vision

Summary of Notes and Diagrams on the Eye

Structure of the Eye

The eye, the sensory organ for vision, is a spherical, fluid-filled structure protected by several mechanisms.12 These include the bony socket, eyelids acting as shutters, eyelashes trapping debris, and tears from the lacrimal glands and conjunctiva for lubrication, cleansing, and bactericidal action.1 The eye is enclosed by three tissue layers:

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Sclera/Cornea: The sclera is the tough, white outer layer of connective tissue.2 The cornea is the transparent anterior part, allowing light passage.2

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Choroid/Ciliary Body/Iris: The choroid is the middle layer containing blood vessels to nourish the retina.2 It has dark melanin pigment under the retina to absorb light.3 The choroid forms the ciliary body and suspensory muscles that control lens shape.3

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Retina: The innermost layer under the choroid, containing pigment cells, rods and cones (photoreceptors), and axons of the visual nerve.3

The eye's interior has two fluid-filled cavities separated by the lens:34

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Posterior cavity: Between the lens and retina, filled with gelatinous vitreous humor.3

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Anterior cavity: Between the cornea and lens, filled with aqueous humor similar to extracellular fluid.4

The eye also contains:4

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Iris: Controls light entering the eye by changing pupil size.4 Contains circular (constrictor) and radial (dilator) smooth muscles.4

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Pupil: The opening through which light enters.4

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Lens: Focuses light onto the retina.4

Many vertebrates, but not humans, have a tapetum lucidum, a reflective layer in the choroid that reflects light back to the retina.5 This improves low-light vision but can cause blurriness.5

Pupil Dilation and Constriction

Pupil size controls the amount of light entering the eye, optimizing vision in both light and dark conditions.56 It also controls depth of field:6

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Small aperture (constricted pupil): Large depth of field, keeping objects in focus over a greater distance.6

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Large aperture (dilated pupil): Reduced depth of field, focusing on close objects.6

Pupil size is controlled by the Autonomic Nervous System (ANS):7

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Sympathetic stimulation: Activates the radial muscle, dilating the pupil.8

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Parasympathetic stimulation: Activates the circular muscle, constricting the pupil.8

This is an example of antagonistic control by the ANS.8

Diagrams Summary

The diagrams in Source 8 illustrate the actions of the radial and circular muscles of the iris on pupil size:

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Pupillary constriction: The circular muscle contracts, making the pupil smaller.7

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Pupillary dilation: The radial muscle contracts, pulling the pupil open wider.7

Source 9 further shows the involvement of the ANS:

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Sympathetic stimulation: Norepinephrine from the superior cervical ganglion activates the radial muscle.8

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Parasympathetic stimulation: Acetylcholine from cranial nerve III activates the circular muscle.