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Lecture 15

Sense Organs I: The Visual System

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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

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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– Thin mucous membrane

over inner surface of eyelids and anterior side of eyeballFig. 19.10

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Anatomy of the Eye

• Three coats or tunics– Fibrous: Consists of sclera and cornea

– Vascular: Consists of choroid, ciliary body, iris

– Neural: Consists of retina

Fig. 19.12

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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

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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

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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

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Anatomy of the Eye• Lens

– Held by suspensory ligaments attached to ciliary muscles

– Transparent, biconvex

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Focus and Accommodation

• Far vision: 20 feet or more from eye• Near vision: Closer than 20 feet

– Accommodation

Fig. 19.16

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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

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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.

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Questions?