Sense Organs: Eyes and Ears

Types of Sense Organs

• There are five types of sense organs: photoreceptors (sight), photoreceptors combined with statoreceptors (hearing-cum-equilibrium), mechanoreceptors (touch), gustatoreceptors (taste), and olfactory receptors (smell)

The Eyes

• The eyes are attached to the eye orbits of the skull and can rotate with the help of six muscles
• The eyes are protected by:
  • Adipose tissue (cushion-like, protects from mechanical injury and shock)
  • Eyelids (protect from injury, clean the eyeball, close during night and bright light, spread lubricating secretions)
  • Eyelashes (prevent dust and foreign particles from entering the eye)
  • Eyebrows (protect from rain, dust, and sweat)
  • Lacrimal glands (produce tears, wash away dust, clean and lubricate the eyes, kill bacteria)
• The structure of the eyeball consists of three layers:

  Sclerotic Layer (Sclera)

  • Made up of tough, non-elastic, fibrous tissue
  • White in color
  • Surrounds the entire eyeball
  • Function: Provides and maintains the shape of the eyeball
  • Cornea: a transparent, visible part of the sclera; refracts light and helps in focusing
  • Cornea facts:
    • Derives oxygen by diffusion
    • Can become opaque and non-functional
    • Remains alive up to 40 hours after death

  Conjunctiva

  • A thin membrane covering the entire front part of the eye
  • Continuous with the inner lining of eyelids
  • Over the cornea, it is reduced to a single layer of transparent epithelium
  • Function: Protects the surface of the eye
  • Viral infection causes conjunctivitis (reddening of the eyes)

  Choroid Layer

  • Supplied with blood vessels to provide nourishment to the eye
  • Dark black in color
  • Contains melanin
  • Functions:
    • Absorbs light
    • Prevents total internal reflection, causing blurring of image
    • Blood vessels nourish the inner layer of the retina
  • Ciliary body: an extension of the choroid layer; secretes aqueous humor and helps in accommodation
  • Iris: an extension of the ciliary body, partially covering the eye and leaving an opening called the pupil
  • Iris functions:
    • Regulates the amount of light entering the eye by widening and constricting the pupil
    • The blue, black, or brown color of the eye is the color of the iris
  • Pupil: light enters the eye through the pupil