FACTS & HINTS

High-Yield Facts

Clinical Points

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Severance of recurrent laryngeal nerve

Recurrent laryngeal nerve (supplies intrinsic muscles larynx)

Is closelyassociated with inferior thyroid arteryand needs to be avoided during neck surgery

If unilateral damage, voice hoarseness mayresult because one vocal fold cannot approximate the other.

If bilateral damage, loss of voice will result because vocal folds cannot approximate each other (be adducted)

Clinical Points

Thyroid lumps

Lumps in thyroid can be single, multiple Solitarynodules are likelyto be benign (80%)

Investigation includes: history, examination, and fine-needle aspiration of the gland for cytologyand radionucleotide imaging Most common malignant is papillarythyroid cancer

Treatment is total thyroidectomy

Clinical Points

Hyperthyroidism

Medical condition with increased activityof the thyroid gland

Results in excessive amount of circulating thyroid hormones

Leads to increased rate of metabolism

Affects about 1% of women and 0.1% of men

Thyrotoxicosis is a toxic condition caused byan excess of thyroid hormones from anycause.

Hyperthyroidism with diffuse goiter (Graves' disease)

Most commons cause of hyperthyroidism in patients younger than 40 years.

Excess synthesis and release of thyroid hormone (T3 and T4) result in thyrotoxicosis,

Thyrotoxicosis upregulates tissue metabolism and leads to symptoms indicating increased metabolism.

Mnemonics

Memory Aids

Note: TEAC is a brand name of a home stereo. Associate the TEAC sound with the vocal cords and you can make a connection.

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9 Orbit and Contents

STUDYAIMS

At the end of your study, you should be able to:

Define the boundaries, content, and function of the bonyorbit

Know the foramina of the bonyorbit and what theytransmit

Describe the anatomyof the eyelids

Describe the anatomyof the lacrimal apparatus and know its functions

Know the anatomyof the eyeball and the composition of its three layers

Understand the roles of the refractive structures and media of the eyeball

Outline the keyextraocular and intraocular muscles and their function

Know the vascular supplyof the eye

Outline the innervation of the eye

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GUIDE

Head and Neck: Orbit and Contents

Bony Orbit

Cavitycontaining and protecting five sixths of eyeball, associated muscles, nerves, and vessels.

Opening is protected bya thin moveable fold: the eyelid.

Supports, protects and maximizes the functions of the eye

Pyramidal shape with apexdirected posteriorlyand base anteriorly

Boundaries

Roof

Orbital plate frontal bone

Lesser wing sphenoid

Fossa for lacrimal gland found in orbital part

Floor

Orbital plate of maxilla

Some contributions from zygomatic and palatine bones

Contains inferior orbital fissure from apexto orbital margin

Medial wall

Paper thin

Orbital plate of ethmoid bone

Some contributions from frontal, lacrimal, and sphenoid bones

Indented bylacrimal fossa for lacrimal sac

Lateral wall

Frontal process of zygomatic bone

Greater wing of sphenoid

Apex

Lesser wing of sphenoid

Contains optic canal medial to superior orbital fissure

Foramina of the orbital cavity

Eyelids and Lacrimal Apparatus

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[Plate 81, Eyelids]

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Eyelids and tears (lacrimal fluid) protect cornea and eyeball from dust and particulate matter. Eyelids

Two moveable folds of skin that cover the eye anteriorly

Protect the eye from injuryand excessive light and keep the corneas moist.

Eyelids separated byan elliptical opening, the palpebral fissure.

Covered bythin skin externallyand palpebral conjunctive internally

Palpebral conjunctive continuous with bulbar conjunctive of eyeball

Lines of reflection of palpebral conjunctiva onto eyeball are deep recesses: superior and inferior conjunctival fornices

Strengthened byplates of dense connective tissue: tarsal plates

Tarsal glands embedded in plates Produce a lipid secretion

a.Lubricates edge of eyelids to prevent then from sticking together

b.Barrier for lacrimal fluid

Medial palpebral ligaments

a.Attach tarsal plates to medial margin of orbit

b.Orbicularis oculi attaches to this ligament

Lateral palpebral ligaments attach tarsal plates to lateral margin of orbit

Orbital septum from tarsal plates to margins of orbit, continuous with periosteum of bonyorbit

Skin around the eyes devoid of hair except for eyelashes

Are arranged in double or triple rows on the free edges of the eyelids

Ciliaryglands associated with eyelashes: sebaceous glands.

Muscles of the eyelids

Orbicularis oculi

Levator palpebrae superioris

Lacrimal apparatus

Functions

Secretes tears

Prevents desiccation of cornea and conjunctiva

Lubricates eye and eyelid

Antibacterial

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Consists of

Lacrimal glands

Lacrimal ducts

Lacrimal canaliculi

Nasolacrimal ducts

Lacrimal gland

Lies in fossa for lacrimal gland in superolateral orbit

Consists of two parts

a.Larger orbital

b.Smaller palpebral

c.Divided byexpansion of tendon of levator palpebrae superioris

Twelve lacrimal ducts open from deep surface of gland into superior conjunctival fornix

Secrete lacrimal fluid upon stimulation byparasympathetic secretomotor fibers from CN VII

Lacrimal canaliculi

Drain tears from lacrimal lake at medial angle of eye

Drain to lacrimal sac

Lacrimal sac drains to nasal cavityvia nasolacrimal duct

Contents of the Orbit

[Plate 87, Eyeball]

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Eyeball

Surrounded byfascial sheath (Tenon's capsule)

From optic nerve to junction of cornea and sclera

Forms socket

Pierced bytendons of extraocular muscles

Three layers

Outer fibrous = sclera and cornea

Middle vascular = choroid, ciliarybodyand iris

Inner pigmented and nervous = retina

Fibrous coat

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Sclera = opaque part of fibrous coat

a.Covers posterior five sixths of eyeball

b.Visible through conjunctiva is the white of the eye

c.Pierced posteriorlybyoptic nerve

Cornea

a.Transparent part of fibrous coat

b.Transmits light

Middle vascular layer

Choroid

a.Outer pigmented layer

b.Inner vascular layer

c.Lies between sclera and retina

d.Lines most of sclera

e.Terminates anteriorlyas ciliarybody

Ciliarybody

a.Connects choroid with iris

b.Contains smooth muscle that alters the shape of lens

c.Folds on internal surface (ciliaryprocesses) produce aqueous humor and attach to suspensoryligament of lens

Iris

a.Pigmented diaphragm with central aperture: the pupil

b.Contains smooth muscle that alters the size of the pupil to regulate the amount of light entering the eye

c.Radial fibers of the dilator pupillae open the pupil

d.Circular fibers of the sphincter pupillae close the pupil

Inner (retinal) layer

Consists of three parts

Optic part (1)

a.Receives light

b.Composed of two layers: inner neural layer and outer pigmented layer

c.Inner neural layer contains photosensitive cells: rods for black and white and cones for color

Ciliaryand iridial parts (2 and 3)

a.Continuation of pigmented layer plus a layer of supportive cells

b.Cover ciliarybodyand posterior surface of retina

Fundus

a.Is posterior part of eye

b.Contains optic disc = depressed area where optic nerve leaves and central arteryof the retina enters

c.Optic disc contains no photoreceptors = "blind spot"

Macula lutea

a.Small oval area of retina

b.Contains concentration of photoreceptive cones for sharpness of vision

c.Depression in center = fovea centralis, area of most acute vision

Neural retina ends anteriorlyat ora serrata

Serrated border posterior to ciliarybody

Termination of light receptive part of retina

Vasculature of retina

Central arteryof retina from ophthalmic artery

Retinal veins drain to central vein of retina

Rods and cones receive nutrients directlyfrom vessels in the choroid

Chambers of the eye

Anterior chamber

a.Between cornea anteriorlyand iris/pupil posteriorly

b.Contains aqueous humor

Posterior chamber

a.Between iris pupil anteriorlyand lens and ciliarybodyposteriorly

b.Contains aqueous humor

Vitreous chamber

a.Between lens and ciliarybodyanteriorlyand retina posteriorly

b.Contains vitreous bodyand vitreous humor

Light refraction

Cornea

Refracts light that enters eye

Transparent and sensitive to touch (ophthalmic nerve = CN V1)

Aqueous humor in anterior chamber

Refracts light

Provides nutrients for cornea

Produced byciliarybody

Circulates through Canal of Schlemm in iridocorneal angle

Lens

Transparent, enclosed in capsule

Shape changed byciliarymuscles via suspensoryligaments attached around periphery

Convexityvaries to adjust for focus on near or far objects

Parasympathetic stimulation of ciliarymuscle reduces tension of suspensoryligaments and lens rounds up for near vision Absence of parasympathetic stimulation relaxes ciliarymuscle, increases tension on suspensoryligaments and flattens lens for far vision

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Muscles of the Orbit

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Intrinsic (intraocular) muscles

Ciliarymuscle

Constrictor pupillae of iris

Dilator pupillae of iris

Extrinsic (extraocular) muscles

Sixmuscles

Four arise from common tendineus ring surrounding optic canal and part of superior orbital fissure

Lateral and medial rectus (2)

a.Lie in same horizontal plane

b.Rotate eyeball laterallyand medially, respectively

Superior and inferior rectus (2)

a.Lie in same vertical plane

b.Pull eyeball superiorlyand inferiorly, respectively

Inferior oblique

a.Works with superior rectus

b.Pulls eyeball superiorlyand laterally

Superior oblique

a.Works with inferior rectus

b.Pulls eyeball inferiorlyand laterally

Sheathed byreflection of fascial sheath around eyeball (Tenon's capsule)

Medial and lateral check ligaments

a.Triangular expansions of sheath of medial and lateral rectus muscles

b.Attached to lacrimal and zygomatic bones

c.Limit abduction and adduction

Suspensoryligament

a.Union of check ligaments with fascia of inferior rectus and inferior oblique muscles

b.Forms sling that supports eyeball

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V1)

Vasculature of the Orbit

[Plate 91, Vascular Supply of Eye]

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Arteries

Ophthalmic artery(main supply)

Enters orbit through optic canal

Lateral to optic nerve

Infraorbital arteryfrom maxillary

Branches of ophthalmic artery

Supraorbital

Supratrochlear

Lacrimal

Dorsal nasal

Ethmoidal-anterior and posterior

Central arteryof the retina

a.Branch of ophthalmic

b.Runs within dural sheath of optic nerve

c.Emerges at optic disc and branches over retina

Posterior ciliaryarteries

a.Branches of ophthalmic

b.Sixshort to choroid

c.Two long to ciliaryplexus

Anterior ciliary

a.From muscular branches of ophthalmic

b.Anastomoses with posterior ciliaryarteries

Distribution of Branches of Ophthalmic Artery

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Venous drainage

Superior ophthalmic vein

Formed byunion of supraorbital and angular vein of face

Receives blood from anterior and posterior ethmoid, lacrimal and muscular branches, central vein of retina, and upper two vorticose veins of retina

Drains to cavernous sinus

Inferior ophthalmic vein

Forms in floor of orbit

Receives blood from lower extraocular muscles and lower two vorticose veins of retina

Drains to cavernous sinus

Communicates with pterygoid plexus of veins through inferior orbital fissure

Innervation of the Orbit

[Plate 120, Oculomotor (III), Trochlear (IV), and Abducent (VI) Nerves: Schema]

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Optic nerve

Formed from axons of retinal ganglion cells

Exits through optic canal

Fibers from medial half of each retina cross at optic chiasm and join uncrossed fibers from lateral half of contralateral retina to form

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optic tract Oculomotor nerve (CN III)

Runs in lateral wall of cavernous sinus

Enters orbit through superior orbital fissure

Contains parasympathetic fibers to sphincter pupillae and ciliarymuscles

Supplies

Levator palpebrae superioris

Superior rectus

Medial rectus

Inferior rectus

Inferior oblique Trochlear nerve (CN IV)

Runs in lateral wall of cavernous sinus

Passes through superior orbital fissure

Supplies superior oblique muscle Abducent nerve (CN VI)

Courses through cavernous sinus

Enters orbit via superior orbital fissure

Innervates lateral rectus muscle Branches of the ophthalmic nerve (CN V1) Lacrimal nerve to lacrimal gland

Frontal nerve

Divides into supraorbital and supratrochlear

Supplies upper eyelid, forehead, and scalp

Nasociliarynerve and its branches

Infratrochlear to eyelids, conjunctiva, and nose

Anterior and posterior ethmoidal nerves to sphenoid and ethmoid sinuses and anterior cranial fossa

Long ciliarynerves to dilator pupillae Short ciliarynerves

Branches from ciliaryganglion

Carryparasympathetic and sympathetic fibers Innervate ciliarybodyand iris

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