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Lecture 15 Sense Organs I: The Visual System 15-1 Visual System • Eye • Accessory structures – Eyebrows, eyelids, eyelashes, lacrimal (tear) glands • Optic nerve (II) – Sensory information travels along optic nerve to thalamus then to occipital lobe 15-2 Accessory Structures of Eye • Eyebrows – Prevent running perspiration into eyes – Shade • Eyelids or palpebrae – Protect and lubricate – Lacrimal caruncle has sebaceous and sweat glands • Extrinsic eye muscles (6) • Lacrimal apparatus – Lacrimal gland (facial nerve VII) • Conjunctiva Fig. 19.10 – Thin mucous membrane over inner surface of eyelids and anterior side of 15-3 eyeball Anatomy of the Eye Fig. 19.12 • Three coats or tunics – Fibrous: Consists of sclera and cornea – Vascular: Consists of choroid, ciliary body, iris – Neural: Consists of retina 15-4 Anatomy of the Eye • Fibrous tunic: Outer – Sclera: White outer layer, maintains shape, protects internal structures, provides muscle attachment point, continuous with cornea – Cornea: No blood vessels, transparent, allows light to enter eye and refracts (bends) light Fig. 19.12 15-5 Anatomy of the Eye • Vascular tunic: Middle – Iris: Controls light entering pupil; smooth muscle – Ciliary muscles: Control lens shape; smooth muscle – Choroid: pigmented layer that contains melanin 15-6 Anatomy of the Eye • Retina: Inner – Contains neurons sensitive to light – Macula lutea & Fovea centralis: Area of greatest visual acuity – Optic disc: Blind spot • Cavities – Anterior: Aqueous humor – Posterior: Vitreous humor 15-7 Anatomy of the Eye • Lens – Held by suspensory ligaments attached to ciliary muscles – Transparent, biconvex 15-8 Focus and Accommodation Fig. 19.16 • Far vision: 20 feet or more from eye • Near vision: Closer than 20 feet – Accommodation 15-9 Review Question Damage to the retina due to excessive light entering the eyeball could indicate a problem with the (a) Iris (b) Ciliary body (c) Cornea (d) Conjunctiva (e) Choroid 15-10 Points to Remember • Accessory eye structures protect and lubricate the eye. • There are 3 basic layers to the eye: fibrous tunic, vascular tunic and neural tunic. • Image formation on retina by bending of light rays through cornea and lens. • To view close objects, the lens increases its curvature. 15-11 Questions? 15-12