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The Special Senses

Learn about the components of the eye and their role in gathering, focusing, and processing light into precise images. Explore the fibrous, vascular, and inner layers of the eyeball, as well as the specialized regions of the retina. Understand the blood supply and internal chambers and fluids of the eye.

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The Special Senses

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  1. PART 2 The Special Senses

  2. Anatomy of the Eyeball • Components of the eye • Protect and support the photoreceptors • Gather, focus, and process light into precise images • Anterior pole – most anterior part of the eye • Posterior pole – most posterior part of the eye • External walls – composed of three tunics • Internal cavity – contains fluids (humors)

  3. The Fibrous Layer • Most external layer of the eyeball • Composed of two regions of connective tissue • Sclera – posterior five-sixths of the tunic • White, opaque region • Provides shape and an anchor for eye muscles • Cornea – anterior one-sixth of the fibrous tunic • Limbus – junction between sclera and cornea • Scleral venous sinus – allows aqueous humor to drain

  4. Medial View of the Eye Figure 16.7a

  5. The Vascular Layer • The middle coat of the eyeball • Composed of choroid, ciliary body, and iris • Choroid – vascular, darkly pigmented membrane • Forms posterior five-sixths of the vascular tunic • Brown color – from melanocytes • Prevents scattering of light rays within the eye • Choroid corresponds to the arachnoid and pia maters

  6. The Vascular Layer • Ciliary body – thickened ring of tissue, which encircles the lens • Composed of ciliary muscle • Ciliary processes – posterior surface of the ciliary body • Ciliary zonule (suspensory ligament) • Attached around entire circumference of the lens

  7. The Vascular Layer Figure 16.8

  8. The Iris • Visible colored part of the eye • Attached to the ciliary body • Composed of smooth muscle • Pupil – the round, central opening • Sphincter pupillae muscle • Dilator pupillae muscle • Act to vary the size of the pupil

  9. The Inner Layer (Retina) • Retina – the deepest tunic • Composed of two layers • Pigmented layer – single layer of melanocytes • Neural layer – sheet of nervous tissue • Contains three main types of neurons • Photoreceptor cells • Bipolar cells • Ganglion cells

  10. Microscopic Anatomy of the Retina Figure 16.9a

  11. Photoreceptors • Two main types • Rod cells – more sensitive to light • Allow vision in dim light • Cone cells – operate best in bright light • Enable high-acuity, color vision • Considered neurons

  12. Photoreceptors Figure 16.10

  13. Regional Specializations of the Retina • Ora serrata retinae • Neural layer ends at the posterior margin of the ciliary body • Pigmented layer covers ciliary body and posterior surface of the iris • Macula lutea – contains mostly cones • Fovea centralis – contains only cones • Region of highest visual acuity • Optic disc – blind spot

  14. Medial View of the Eye Figure 16.7a

  15. Blood Supply of the Retina • Retina receives blood from two sources • Outer third of the retina – supplied by capillaries in the choroid • Inner two-thirds of the retina – supplied by central artery and vein of the retina Figure 16.9c

  16. Internal Chambers and Fluids • The lens and ciliary zonules divide the eye • Posterior segment (cavity) • Filled with vitreous humor • Clear, jelly-like substance • Transmits light • Supports the posterior surface of the lens • Helps maintain intraocular pressure

  17. Internal Chambers and Fluids • Anterior segment • Divided into anterior and posterior chambers • Anterior chamber – between the cornea and iris • Posterior chamber – between the iris and lens • Filled with aqueous humor • Renewed continuously • Formed as a blood filtrate • Supplies nutrients to the lens and cornea

  18. Internal Chambers and Fluids Figure 16.8

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