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Biology LibreTexts

Part 1: The Eye

  • Page ID
    20770
  • The eye can focus light images on the retina using the cornea and the lens. Visible light only occupies a small portion of the electromagnetic spectrum. Other species and artificial technologies can detect other parts of this energy spectrum. 

    Sensory System 1.PNG

    Figure \(\PageIndex{1}\) In the electromagnetic spectrum, visible light lies between 380 nm and 740 nm. (credit: modification of work by NASA)

    Light passes through the eye ball via the cornea, pupil, and lens. The humors are fluids filling the anterior and posterior chambers of the eye.

    Sensory System 2.PNG

    Figure \(\PageIndex{2}\)  (a) The human eye is shown in cross section. (b) A blowup shows the layers of the retina.

    The focused image is directed toward the fovea (or fovea centralis), which contains the highest density of photoreceptor neurons. Muscles in the iris alter pupil size to vary light entering the eye.  Choroid body muscles surround the lens. They alter the lens to aid focusing.

    Anatomy terms to know:

    • Extra-ocular muscles
    • sclera
    • choroid
    • pigmented epithelium
    • fovea
    • vitreous humor
    • aqueous humor
    • iris
    • lens
    • ciliary body
    • cornea
    • conjunctiva
    • optic nerve
    • blind spot (or optic disc)
    • retina
      • retinal neurons
        • ganglion cells
        • amacrine cells
        • bipolar cells
        • horizontal cells
        • photoreceptors (rods, cones)
    • occipital (visual) cortex of the brain.

    Possible Specimens & Models for examination: (sheep or cow) eyeballs for dissection, microscope slides of retina, models of eyes.

    You are responsible for identifying these major anatomical structures of the eye:

    • sclera (tough, outer layer)
    • choroid (dark, middle layer)
    • pigmented epithelium
    • fovea
    • vitreous humor
    • aqueous humor
    • iris
    • lens
    • ciliary body
    • cornea
    • conjunctiva
    • optic nerve
    • blind spot (or optic disc)
    • retina (thin, and pale inner layer).

    You will have to identify these structures using both the sheep or cow eye and the models.

    Dissection:
    You will work in groups of two or three to dissect an eye. There are several ways to slice through an eye. Check with your instructor to determine which way they want you to cut the eye in half. Possible cuts include:

    Dissection 1.PNG

    Note: To be able to best see the eye’s structures, you should work very carefully. Many internal structures are delicate and tear easily such as the retina. Some structures are quite tough including the lens and sclera (in Latin, sclera means ‘tough’!). Practicing careful dissecting is an important skill. Take your time! In order to get to the first layer, the sclera, you may have to clear away some fat and connective tissue. Ask for help if you need it.

    Physiology: Light striking photoreceptor neurons activates networks of retinal neurons.  One network of neurons in the retina sends signals to one ganglion cell.  Action potentials from ganglion cells, whose axons form the optic nerves, represent patterns of light. Perception of the network of interconnected neuron signals is eventually perceived in the occipital cortex.

    Figure \(\PageIndex{3}\) The receptive field for 1 ganglion cell.

    Dissection 2.PNG

    In this example, the ON-center cell will send a maximum rate of action potentials along its axon (in the optic nerve) to the brain if the brightest light is striking photoreceptors near the center of its portion of the retina, and the surrounding photoreceptors in that portion are receiving minimal light. Eventually, patterns of ganglion cells signals are integrated in the brain (in the occipital cortex) to generate the perception of complex images.

    Color vision results from the interaction of 3 sub-types of cone photoreceptors. They preferentially absorb light at different wavelengths, shown in the figure below. 

     Figure \(\PageIndex{4}\)  Human rod cells and the different types of cone cells each have an optimal wavelength. However, there is considerable overlap in the wavelengths of light detected.

    Dissection 3.PNG