Caution: Organs at Work - Part V: Vision
Everybody knows that the gauges on the dashboard are put there to tell the driver about what is going on in his car. But how do they work? Each device is essentially a sensory transducer (L. transducere = to lead across) with a mechanism in place that allows it to detect a physical phenomenon and convert it into useful information which, depending on the situation, the driver ignores at his peril. One type of sensor, placed in the fuel tank, informs the fuel gauge on the dashboard about how much fuel is left, and on seeing it, the driver must decide how soon to fill up. Another type of sensor, placed within the engine block, informs the temperature gauge on the dashboard of how hot the engine is, and on seeing it, the driver must decide whether or not the car is safe to drive. To work properly, the car must have the right types of sensors, in the right places, providing the right gauges on the dashboard with the right information, otherwise the driver will not know the true situation and may end up running out of gas or permanently damaging his car.
This same principle (with the car) can be applied to the body and its ability to survive within the laws of nature. Previous articles in this series have shown that the body has a whole host of sensory transducers located in exactly the right places which provide the right information to the right organs to allow it to control its metabolism and internal environment. In general, these sensory transducers can be divided into three main types; chemoreceptors; which respond to chemicals, like the glucosensors in the alpha and beta cells of the pancreas that control the blood glucose, mechanoreceptors; which respond to motion and stretch, like the baroreceptors in the walls of the main arteries that supply blood to the brain for blood pressure control; and physical sensors; which respond to natural phenomena, like the thermoreceptors in the hypothalamus which controls the body's core temperature.
How all of this came about by just chance and laws of nature alone, as evolutionary biologists would have us believe, goes against all common sense, human invention and reverse engineering. After all, just like the sensors and the properly calibrated gauges on the dashboard, the systems needed to control things like blood glucose, blood pressure and core temperature are irreducibly complex in that each of them consists of different components each of which must be present and working properly to allow for life. Moreover, just as the driver has an inherent knowledge of what the gauges on the dashboard tell him about his car's function, since the blood glucose, blood pressure and core temperature must stay within a certain range for the body to survive, this means that these systems must also have an inherent knowledge of what these parameters must be for the body to survive, something I call natural survival capacity. Evolutionary biologists are great at imagining how all of these parts came together because they only deal with how they look but not how they must work within the laws of nature to allow for survival.
Like the rest of the body, the nervous system, which makes us aware of our surroundings, controls respiratory and cardiovascular function and lets us move about and manipulate things, needs different sensory receptors, located in the right places, to tell it what is going on inside and outside the body. After all, if the body could not feel the ground or balance itself because it didn't know where its arms and legs are in space or whether it was upside down or not, then how could our ancient ancestors have survived?
The nervous system has chemoreceptors that send information to the brain where it is interpreted as different tastes and smells. Mechanoreceptors in the skin send information to the brain on things like touch, pressure and vibration while others in the muscles and joints send information to the brain on limb position and muscle movement. This article will focus on the physical sensors found in the retina of the eye, which, when stimulated by electromagnetic energy rays (light) within a certain wavelength range, results in nerve impulses to the occipital lobes of the brain which are interpreted as vision.
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 electromagnetic energy has a wide variation in wavelength and frequency (number of waves per second). 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.
See: Electromagnetic Spectrum
The amount of energy within a given type of radiation is directly related to its frequency. Therefore, the energy level of gamma rays far exceeds the energy level of the visible light spectrum, which in turn far exceeds the energy level of radio waves. The visible light spectrum consists of the colors of the rainbow: red, orange, yellow, green, blue and violet which represent wavelengths from about 400 nm (10-9 m) (violet) to about 700 nm (red).
As light waves from a distant source travel through the air they remain parallel to each other, moving in a straight line. If the rays of light pass through a substance of different density, only the rays hitting perpendicularly will remain straight while all of the others will bend (refract) based on their striking angle. Experience teaches that when light passes from air through a clear curved glass convex lens all of the rays passing through bend and meet up on the other side at what is called the focal point. The distance of the focal point from the lens (focal length) depends on its degree of curvature. The more powerful (more curved) the lens, the shorter the focal length and the closer to the lens all the light rays will meet, and the weaker (less curved) the lens, the longer the focal length and the further away from the lens the light rays will meet.
This means that the refractive power of a lens is inversely related to the distance it takes to bring light together at a focal point. The higher the refractive power of a lens the shorter the focal length and the lower the refractive power, the longer the focal length. The unit used to measure the refractive power of a lens is called a diopter. By definition, a lens with the refractive power of one diopter has a focal length of one meter. So, a lens with the refractive power of two diopters has a focal length of one-half meter and a lens with refractive power of one-half diopter has a focal length of two meters.
See: Principal Focal Length
Finally, experience teaches that when light travels through a convex lens the image that forms at the focal point on the other side is inverted and reversed. This means that it is upside down and turned around from left to right and right to left.
See: How an inverted image is formed by a convex lens
All of these properties of light, its different energy levels and that it takes a certain distance for its rays to come to a focal point behind a convex lens in addition to forming an inverted and reversed image, must have been taken into account by human life as it evolved to experience the sensation of vision. Evolutionary biologists claim that our ability to see well enough to survive came about by chance and the laws of nature alone. However, I suspect that after you understand how the parts of the eye work together in an irreducibly complex way and have the natural survival capacity to contend with the laws of nature so that our ancient ancestors could see well enough to survive, you will come to see that when Darwinists teach their acolytes about the evolution of vision; it's the blind leading the blind.
The Parts of the Eye
The human eye is a very complex sensory organ in which all of its parts work together to focus light onto its 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.
See: Diagram of the Eye
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 lashes 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 is absent blood vessels (avascular) and receives 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. Light rays that reflect from an object more than twenty feet away enter parallel to each other and must be bent (refracted) to focus them on the area in the retina for central (macula) and sharp vision (fovea). The cornea’s curvature plays a major role in focusing the light that enters the eye onto the retina.
The lens is a transparent elastic biconvex structure that is kept in place by suspensory ligaments. Like the cornea, it too is avascular and obtains its oxygen, water and nutrients from the aqueous humor in the anterior chamber. As noted above, light rays from a distance (greater than twenty feet) enter the eye in parallel whereas those from nearby (generally less than twenty feet away) spread out. To focus the light on the macula and fovea this diverging light must be further refracted and the biconvex curvature of the lens accomplishes this task. Also, since what the eye focuses on close-up is always changing, the curvature of the lens can be reflexively adjusted (accommodation) so that the light rays will strike the retina in the area for sharp vision.
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 choroid also contains the retinal pigmented epithelium (RPE) which sits behind the retina and absorbs light thereby preventing it from reflecting back on the photoreceptors of the retina which would cause visual blurring. 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 (pupil). Finally, 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, which respond to specific wavelengths within the visible light spectrum and converts this energy into nerve impulses. The retina of each eye has about one hundred twenty million rods which are strewn throughout the periphery. The rods contain a photopigment called rhodopsin which is very sensitive to all the wavelengths of the visible light spectrum. In contrast, there are only about six million cones that are mostly concentrated in the macula, with the highest amount in the cone-only fovea. Each cone contains one of three different photosensitive pigments, called photopsins, which tend to react stronger to either the red, green or blue wavelengths of light. Both rhodopsin and the photopsins are dependent on Vitamin A which is digested and absorbed in the gastrointestinal system, metabolized in the liver and fat and is also important for skin and mucous membrane development and growth and immune function.
When photons of light strike the retina they interact with the photoreceptor cells causing an electrical change and the release of a neurotransmitter. The messages are passed through interconnecting neurons within the retina. These retinal interneurons process the information and send the resulting nerve signals along the optic nerve to the brain. About eighty percent of the optic nerve impulses travel to neurons within the brain that pass on the sensory information to the visual cortex in the occipital lobes. The remaining twenty percent veer off and 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 of brainstem function.
What the Brain Sees
In considering the nature of the sensory data being presented from the eyes to the visual cortex several points must be kept in mind. First, as noted above, the use of the cornea and lens to refract and focus light on the retina results in an inverted and reversed image that is turned around and upside down. This means that what appears in the right upper half of the visual field is detected by the left lower half of the retina and what appears in the left lower half of the visual field is detected by the right upper half of the retina etc.
Second, by looking through one eye at a time experience teaches that there is an overlap in the nasal visual fields (the right half of the left eye and the left half of the right eye). This overlap provides the visual cortex with two different perspectives and allows for depth perception.
Finally, impulses sent along each optic nerve split-up on their way to the brain. The messages from the nasal half of the retina (the right half of the left eye and the left half of the right eye) cross over from right to left and from left to right through what is called the optic chiasm. However the impulses from the temporal half of the retina (the left half of the left eye and the right half of the right eye) stay on the same side. This means that everything seen by the right half of each eye (the nasal field of the left eye and the temporal field of the right eye) goes to the left occipital lobe and everything seen by the left half of each eye (the nasal field of the right eye and the temporal field of the left eye) goes to the right occipital lobe.
Our brain then takes this upside down, turned around, split-up and overlapping collection of photon-generated nerve impulses and provides us with what we experience as vision. How it is able to accomplish this feat is as yet entirely unknown.
Real Numbers Have Real Consequences
Previous articles have shown that when it comes to the various parameters of life, like the blood glucose, blood pressure and core temperature (among dozens of others), real numbers have real consequences. Not just any blood glucose, blood pressure or core temperature will do. Each of them (along with dozens of others) must stay within a certain objective range that medical science can measure and express in a digital form.
As one example; blood glucose below 60 units or above 400 units usually results in weakness and severe fatigue and as the blood glucose drops toward 20 units, or rises up toward 1,000 units, this results in worsening organ malfunction and usually death. When it comes to real numbers having real consequences, the same can be said for eye function and its ability to have allowed our earliest ancestors to have had adequate distance and near vision for survival.
As noted above, light rays reflecting from an object more than twenty feet away enter the eye parallel to each other. As it enters the eye the light is first refracted (bent) by the cornea, and then by the lens so that it can be focused on the area in the retina for sharp vision. The ability for the eye to provide clear and sharp distance vision is therefore dependent on the combined refractive power of its cornea and lens to focus light from a distant object onto its retina.
Remember, as noted above, the refractive power of a convex lens is inversely related to the distance it takes to bring light together at a focal point behind it. The more curvature and higher the refractive power, the shorter the focal length, and the less curvature and lower the refractive power, the longer the focal length. By definition, a convex lens that can bring parallel light rays traveling through it to a focus one meter behind is said to have a refractive power of one diopter. This means that a two diopter lens has a focal length of one-half meter and a one-half-diopter lens has focal length of two meters. It also means that if the combined refractive power of the cornea and the lens were only one diopter, the human eye would have to be one meter long for light from a distance to focus on its retina. More to the point, medical science’s understanding of optics verifies that for our earliest ancestors to have had normal distance and near vision, the size of the eye must have matched up perfectly with the combined refractive power of the cornea and lens, otherwise the eye would not be able to focus light on the area for sharp vision in the retina and everything would have been blurry.
In reality, the refractive power of the cornea is about 43 diopters and for the lens (at rest) is about 15 diopters. Therefore the combined refractive power of the cornea and lens is about 58 diopters. And what do you know? As luck would have it, when taking into account that light entering the eye is affected not only by the refractive powers of the cornea and lens but also going from air to water, 58 diopters correlates with a focal length of about 23 millimeters, the normal distance from the cornea to the retina. What a coincidence!!!
Once having found food and water and come close enough to handle and prepare it for ingestion, the eyes of a primitive human would have had to have made adjustments to focus on what is to be eaten and drunk. As noted above, light rays from a distance enter the eye in parallel, but light rays from objects that are near (generally less than twenty feet away) spread out (diverge). This divergence of light rays from near objects gives them the tendency to come to a focus behind the retina, resulting in blurry vision. Since light rays from close-up objects diverge as they enter the eye it must be able to increase its power to bend them so that they come together at the area for central vision (macula) and in particular, for sharpest vision (fovea).
Look at something far away and then focus on something only a foot away, like your hand. You immediately feel a tugging sensation in the eyes. This is the ciliary muscle reflexively contracting as it makes the lens change shape, increasing its convexity and refractive power, in what is called accommodation. The refractive power of the cornea is a fixed constant, but the refractive power of the lens can be adjusted depending on how far away an object of interest is located. The refractive power of the lens at rest is about 15 diopters but can rise up to about 30 diopters during accommodation to allow us to see things up close. Since there is a limit to how much the lens can accommodate, this means that if the eye is not the right size, near vision may be impossible as well.
Take a moment now and consider what it would have been like for our earliest ancestors to have found what they needed to live and defended against danger if their distance vision was such that they could barely see the big E on the eye chart. And how could they have prepared their food or made the things they needed to survive if they couldn't focus on things within their hands? Eye doctors know that about a four percent increase in the combined refractive power of the cornea and lens (or a lengthening of the eye) results in severe myopia (not being able to see the big E on the eye chart clearly). And a twenty five percent decrease in both of these leads to difficulties with distance and near vision.
When evolutionary biologists talk about vision, not only do they leave out how it is irreducibly complex (all of the parts of the eye and the brain are needed for proper function) but also that it demonstrates natural survival capacity, in that the combined refractive power of the cornea and lens and the latter's ability to adjust to close-up objects perfectly matches the size of the eyeball. Remember, when it comes to life and the laws of nature, real numbers have real consequences. Without the right refractive power or size of the eyeball our earliest ancestors would have been as blind as bats. But of course, as most people who believe neo-Darwinism mistakenly teach, evolution would then have just made them develop sonar instead (like bats), because that would have been what they needed to survive.
Be sure to catch all of the articles in Dr. Glicksman's series, "Beyond Irreducible Complexity."
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 about this article or any of the previous ones are welcome.
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