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Basic ocular anatomy and diagnostics (Proceedings)


A review of the basic anatomy of the eye in dogs and cats.

Basic Anatomy of the Eye

  • Orbit

  • Eyelids

  • Nasolacrimal System

  • Cornea/Sclera

  • Lens

  • Uvea

  • Aqueous/Vitreous

  • Iridocorneal Angle

  • Retina

  • Optic Nerve & Brain

The Orbit

  • Orbit= the "camera case" enclosing the eye

  • Made of bone, ligament, fat, muscle, salivary gland, roof of mouth

  • A mass behind the eye can push the eye out and/or protrude through soft tissue into the mouth*

The Eyelids

  • Eyelids= "lens cover" or "windshield wipers" of the eye

  • Act to protect and lubricate the surface of the eye

o Mechanical protection

o Meibomian glands of eyelids produce the oily component of the tears

o Spread tears (and foreign material) across surface & toward drainage system

  • Outer layer is thin skin

  • Muscles to close (orbicularis oculi) and open (levator palpebrae superioris) lids

  • Poorly developed supportive tarsal plate in dogs/cats

  • Meibomian glands in tarsus

  • Inner surface lined with conjunctiva

  • Cilia (eyelashes) arise from the lid margins*

  • Eyelids= skin, so can get all the skin diseases

o Bacterial

o Parasitic

o Allergic

o Auto-Immune

o Neoplastic

  • Eyelid glands can also become infected and/or inflamed, develop tumors

The Third Eyelid

  • Most domestic species have some variation of this structure

  • Acts to protect the eye when it is drawn over its surface passively*

  • T-shaped cartilage core coated by conjunctiva

  • Important tear-producing gland located behind and at its base

  • Von Graefe or other atraumatic forceps are used to look behind the third eyelid for foreign bodies, follicles

The Nasolacrimal System

  • Tear film coats cornea

o Meibomian glands produce thin fatty outer portion of tear film

o The lacrimal & third eyelid glands produce the thick middle watery tear layer

o Goblet cells within conjunctiva produce inner mucus layer

  • After tear production, tears are flushed by lids toward upper & lower punctae

  • Tears drain from lacrimal punctae→ canaliculi→ sac→ NL ducts→ nose/mouth

  • Obstruction at any point causes failure of drainage (tearing)*

Cornea & Sclera

  • Together make up the outer, fibrous coat of the eye

  • CORNEA= clear "windshield" and most important focusing structure of eye

  • SCLERA= the support mechanism


  • Cornea is a multilayered structure

  • Pumps in the innermost endothelium (esp) and outermost epithelium act to dehydrate it

  • The middle stroma is hydrophilic (water-loving)*


  • Corneal clarity achieved due to:

o Relative dehydration

o Lack of blood vessels

o Strict organization of collagen in stroma

  • The sclera is opaque because it lacks these qualities

  • Scarred cornea is also opaque to varying degrees b/c of fibrosis

The Lens

  • LENS= "fine- focus" mechanism of eye

  • Spherical structure responsible for focusing images on retina

  • Similar to cornea, clarity achieved through:

o Relative dehydration

o Acellularity

o Lack of blood vessels

  • Production of new lens fibers occurs only from anterior lens epithelium

  • Individual lens fibers do not reach all the way around; where they meet is staggered, forming lens sutures

o Y-shaped in dog/cat

  • Cells lose organelles with maturity

  • Layers added consecutively throughout life ("onionskin")

o Volume stays same, so compression of inner layers occurs with age

o Oldest layers are innermost

  • Normal lens held in place by numerous peripheral lens zonules

  • When all zonules rupture, lens luxation occurs

  • Without a lens, near images appear

o Out of focus

o Larger than life

The Uvea

  • Pigmented & highly vascular middle coat of eye

  • Three segments

o Iris

o Ciliary body

o Choroid

  • Iris and CB= anterior uvea

  • Because of its intense vascularity, uvea sensitive to systemic inflammation and infections

  • A blood-aqueous barrier (BAB) is intended to dampen this response, but can be broken down and takes months or longer to re-establish

The Uvea- Iris

  • Iris= pigmented "sunshade"

  • Pupil is "aperture"

o Controls amount of light entering eye

  • Pupil shape differs with species

The Uvea- Ciliary Body

  • Ciliary body= Aqueous-producing factory

  • Target of IOP-lowering drugs and procedures which reduce aqueous production

o Methazolamide

o Trusopt

o Laser CPC

o Cryoablation

o Gentocin injection

The Uvea- Aqueous Humor

  • Aqueous is normally clear liquid

  • Fills anterior & posterior chambers

  • Normally low in protein

o Increase in protein content with BAB breakdown (seen as aqueous flare)

  • Drains through iridocorneal angle

The Uvea- Choroid

  • Blood and oxygen supply to the retina

o Exclusive supply to some species' retinas

  • Located between visual retina and supportive sclera

  • Contains reflective tapetum


  • Vitreous is normally gelled and fills vitreous chamber between lens and retina

  • Even degenerated vitreous held in back of eye by intact lens zonules*

  • Places pressure on neuroretina to help keep it in place

o Vitreal degeneration may result from or predispose to retinal detachment

The Iridocorneal Angle

  • IRIDOCORNEAL ANGLE= the eye's "drain"

  • If this is abnormally formed or damaged, drainage occurs less freely and intraocular pressure builds up→ glaucoma

The Retina

  • RETINA= the "film" in the "camera"

  • Responsible for converting light energy into chemical energy and passing it as a neuro signal to the brain

  • Ten layered structure

o 9 layers of neuroretina

o Outer retinal pigment epithelium (RPE)

o Reflective tapetum actually within choroid

o Retinal detachment not entirely accurate term, since separation occurs b/w neuroretina and RPE

The Retina-the Cells

  • Photoreceptors

o "rods" and "cones"

o Absorb incoming light

o Narrow range of specific wavelengths absorb by each cone- varies by species

  • Bipolar cells connect PRs to...

  • Retinal ganglion cells

  • RGCs converge to ON

Optic Nerve & Brain

  • OPTIC NERVE= "cable" connecting the eye to the "computer" (BRAIN)

  • The optic nerve is the only nerve in the body that can be directly examined (by ophthalmoscopy)

  • If the ophthalmic exam and ERG are normal, blindness must be secondary to a ON or brain problem

  • The nerve is a collection of axons of the retinal photoreceptors

  • Takes visual message to the brain

o Travels through weak point in sclera called lamina cribrosa

o Padded in orbit by fat

  • Passes thru the optic foramen

  • Considered cranial nerve II

  • Brain is responsible for interpreting the signal from the eye so that it forms a meaningful image

  • Damage to the occipital or "visual cortex" can result in blindness even if the eye is completely normal

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