Eye

• Eye


  Two eyes (left and right) located on the lateral sides of the skull
• Eye


  • Sits in a protective bony socket/bony cavity called the orbit which is located in the skull at the junction of the bones of the cranium and the nose
  • Found within the orbit are structures like the eyeball, extraocular muscles, nerves, blood vessels and the lacrimal apparatus (gland and the duct that drain tears)
• Eyeball


  Spherical in shape comprising of two spheres: the transparent segment (anterior cavity) and the opaque segment (posterior cavity)
• Parts of the eyeball

  • Fibrous tunic
  • Vascular tunic
  • Nervous tunic
• Fibrous tunic


  • Forms the external coat of the eye
  • Composed of an opaque posterior pars known as the sclera (the white part of the eye) and a transparent anterior part known as the cornea
• Sclera


  Opaque, fibrous outer coat of the eye formed mainly by collagen fibres and a few elastic elements
• Sclera


  • The posterior part is continuous with the dura matter
  • The anterior part is continuous with the cornea at the corneal-scleral junction
  • It is perforated by the optic nerve
  • Its function is to maintain the shape of the eye
• Cornea


  • Oval in shape and blends smoothly with the sclera at the corneal-scleral junction or the limbus
  • Transparent, colorless and non vascular
  • Formed by five layers from outside to the inside: corneal epithelium, Bowman's membrane, stroma, Descemet's membrane, corneal endothelium
• Cornea


  • Provides protection of the anterior part of the eye
  • Allows light transmission
• Vascular tunic


  • Lies internal to the fibrous tunic and external to the nervous tunic
  • Made up of blood vessels, intrinsic muscles and the stroma
  • Composed of the choroid, ciliary body and the iris
• Choroid


  • Thin opaque membrane that lies between the retina and the sclera
  • Loosely attached to the sclera but in tight adherent at point of entrance of the optic nerve
  • Its inner surface is in contact with the pigmented epithelium of the retina
• Ciliary body


  • Middle portion of the vascular tunic
  • Separates the opaque and transparent regions of the eye
  • Surrounds the lens
  • Anterior part is attached to the periphery of the iris while posteriorly the ciliary body is attached to the retina
• Parts of the ciliary body

  • Ciliary ring
  • Ciliary process
  • Ciliary muscles
• Ciliary ring


  Made up of a smooth band of tissue with folds on its inner surface
• Ciliary processes


  Numerous, forming folds that surround the lens and act as points of attachment for the suspensory ligaments of the lens
• Ciliary muscle


  • Also called the muscle of accommodation
  • Lies between the sclera and the ciliary processes
  • Circular band of involuntary muscle whose contraction causes the relaxation of the suspensory ligaments and its relaxation tightens the suspensory apparatus
• Iris

  • Forms the anterior part of the vascular coat
  • Visible through the cornea
  • Muscular diaphragm that incompletely divides the anterior and posterior chambers of the eye
  • Perforated centrally by the pupil
  • Periphery is attached to the base of the ciliary body
• Iris


  • Free border forms the margin of the pupil
  • Has a pigmented layer that gives the color of the eye based on the amount of melanin cells present
  • Contains smooth muscles arranged in two groups: the dilator muscle whose fibers are arranged radially and the sphincter muscle arranged circularly
• Nervous tunic (Retina)


  • Innermost light sensitive part of the eye
  • Receives images and sends them to the brain for interpretation
  • Contains special cells called photoreceptors that change light into energy transmitted to the brain
  • Two types of photoreceptors: rods that perceive black and white and enable night vision, and cones that perceive color and provide detailed vision
• Chambers of the eye

  • Anterior chamber marked anteriorly by the cornea and posteriorly by the iris
  • Posterior chamber marked anteriorly by the iris and posteriorly by the lens and ciliary process
  • The two chambers communicate via the pupil and contain clear/transparent fluid
• Vitreous body


  • Fills the space between the lens and the retina
  • Fresh vitreous is semifluid and highly transparent, jelly-like formed mainly by water
  • Anteriorly it has a cup shaped depression to conform with the posterior surface of the lens
• Lens


  • Transparent, biconvex body suspended from the ciliary body by the suspensory ligament
  • Anterior surface is related to the iris and the posterior surface lies on the depression formed by the vitreous body
  • Enclosed by a transparent elastic membrane known as the lens capsule
  • The convexity of the surfaces of the lens vary due to the tension created by the suspensory ligaments, therefore changing the focal length to accommodate near or far vision
• Adnexia


  • Structures that protect and move the eyeball
  • Include orbital fascia, extra ocular muscles, the eyelids and conjunctiva and the lacrimal apparatus
• Extraocular muscles


  • The muscles of the eye that are located outside the eye, within the orbit
  • Extrinsic and separate from the eyeball itself
  • Act to control the movements of the eyeball
  • There are six extraocular muscles in the orbit that move the eye up and down, side to side, and rotate the eye
• Types of extraocular muscles

  • Four recti (straight) muscles: dorsal, ventral, medial, and lateral rectus
  • Two oblique muscles: dorsal oblique and ventral oblique
  • Retractor bulbi muscle
• Rectus muscles


  • Originate from the orbital apex (posterior part of the orbit) to insert on the sclera by a thin flat tendon
  • Each muscle pulls the eye in the direction of its name
• Oblique muscles


  • Pull from the direction of the medial or nasal wall of the orbit
  • Dorsal oblique muscle originates from the orbital apex and takes an oblique position by passing through the trochlea on the medial orbital wall, its contraction moves the eye in the down-and-out position
  • Ventral oblique muscle is a thin narrow muscle located along the anterior margin of the floor of the orbit, originates from the medial wall of the orbit to insert on the lateral surface of the eye, its contraction rotates and elevates the eye
• Retractor bulbi


  • Not present in all domestic animals
  • Originates from the rear orbital wall and inserts onto the posterior half of the eyeball
  • Withdraws the eye into the orbit
  • Well developed in herbivores
• Eyelids


  • Two, upper and lower, for each eye
  • Serve to close or cover the eye, protecting the eyeball and keeping its surface moist
  • External surface covered by skin with fine hairs, with dense irregular connective tissue in the subcutaneous tissue and the striated muscle of the muscularis orbicularis
• Oblique muscles


  • The two muscles pull from the direction of the medial or nasal wall of the orbit
• Dorsal oblique muscle


  1. Originates from the orbital apex

  2. Takes an oblique position by passing through the trochlea on the medial orbital wall

  3. Its contraction moves the eye in the down-and-out position
• Trochlea


  A saddle-like cartilaginous structure in the dorsal and medial part of the orbit
• Ventral oblique muscle


  1. It is a thin narrow muscle located along the anterior margin of the floor of the orbit

  2. Originates from the medial wall of the orbit to insert on the lateral surface of the eye

  3. Contraction of this muscle rotates and elevates the eye
• Retractor bulbi


  • It is not present in all domestic animals
  • Originates from the rear orbital wall and inserts onto the posterior half of the eyeball
  • This muscle withdraws the eye into the orbit
  • It is a well developed muscle in the herbivores
• Eyelids


  • There are two, upper and lower, for each eye
  • They serve to close or cover the eye, therefore protecting the eyeball
  • They also aid in keeping the surface of the eye moist
  • External surface is covered by skin with fine hairs
  • The glands are well developed in the pig
  • In the subcutaneous tissue are dense irregular connective tissue
  • There is also the striated muscle of the muscularis orbicularis palpebrae
  • A plate of dense connective tissue forms the skeleton of the eyelid and is called the tarsus or tarsal plate
• Tarsal glands of MELBOM


  • Modified sebaceous glands, located at the base of the hair follicles of the eye lashes
  • They open at the line of transition from skin to conjunctiva
• Goblet cells and tarsal glands

  Add a film of mucous and oil to the tears to prevent excessive evaporation
• Eyelashes

  • Special hairs found at the edges of the eyelids
  • Serve as protective screens and dust filters
  • In ruminants and horse, the eyelashes are present in both eyelids
  • In pig, cat and dog no eyelashes are present in the lower eyelid
• Intrinsic muscles of the eye

  • Control the movements of the lens and size of the pupil, playing a role in the accommodation of light
  • They are three groups of smooth muscles including: Ciliary muscle, Dilator pupillae muscle, Sphincter pupillae muscle
• Ciliary muscle

  A smooth muscle that controls accommodation by altering the shape of the lens