Home Page Overview Site Map Index Appendix Illustration About Contact Update FAQ


Medical Science


Evolution of Vision Compound Eye Ophthalmmology The first light-sensitive cells appeared in Cnidarians animals such as the hydras. Figure 24a illustrates the sequence of vision evolution with an insert to indicate the kind of light perception, and a short description for each step below.

Figure 24a Evolution of Vision [view large image]

Figure 24b Compound Eye

Figure 25 Ophthalmology and Eye Anatomy
[view large image]

  1. The box jellyfish is an amazing animal with more than 20 eye pits and many eyes similar to ours. They seem to double for ears as well.
  2. Hydras are among those animals that are limited to light-detecting cells spread around their bodies to pick up change in brightness. The eyespots consist of a single photoreceptor cell and a surrounding pigment cell.
  3. In animals like flatworms, the eyespots grew inward to form cup-shaped structures to receive light from different angles.
  4. Over time the cup became a deep chamber. As the opening to the chamber became narrower, the pinhole-eye developed. This type of eye can provide coarse images, allowing an animal to discern the shape of objects. Optic nerve to convert light energy to electric impulse also appeared in this type of eye.
  5. Eventually the chamber opening closed over with transparent cells, which helped to prevent eye infections, creating a lens that focuses light onto the retina (a layer of photoreceptor cells). Marine snails have this type of eye, which provides the ability to see more clearly through water.
  6. The lens evolved into a refractive lens that could change shape to focus many rays of light onto one spot on the retina. Cephalopods such as cuttlefish, squid and octopuses have eyes that are similar to those of vertebrates like humans (with small variation in the orientation of the retinal cells). Insects have compound eye with multiple facets, each with a fixed lens and crystalline cone that focuses light on the photoreceptor cells (Figure 24b).
Figure 25 shows the structure of human eye, see "sight" for more information. Table 06 lists some eye diseases treated by Ophthalmology.

Disease Symptom(s) Cause(s) Treatment(s)
Cataracts Cloudy, blurry, foggy, or filmy vision, color change, uncomfortable with glare, double vision, sudden changes in glasses prescription Cloudy lens due to aging, drug, trauma, radiation, genetics, skin diseases, diabetes Eye glasses change, surgery
( see link)
Color Blindness Trouble seeing red, green, or blue or a mix of these colors Retinal cones deficiency by genetic, aging, injury, eye diseases, medicine No cure, try specialized eye glasses to minimize impact ( see link)
Corneal Diseases Blurred vision, light sensitivity, pain Inflections, trauma, genetic, autoimmune, nutrition deficiencies, allergies Medications, laser treatment, surgery
( see link)
Glaucoma Dark rooms adjustment or focusing problem, sensitivity to glare, color of iris change, recurrent pain around eyes, double vision, distorted lines, tearing or "watery eyes", itching eyes, seeing things Aging, eye pressure, inherited, nearsightedness, injuries, severe anemia, diabetes Medicines, surgery
( see link)
Macular Degeneration Dark, blurry areas in the center of vision, diminished or changed color perception Deterioration of the central portion of the retina (macula) Drug, vitamins, therapy, vision aids ( see link)
Myopia/Hyperopia Blurry vision Inherited, improper viewing, not enough day light for development Eye glasses ( see link)
Uveitis Eye redness, blurred vision, eye pain, sensitivity to light, spots before the eyes Infection with virus, fungus, bacteria, parasite, eye injury Antibiotics, drug
( see link)

Table 06 Ophthalmology

Talking about color blindness, actually human and mammals are not color perfect. Color vision of vertebrates depends on the pigments in the cone cells. It turns out that birds, as well as reptiles, and many fish, have four types of cone pigments, whereas most mammals have only two types (Figure 26a). Mammals lost two of the pigments during their early evolution, very likely because these animals were
Colours UV Photo nocturnal and cones are not needed for vision in dim light. After the dinosaurs died out, mammals began to diversify, and the lineage that gave rise to the Old World primates of today reclaimed a third cone through duplication and subsequent mutation of the gene for one of the remaining pigments. Thus, mammalian colour vision distinctly limited when compared with the visual world of birds and other vertebrates especially in the near ultraviolet region of the spectrum. We cannot comprehend the sensation of colour in these animals, but a camera equipped to detect only ultraviolet light "sees" patterns invisible to us as shown in Figure 26b.

Figure 26a Colour Vision

Figure 26b UV Photo
[view large image]

See a color blindness simulator (click me) to experience the perspective of color challenged individuals. The "abnormal" view is created by removing the corresponding part of the optical spectrum while taking the photo image. It has nothing to do with the "Hard Problem in Consciousness".

Go to Next Section
 or to Top of Page to Select
 or to Main Menu

.