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Chapter 15

BLG10A/B Chapter 15: BLG10A WEEK 10 TEXTBOOK NOTES.pdf

Course Code
Emily Agard

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Chapter 15: Special Senses
Special senses are vision, taste, smell, hearing, and equilibrium.!
Special sensory receptors are distinct receptor cells — confined to head region, highly local-
ized within sensory organs (eyes and ears) or epithelial structures (taste buds and olfactory ep-
Accessory Structures of the Eye
- The accessory structures of the eye incl.:
• Eyebrows
• Eyelids
• Conjunctive
Lacrimal apparatus
Extrinsic eye muscles
- Short, coarse hair overlying supraorbital margins of skull
- Helps shade eyes from sunlight and prevent perspiration from trickling down forehead
and reaching eyes
-Eyelids, or palpebrae, protect eyes
- Eyelids separated by palpebral fissure and meet at medial and lateral angles of eye —
medial and lateral commissures (canthi)
Medial commissure has fleshy elevation called lacrimal caruncle — contains seba-
ceous and sweat glands, produces whitish, oily secretion
Most Asian peoples, vertical fold of skin (epicanthic fold) appears on both sides of
nose and sometimes covers medial commissure
- Supported internally by connective tissue sheets called tarsal plates
Also anchors:!
Orbicularis oculi (muscle encircles eye and allows for contraction)!
Levator palpebrae superioris (muscle opening eye)!

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-Eyelashes protects free margins of eyelid; hair follicles richly innervated by nerve end-
ings (hair follicle receptors); anything touching eye triggers reflex blinking
- Many glands associated with eyelids!
Tarsal glands embedded in tarsal plates — ducts open at eyelid edge just posterior
to eyelashes; produces oily secretion lubricating eyelid and eye and prevents eye-
lids from sticking together
Sweat glands called ciliary glands lie between hair follicles
- Transparent mucous membrane
- Lines eyelids as palpebral conjunctiva
- Folds back over anterior surface of eyeball as bulbar conjunctiva — covers white of
eye, not cornea; thin, blood vessels clearly visible beneath it
- When eye is closed, slit-like space occurs between conjunctiva-covered eyeball and eye-
lids — conjunctival sac, where contact lens lies and eye medications administered
- Major function is to produce lubricating mucus that prevents eye from drying out
Lacrimal Apparatus
- Consists of lacrimal gland and ducts draining lacrimal secretions into nasal cavity!
-Lacrimal gland lies in orbit above lateral end of eye and visible through conjunctiva
when lid is everted
Continually releases dilute saline solution called lacrimal secretion (tears) into su-
perior part of conjunctival sac through small excretory ducts
- Blinking spreads tears down to medial commissure, where they enter lacrimal canaliculi
via two tiny openings called lacrimal puncta — visible red dots on medial margin of
each eyelid
Tears drain into lacrimal sac then into nasolacrimal duct
Contains mucus, antibodies, and lysozyme — enzyme destroying bacteria
Cleanses, protects, moistens, and lubricates eye surface
- When lacrimal secretion incr. substantially, tears spill over eyelids and fill nasal cavities,
causing congestion “the sniffles”
Extrinsic Eye Muscles
- 6 strap-like extrinsic eye muscles control each movement of each eyeball, as well
maintain shape and hold it in orbit
- Four rectus muscles originate from common tendinous ring (or annular ring) at back of
orbit and run straight to insertion on eyeball
Locations and movements indicated by their names: superior, inferior, lateral, and
medial rectus muscles

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-Two oblique muscles — move eye in vertical plane when eyeball already turned medially
by rectus muscles!
Superior oblique muscle runs along medial wall of orbit, makes right-angle and
passes through fibrocartilaginous loop called the trochlea suspended from frontal
bone before inserting on superolateral aspect of eyeball; rotates eye downward and
somewhat laterally !
Inferior oblique muscle — rotates eyes up and laterally
Structures of the Eyeball
-Eyeball is a slightly irregular hollow sphere; said to have poles
Anterior point is the anterior pole, and posterior point is posterior pole
- Walls composed of three layers:
1. Fibrous
2. Vascular
3. Inner
- Internal cavity filled with fluids called humours that help maintain its shape
- The lens supported vertically within eyeball, dividing into anterior and posterior segments
Fibrous Layer
- Outermost coat of eyeball
- Composed of dense avascular connective tissue
- Has two regions:
• Sclera
• Cornea
Sclera — glistening white and opaque; forms bulk of fibrous layer; protects and shapes eyeball;
provides sturdy anchoring site for extrinsic eye muscles; posteriorly, where pierced by optic
nerve, it is continuous with dura mater of brain
Cornea — crystal-clear, forms window letting light enter eye and major part of light-bending ap-
paratus of eye
- Epithelial sheets cover both faces of cornea
- External sheet — stratified squamous epithelium; protects cornea from abrasion, merges
with bulbar conjunctiva at corneoscleral junction; renewing epithelial cells located here
-Deep corneal endothelium — composed of simple squamous epithelium; lines inner face
of cornea; cells have active sodium pumps maintaining clarity of cornea by keeping water
content low
- Cornea well supplied with nerve endings, most are pain receptors; no blood vessels so
not related to immune system — only tissue in body transplanted from person to person
with little risk of rejection
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