Ocular Block 2

Orbit

Eye socket

Globe

Eyeball

Adnexa

Everything except the globe

Includes: orbit, extracellular muscles, eyelids, conjunctiva, third eyelid/nictitating membrane, and lacrimal system

Anterior segment of eye includes…

the cornea, sclera, anterior chamber, posterior chamber, iris, iridocorneal angle, ciliary body, and lens

Anterior Chamber

Space between the cornea and iris/lens

Cornea

transparent, avascular, 0.5-1mm thick rostral portion of eye; transmits light, refracts light, and protects internal contents

Sclera

White of eye

Iris

Constricts to control size of pupil

Iridocorneal Angle

Angle between iris and cornea

Pupil

Lens

Ciliary Body

Vitreous

Retina

Choroid

Optic Nerve

Posterior Segment of the eye includes…

Vitreous humor, retina, choroid, optic nerve, and posterior sclera

How is the orbit different in herbivores vs carnivores?

Herbivores have a complete bony rim while carnivores have an incomplete bony rim with the orbital ligament.

What is in the orbit?

It is lined with fascia and contains the conjunctiva, nictitans, fat, extraocular muscles, muscles of mastication, blood vessels, glands, cranial nerves 2-8, and autonomic nerves.

What bone of the skull contacts the floor of the orbit?

Ramus

What structures can cause secondary orbital disease due to close nature?

Nose, sinuses, and teeth

What is the purpose of the extraocular muscles?

To move the globe in the orbit and the muscle cone anchors in the back of the orbit.

What do the dorsal/superior and ventral/inferior oblique muscles do?

Rotate the globe

What does the retractor bulbi muscle do?

Pulls globe back in orbit

What structures are on the eyelid?

Skin, hair, mucocutaneous junction, conjunctiva, glands, and muscles

Skin of the eyelid

Very thing to provide mobility and elasticity varies. Made up of epidermis and contains hair follicles

Cilia

Eyelashes!  Varies among species and originate outside the meibomian gland openings

Meibomian glands definition

Row of sebacous glands on eyelid margin that open at the mucocutaneous junction and secrete lipid layer of the tear film

Tarsus

Collagen sheet that encompasses structures of eyelid margin (3-4mm) and gives rigidity and a surface for muscle attachment

Levator palpebrae superioris muscle

Elevates the upper eyelid, innervated by CN III

Orbicularis oculi muscle

Surrounds margins and closes upper and lower lids, innervated by CN VII

Malaris muscle

Lowers the inferior eyelid, innervated by CN VII

Mueller’s muscle

smooth muscle that provides tone to the tarsus (keeps eyes open), innervated by Sympathetic nerve

Medial and Lateral Canthal Ligaments

Ligamentous bands, anchor medial (wider and shorter) and lateral (longer and flexible) canthus to periosteum of orbital rim

Vascular supply of eyelids

Well vascularized by longitudinal vessels parallel to eyelid margins

Palpebral Conjunctiva

Lines eyelids

Bulbar Conjunctiva

Lines exposed surfaces of globe

Fornix Conjunctiva

cul-de-sac formed by reflections of conjunctiva at transition from lid to globe

Nictitans Conjunctiva

Convers palpebral and bulbar surfaces of the nictitating membrane

Conjunctiva appearance

Pale pink with some pigment is normal and a few lymphoid follicles

Histology of conjunctiva

Non-keratinized stratified squamous epithelium with goblet cells between to secrete mucus

Conjunctiva functions

Allows smooth gliding of nictitans and eyelids over globe, secretes mucus layer of tear film, offers immune protection for ocular surface, and helps with corneal repair surgery

Nictitating membrane definition

Triangular piece of tissue in medial fornix/canthus, leading edge is pigmented

Nictitating membrane function

Moves dorsolaterlaly as globe is retracted, spreads tear film, protects globe, and removes particulate matter from surface

Structures of the nictitans

Stroma (fibrous connective tissue)

T-shaped cartilage for strucural support

Lacrimal gland at base (galnd of nictitans)

Covered in goblet cell rich conjunctiva

How do you exam the nictitans?

To expose nictitans during examination, retract the eyelids while applying digital pressure to the globe

What is a common name for a prolapsed gland of the nictitating membrane?

Cherry eye

Function of the lacrimal apparatus

Produce, distribute, and drain pre-corneal tear film to maintain lubrication and health of the ocular surface

Tear film layers

Lipid layer - produced by Meibomian gland

Aqueous layer - produced by lacrimal glands

Mucin layer - produced by goblet cells

Lacrimal Glands

Orbital lacrimal glands (dorsolateral orbit) and Gland of the nictitans (base oof nictitans)

Lacrimal Outflow Apparatus

Drains tears from eyelids to nose

1. Ocular puncta in eyelids

2. Canaliculi in eyelids

3. Lacrimal sac in medial canthus

4. Nasolacrimal duct

5. Nasal puncta

What are the 3 concentric tunics?

1. Fibrous

2. Vascular

3. Nervous

What structures make up the fibrous tunic?

1. sclera

2. cornea

What structures make up the vascular tunic?

1. ciliary body

2. iris

3. choroid

What structures make up the nervous tunic?

1. retina

2. optic nerve

Corneal Innervation

Sensory innervation from the opthalmic branch of the trigeminal nerve (CN V) in the anterior 1/3 of cornea

Corneal Anatomy

4 Layers:

1. epithelium

2. stroma

3. descemet’s membrane

4. endothelium

Corneal Epithelium

non-keratinized, non-pigmented squamous epithelium that is 8-15 cell layers thick

1. squamous cells

2. wing cells

3. basal cells

4. basement membrane

desmesomes

junctions between cells

hemidesmesomes

junctions between cells and basement membrane

What special structure are on the superficial cells on the corneal epithelium?

They have microvilli to help stabilize the tear film

What is the turnover rate of the cornea epithelium?

It takes 7 days from basal to superficial cells

How much of the epithelium makes up the stroma?

90% of the thickness

Composition of the cornea stroma?

collagen (regularly arranged), keratocytes, glycosaminoglycans, water (78%), and no blood vessels

Why is normal distance between fibrils important?

It is required for light to transmit well through the cornea, edema causes spreading of the fibrils from the normal arrangement

What is important about the corneal stroma?

It is hydrophilic and will imbibe water if there is a break in epithelial or endothelial barriers; needs to maintain deturgescence (dehyration) to be clear

Descemet’s membrane

basement membrane of the endothelium; modified basal lamina

What does descemet’s membrane do as a dog ages?

It gets thicker

Corneal endothelium

A single cell layer at the inner-most layer of the cornea made of hexagonal cells

Corneal endothelium function

actively pumps fluid out of cornea to maintain deturgescence and is mechanical barrier to stroma

How do cells heal in the corneal endothelium?

Cells do not undergo mitosis and heal by cellular enlargement and migration

polymegathism

cellular enlargement

pleomorphism

variable cell shape

Limbus

corneal-scleral junction; zone of transition from regularly arranged collagen and straight basement membrane of cornea

Sclera composition

Irregularly arranged densely packed collagen fibrils, vascularized, contains pigment and nerves, opaque, covered by bulbar conjunctiva, contains posterior opening for optic nerve

Lamina Cribosa

collagen sieve supporting axons of optic nerve, site of weakness