works. After all, thats what the light rays are being focused on.
Except when striking an interface perpendicularly, light rays are bent, or refracted, when they pass between matter of different densities, such as air and water. Therefore, except for light that passes directly through the dead-center of the cornea and the lens (pin hole vision), all of it will be bent toward a principal focus some distance behind them. (focal length) This distance will be dependent on the combined power of both the cornea and the lens to refract light which is directly related to their curvatures.
For the purpose of understanding how and why the eye must focus light in order for us to see clearly, it is important to know that, in general, all light rays entering the eye from a source more than 20 feet away are traveling parallel to each other. In order for the eye to be able to experience central vision, the cornea and the lens must be able to bend these rays in such a fashion that they all converge on the fovea and the macula. (See Figure 4)
Figure 4. Figure 4 demonstrates how the eye focuses on objects that are more than 20 feet away. Notice how the light rays are parallel to each other as they come into the eye. The cornea and the lens combine to refract the light to a focal point on the retina which coincides with the location of the fovea and the macula that surrounds it (See Figure 1). Illustration obtained from www.health.indiamart.com/eye-care.
The refractive power of a lens is measured in diopters. This power is expressed as the reciprocal of the focal length. For example, if the focal length of a set of lenses is one meter, then the refractive power is designated as 1/1 = 1 diopter. Therefore, if the combined power of the cornea and lens to converge light rays were to be 1 diopter, the size of the eye from front to back would have to be about 1 meter for light to be able to focus on the retina. That would be a pretty big eye!
In fact, the refractive power of the cornea is about 43 diopters and the refractive power of the lens, at rest when viewing an object that is more than 20 feet away, is about 15 diopters. When totaled together, one sees that the combined refractive power of the cornea and the lens is about 58 diopters. This would mean that the distance from the cornea to the retina would have to be about 1/58 = 0.017 meters = 17 mm for the proper focusing of light on the fovea. And what do you know? Thats about how far it is in most people. Of course this is an approximation of the average and certainly a given person may have a cornea or lens with different curvatures resulting in different dioptic powers and eyeball lengths.
The point here being that the combined refractive power of the cornea and lens, together, correlates perfectly with the size of the eyeball. Macroevolution must explain the genetic mutations that were responsible for, not only the primitive light-sensitive tissue becoming enclosed in a well-protected gelatin-filled ball, but also the different tissues and fluid that allow for the transmission and focusing of light at a power that is matched to the order of the size of that ball.
People who experience myopia (near-sightedness) have difficulty seeing clearly because their eyeball is too long and the cornea and lens focus the light from a distant object in front of the retina. This allows the light to continue past the focal point and to spread out over the retina resulting in blurring of vision. The wearing of corrective lenses can solve this problem.
But now lets consider what happens when the eye attempts to focus on something that is close by. By definition, light that enters the eye from an object that is less than 20 feet away does not come in parallel but is divergent.(See Figure 5) Therefore, in order to be able to focus on an object that is close to our eyes they must somehow be able to refract that light and converge it more than what the cornea and the lens at rest can do.
Figure 5. Figure 5 demonstrates how the eye focuses on an object that is less than 20 feet away. Notice that the light rays that enter the eye are not parallel, but divergent. Because the refractive power of the cornea is fixed, it is then left to the lens to make the adjustments necessary to be able to focus in on the near object. See text for how it does it. Illustration obtained from www.health.indiamart.com/eye-care.
Go ahead and look far away again and then focus on the back of your hand. Youll feel a bit of tugging in the eyes as they focus close up. This process is called accommodation. What actually happens is that the ciliary muscle under nervous control is able to contract which allows the lens to bulge more. This motion increases the refractive power of the lens from its resting 15 diopters up to a maximum of 30 diopters. This action causes the light rays to converge more and allows the eye to focus the light from the near object of interest onto the fovea and the surrounding macula. Experience has shown us that there is a limit to how close the eye can focus. This is called the near point.
As people age, usually by their early 40s, they develop a condition called presbyopia in which they have difficulty focusing on objects that are close up because the lens has hardened and has lost its elasticity. This is why youll often see older people hold objects far away from their eyes in order to try to focus on them. Youll also see them wearing bifocals or reading glasses which will allow them to read comfortably.
Macroevolution must be able to explain the independent development of each component that is necessary for accommodation. The lens must have a certain elasticity that allows it to change shape. It needs to be suspended in a way that allows for this action. The ciliary muscle and its nervous control must come into existence. The whole process of neuromuscular function and reflex activity must be explained on a step by step basis at the biomolecular and electrophysiological levels. None of this has been accomplished except to make optimistically broad statements about the simplicity of the task without any specifics. This may be sufficient for those who are antecedently wedded to the notion of macroevolution but falls short of what should be required of any truly scientific endeavor.
Finally, one must not forget that in addition to having this complex arrangement in the eye to allow for proper focusing, one also needs to be able to turn the eyes toward what is of interest. There are six extraocular muscles that work in a coordinated fashion to do this and the eyes also work in tandem so that proper depth perception and vision may take place. As one muscle contracts, its opposite relaxes, to allow for smooth movement of the eyes as they scan the environment. This too is under nervous control and requires an explanation from macroevolution. Which muscle came first and what genetic mutations were responsible? How did the eye function without the other muscles? When did nervous control of the muscle develop and how? When and how did coordination take place?
Change in Focus ??
From the information provided in this column, many as yet unanswered questions for macroevolution can be generated. We havent even touched on the biomolecular basis for photoreceptor function, nerve impulse generation, the optic pathways to the brain, and the resulting neuroexcitatory pattern that the brain tells us is vision. Many extra-ordinarily complex parts are needed for the human eye to exist, survive, and function. Science now has new information about macromolecular and tissue formation, the underlying electrophysiological mechanisms behind photoreceptor function, and the interdependent anatomical components of the eye that are necessary for proper function and survival. Macroevolution necessarily must address all of these issues in order to provide an acceptable explanation of how such a complex organ came into being.
Even though Darwin didnt know it at the time, his intuition was indeed most likely correct when he opined in his On The Origin of Species: To suppose that the eye [ ] could have been formed by natural selection, seems, I freely confess, absurd in the highest degree.
For scientists now, given our current understanding of how life really works, more proof than the mere existence of various eyes being present in various organisms should be required to verify a theory of origin. Every aspect of eye function and vision: the genetic coding responsible for the macromolecular structures contained within each necessary part : the physiological interdependence of each component: a detailed explanation for how the electrophysiology behind vision came into being: and the underlying mechanisms within the brain that allow it to take these nerve impulses and convert them into what we call sight: must be presented in a step by step fashion in order for macroevolution to be considered as an acceptable mechanism behind origin.
Taking into account all that is required and has been left wanting by macroevolution regarding a logical and thorough explanation for the development of the human eye, one rational approach for explanation may be to compare eye function to the empirical evidence contained within human invention. It is often said that the eye is like a camera, but indeed this is a misleading assertion. For in human affairs, the convention when making comparison is the universal understanding that to say that if y is like x, then by definition x chronologically preceded y. Therefore the truer statement when comparing the eye to the camera is to say that the camera is like an eye. Having said this, it is evident to most readers that the camera did not come into being but by the work of human intelligence i.e. it was the work of intelligent design.
Is it therefore such a leap of faith to consider that since empirically we know that the camera was intelligently designed and is very similar to the human eye, that it is plausible that the eye was intelligently designed as well? Which makes more rational sense to the mind: What macroevolution proposes? Or what intelligent design proposes?
Next month well delve into the world of the retina with its photoreceptor cells and the biomolecular and electrophysiological basis for photon detection and the resulting transmission of impulses to the brain. Its sure to add one more layer of complexity that requires a detailed explanation from macroevolution which as yet, to my mind, has not been sufficiently presented. See you then.
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 recently left his private practice and has started to practice 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 sciences understanding and promotion of what it means to be a human being. Comments and questions about this column or any of the previous ones are welcome at firstname.lastname@example.org
Copyright 2004 Dr. Howard Glicksman. All rights reserved. International