Anatomy of the Eye

• In many ways, eyes function similarly to cameras, utilizing lenses, adjustable diaphragms, and light detectors.
• Most components of the eye are involved in forming the visual image
• Rods and cones within the retina detect the image.

Key Terms

• Ocular and ophthalmic describes relating to the eye
• Ophthalmology refers to the study of the eye and its diseases

Layers of the Eyeball

• The eyeball has three primary layers:
  • The outer fibrous layer
  • The middle vascular layer
  • The inner nervous layer.

Fibrous Layer

• The outer fibrous layer gives strength and shape to the eye
• The cornea is the transparent window of the eye
  • The transparent cornea consistes of an orderly arrangement of collagen fibers
  • The cornea also features a controlled water content
    • Excessive water content leads to corneal edema and cloudiness.
    • Insufficient water causes corneal dehydration and cloudiness.
• The sclera makes up the white part of the eye
  • The sclera consists of dense fibrous connective tissue
  • The limbus is the junction between the sclera and cornea.

Vascular Layer

• The middle vascular layer is known as the uvea
• The uvea comprises three parts:
  • Choroid
  • Iris
  • Ciliary body.

Choroid

• The choroid is positioned between the sclera and the retina
• Consists of pigment and blood vessels that nourish the retina
• In most animals a highly reflective area the tapetum lucidum exists in the choroid except for in swine.
  • The tapetum lucidum causes bright light reflection from an animal’s eyes in the dark
  • In low-light situations it acts as a light amplifier
  • The tapetum reflection allows light to pass through receptors twice

Iris

• The iris is a modification of the vascular layer
• Forms the colored part of the eye
• Functions as a pigmented muscular diaphragm
• Controls the amount of light entering the eye through the pupil
  • The pupil gets larger in low light conditions
  • Conversely, it becomes smaller in bright light.
• Built from radially arranged and circularly arranged fibers
• The autonomic nervous system provides nerve supply to the smooth muscle cells of the iris

Ciliary Body

• The Ciliary Body is a ring-shaped structure just behind the iris
• Made of ciliary muscles, which adjust the shape of the lens
  • These muscles enable vision at varying distances.
  • The muscles connect to the edge of the lens through suspensory ligaments
• The ciliary body produces aqueous liquid.

Nervous Layer

• The nervous layer lines the back of the eye
• It contains sensory receptors for vision within the retina.

Compartments of Eyeball

• There are two fluid filled compartments to the eye:
  • Aqueous compartment is in front of the lens and ciliary body
    • Anterior chamber is in front of the iris, contains aqueous humor
    • Posterior chamber is behind the iris, aqueous humor here also
      • Aqueous humor is generated by the cells of the ciliary body
      • It flows through the pupil into the anterior chamber
      • Drains through the canal of Schlemm and returns to the bloodstream
      • A ring-like structure at the angle where the iris and cornea meet is where filtration occurs
  • Vitreous compartment fills the back of the eye behind the lens and ciliary body, contains vitreous humor
    • Vitreous humor is soft and gelatinous

Lens

• The lens is a soft, transparent structure of microscopic fibers
• Elastic and biconvex in shape
• It is normally round in shape
• To focus light, muscles of the ciliary body contract
• The lens touches the vitreous humor on its back surface
• Accommodation describes focusing light by changing the lens shape

Retina

• Visual images are formed. sensed, and converted into nerve impulses in the retina
• Lines most of the vitreous compartment
• Layers of the retina from outside in:
  • Pigment layer
  • Photoreceptor layer
  • Bipolar cells Layer
  • Ganglion cell layer
  • Nerve fiber layer
• Integrated and relayed impulses pass from the photoreceptor cells to optic nerve by the bipolar and ganglion cell layers
• The optic disc contains no photoreceptors
• Photoreceptor Cells are neurons with dendrites modified into sensory receptors
  • Rods are more sensitive to light, coarse images, motion, and low light
  • Cones are sensitive to color and detail, yet are low in light sensitivity

Visual Image

• Light refraction focuses images on the retina
• Four refractive media help to focus an image:
  • Cornea does most of the refractive work
  • Aqueous humor
  • Lens
  • Vitreous humor.
• The visual image forms upside down within the retina
• The brain then inverts the image.

Extraocular Structures: Conjunctiva

• The conjunctiva is a thin, transparent membrane on the front of the eyeball
• Lines the inside of the eyelids
• The potion lining eyelids is palpebral conjunctiva
• The conjunctiva allows detection of paleness and jaundice
• The space bulbar and palpebral conjunctiva is the conjunctival sac

Eyelids

• Upper and lower folds of skin lined with the conjunctival membrane
• Lateral and medial eye corners are called canthi
• The meibomian glands are along each eyelid margin
• Each Lid is fringed with eyelashes

Third eyelid

• Domestic animals have a third eyelid, also known as the nictitating membrane
• Located on the interior of the eyelids/eyeball
• T-shaped cartilage plate
• Covered with the conjunctiva
• Ocular surface contains lymph nodules and an accessory lacrimal gland

Lacrimal Apparatus

• Lacrimal apparatus is primarily involved in producing and draining tears
• It moistens and protects the eye surface
• tears are made up of 3 layers
  • Inner mucous layer that contains antibacterial substances
  • Middle tear layer made from lacrimal glands moistens the cornea
  • Outer oily layer is made from the meibomian glands reducing evaporation
• Are constantly produced, needing constant draining
• Tears drain in the nasolacrimal duct into the nasal cavity

Eye Muscles

• Eye muscles are extraocular and attach to the sclera
• Skeletal muscles hold the eye in place and move it
• There are four straight muscles and two oblique muscles
  • Includes the Dorsal, ventral, medial, and lateral rectus muscles
  • Dorsal and ventral oblique muscles.
• Humans lack Retractor bulbi muscles