EYES

• The eye transmits visual stimuli to the brain for interpretation and, in doing so, functions as the organ of vision
• Eye orbit


  • A round, bony hollow formed by several different bones of the skull
  • A cushion of fat surrounds the eye
  • The bony orbit and fat cushion protect the eyeball

  
• External structures of the eye

  • Eyelids (upper and lower)
  • Eyelashes
  • Conjunctiva
  • Lacrimal apparatus
  • Extraocular muscles

  
• Eyelids


  • Two movable structures composed of skin and two types of muscle: striated and smooth
  • Protect the eye from foreign bodies and limit the amount of light entering the eye
  • Distribute tears that lubricate the surface of the eye

  
• Tarsal plates


  • Made up of connective tissue in the upper eyelid
  • Contain the meibomian glands which secrete an oily substance that lubricates the eyelid

  
• Medial canthus


  • Contains the puncta (two small openings that allow drainage of tears into the lacrimal system)
  • Contains the caruncle (a small, fleshy mass that contains sebaceous glands)

  
• Palpebral fissure


  The white space between open eyelids
• Conjunctiva


  • A thin, transparent, continuous membrane
  • Divided into a palpebral portion (lining the inside of the eyelids) and a bulbar portion (covering most of the anterior eye, merging with the cornea at the limbus)

  
• Lacrimal apparatus


  • Glands and ducts that lubricate the eye
  • Lacrimal gland produces tears
  • Tears drain into the puncta, lacrimal canals, nasolacrimal sac, and nasal meatus

  
• Extraocular muscles


  • Six muscles attached to the outer surface of each eyeball
  • Control six different directions of eye movement
  • Four rectus muscles and two oblique muscles
  • Innervated by the oculomotor (III), trochlear (IV), and abducens (VI) cranial nerves

  
• Internal structures of the eye

  • External layer (sclera and cornea)
  • Middle layer (iris, ciliary body, choroid)
  • Innermost layer (retina)

  
• Sclera


  A dense, protective, white covering that physically supports the internal structures of the eye

  
• Cornea


  The transparent "window of the eye" that permits the entrance of light
• Ciliary body


  Consists of muscle tissue that controls the thickness of the lens
• Iris


  • A circular disc of muscle containing pigments that determine eye color
  • The central aperture of the iris is called the pupil
  • Muscles in the iris adjust the size of the pupil to control the amount of light entering the eye

  
• Lens


  • A biconvex, transparent, avascular, encapsulated structure located immediately posterior to the iris
  • Supported by suspensory ligaments attached to the ciliary body
  • Functions to refract (bend) light rays onto the retina
  • Adjustments in refraction are made by changes in the shape of the lens, controlled by the ciliary body

  
• Choroid


  Contains the vascularity necessary to provide nourishment to the inner aspect of the eye and prevents light from reflecting internally

  
• Retina


  • Extends only to the ciliary body anteriorly
  • Receives visual stimuli and sends it to the brain
  • Consists of numerous layers of nerve cells, including rods and cones (photoreceptors)

  
• Optic disc


  • A cream-colored, circular area located on the retina where the optic nerve enters the eyeball
  • Normally round or oval in shape, with distinct margins
  • Contains a smaller circular area called the physiologic cup

  
• Retinal vessels


  • Four sets of arterioles and venules travel through the optic disc, bifurcate, and extend to the periphery of the fundus
  • Venules are dark red and grow progressively narrower as they extend out to the peripheral areas
  • Arterioles carry oxygenated blood and appear brighter red and narrower than the veins

  
• Fovea centralis


  • A retinal depression located adjacent to the optic disc in the temporal section of the fundus
  • Highly concentrated with cones and forms the area of highest visual resolution and color vision

  
• Anterior chamber


  Located between the cornea and the iris, filled with aqueous humor
• Posterior chamber


  Located between the iris and the lens, filled with aqueous humor
• Vitreous chamber


  Located behind the lens to the retina, filled with a clear, gelatinous vitreous humor
• Visual field


  What a person sees with one eye, can be divided into four quadrants: upper temporal, lower temporal, upper nasal, and lower nasal

  
• Binocular vision


  The visual cortex fuses the slightly different images from the two eyes, providing depth perception and three-dimensional vision

  
• Pupillary light reflex


  Causes pupils to constrict immediately when exposed to bright light, a direct reflex in the exposed eye and a consensual reflex in the opposite eye

  
• Accommodation


  A functional reflex allowing the eyes to focus on near objects, accomplished through movement of the ciliary muscles causing an increase in the curvature of the lens

  
• Genetic and biologic variations can be seen in both structure and color of the eyes
• Eye color tends to vary with distance from the equator, with Northern Europeans and Scandinavians often having blue eyes, and most Asians and Africans having brown eyes

  
• Asians often have epicanthic folds, and a narrowed palpebral fissure in a non-Asian may indicate Down syndrome
• Visual impairment varies across age (> 50), gender (more in females), and geography (> 90% of those with visual impairment live in developing countries)
• In all but highly developed countries, cataract is the leading cause of visual disease and blindness, followed by glaucoma and age-related macular degeneration
• Changes in vision are often gradual and go unrecognized by clients until a severe problem develops
• More than 90% of those with visual impairment live in developing countries
• Cataract is the leading cause of visual disease and blindness in all but highly developed countries, followed by glaucoma and age-related macular degeneration (AMD, which is the leading cause in developed countries)
• Other diseases include trachoma, other corneal diseases, diabetic retinopathy, and diseases of children, such as cataract, retinopathy of prematurity, and vitamin A deficiency
• Visual impairment


  Any visual condition that impacts an individual's ability to successfully complete the activities of everyday life
• Visual impairment classification


  • Low vision
  • Functional blindness
  • Total blindness
• Origin of visual impairment

  • Congenital (occurring in fetal development)
  • Adventitious (occurring after having normal vision through a hereditary condition or a trauma)