ch15

Conjunctiva

Transparent mucous membrane that produces a lubricating mucous secretion to
prevent eye from drying out

Lacrimal apparatus

Consists of lacrimal gland and ducts that drain into nasal cavity

ears then drain into

lacrimal sac
and nasolacrimal duct,

Extrinsic eye muscles

Originate from bony orbit and insert on eyeball (sclera)
▪ Enable eye to follow moving objects, maintain shape of eyeball, and hold it in
orbit

Wall of eyeball contains three layers

Fibrous layer
– Vascular layer
– Inner layer
• Internal cavity filled with fluids
• Lens separates internal cavity into anterior and posterior segments

Fibrous layer

Outermost layer; dense avascular connective tissue
– Two regions: sclera and cornea

Sclera

– Opaque posterior region. Seen anteriorly as the “white of the eye”
– Protects and shapes eyeball
– Anchors extrinsic eye muscles

Cornea

ransparent anterior
• Forms clear window that lets light enter and bends light as it enters
eye

Vascular layer

Forms the middle coat of the eyeball
– Three regions: choroid, ciliary body, and iris

choroid region

Supplies blood to all layers of eyeball. Dark brown membrane

Ciliary body

Ciliary body
– Anteriorly, choroid becomes ciliary body
– Thickened ring of tissue surrounding lens
– Consists of smooth muscle bundles, ciliary muscles, that control
shape of lens

iris

Colored part of eye that lies between cornea and lens, continuous with
ciliary body
– Pupil: central opening of iris that regulates amount of light entering eye
– Made of two muscles: sphincter papillae and dilator pupillae

Inner layer (retina)
– Contains:

▪ Millions of photoreceptor cells that transduce (change) light energy into AP
▪ Neurons
▪ Glial cells
– two-layered membrane
▪ Outer pigmented layer
▪ Inner neural layer

Inner layer (retina) (cont.)

– Pigmented layer of the retina
▪ Single-cell-thick lining next to choroid
▪ Functions:
– Absorbs light
– Stores vitamin A needed by photoreceptor cells

Inner layer (retina) (cont.

– Neural layer of the retina
▪ Composed of three main types of neurons
– Photoreceptors, bipolar cells, ganglion cells

Retina has quarter-billion photoreceptors that are one of two types:
– Rods
– Cones

Rods

▪ For dim light and peripheral vision receptors – highly sensitive to light
▪ No color vision and do not provide sharp vision

Cones

▪ Vision receptors for bright light – low sensitivity to light
▪ High-resolution color vision

Internal chambers and fluids
– The lens separate eye into two segments

1. Posterior segment
2. Anterior segment

Internal chambers and fluids (cont.)
– Posterior segment - behind lens

Contains vitreous humor, a fluid that:
– Transmits light
– Supports posterior surface of lens
– Holds neural layer of retina firmly against pigmented layer
▪ Vitreous humor forms in embryo and lasts whole lifetime

nternal chambers and fluids (cont.)
– Anterior segment

Entire anterior segment contains aqueous humor, a plasma like fluid
continuously formed (unlike vitreous humor)
– Supplies nutrients and oxygen mainly to lens and cornea and removes
wastes
– Normally aqueous humor forms and drains at the same rate, maintaining
a constant intraocular pressure of about 16 mm Hg

Lens

Biconvex, transparent, flexible, and avascular
– Changes shape to precisely focus light on retina. Muscle of ciliary body control
lens shape
– Bulge for up close vision
– Flattens for distance vision