Eye Anatomy

Introduction

• The structures of the eye
  • cornea
    • a transparent structure that allows light to enter the eye
  • pupil
  • iris
    • covered by the conjunctiva, a transparent mucous membrane
      • remember that the conjunctiva lines the inside of the eyelids as well, up to the limbus
      • clinical correlate
        • conjunctivitis
          • which describes inflammation of the conjunctiva
  • lens
  • sclera
  • limbus 
    • which is the border of the cornea and sclera
  • medial and lateral canthus

• Light enters the eye through the cornea and lens which results in
  • an image (inverted and reversed) being formed in the retina
    • the area on the retina with the highest visual acuity is the fovea, which is surrounded by the macula
    • medial (nasal) to the fovea is the optic disc, which  
      • is where axons exit forming the optic nerve (cranial nerve II)
        • note that the optic nerve does not have photoreceptors over it, resulting in a small blind spot
    • photoreceptors
      • there are two classes
        • rods
          • provides vision in a low-level light environment
          • does not detect color
          • Retinitis pigmentosa is an inherited disorder characterized by degradation of rod photoreceptors. It presents with progressively deteriorating night vision and peripheral vision 
        • cones
          • highly represented in the fovea
          • detect color
• choroid
  • is a vascular layer of the eye
• ciliary body
  • is found between the choroid and the iris and is composed of the
  • ciliary muscle
    • which is controlled by the parasympathetic fibers in the oculomotor nerve in order to
      • contract, resulting in miosis
  • ciliary processes    
    • which have zonular fibers extending from this structure to the lens forming the suspensory ligament
• Lens
  • transparent biconvex disc behind the pupil that provides additional refractive power. Is composed of:
    • lens capsule
    • subcapsular epithelium
    • lens fibers
  • disease processes that affect the lens include
    • cataracts – opacifiction of the crystalline lens
    • ectopia lentis – lens dislocation
    • presbyopia – decreased elasticity of the lens leading to impaired accomodation 
• Anterior chamber
  • describes the area behind (posterior) to the cornea and infront (anterior) to the iris
• Posterior chamber
  • describes the area posterior to the iris and anterior chamber
• Aqueous humor pathway
  • the ciliary body produces aqueous humor into the posterior chamber which
    • flows through the space between the lens and iris into the
      • anterior chamber and finally drains into the
        • trabecular meshwork and then canal of Schlemm
        • uveoscleral pathway
• Blood supply
  • an arterial source is from the ophthalmic artery 
    • the short posterior, long posterior, and anterior ciliary arteries
    • central retinal artery which
      • supplies the optic nerve
    • occlusion presents as painless, acute loss of vision in the affected eye 
  • venous drainage is from
    • the vorticose veins
    • central retinal veins