Special Senses

• Eye


  A sphere that measures about 1 inch (2.5 cm) in diameter, with only the anterior one-sixth of the eye's surface normally seen
• Accessory structures of the eye

  • Extrinsic eye muscles
  • Eyelids
  • Conjunctiva
  • Lacrimal apparatus
• Eyelids


  Protect the eye anteriorly, meeting at the medial and lateral corners. Palpebral fissure is the space between the eyelids when the eye is open. Tarsal glands associated with the eyelid edges produce an oily secretion that lubricates the eye.
• Conjunctiva


  A delicate membrane that lines the eyelids and covers part of the outer surface of the eyeball, secreting mucus which helps lubricate the eyeball and keep it moist
• Conjunctivitis


  • Inflammation of the conjunctiva which results in reddened, irritated eyes
• Pinkeye


  • Infection caused by bacteria and is highly contagious
• Lacrimal apparatus


  Consists of the lacrimal gland and a number of ducts that drain lacrimal secretions into the nasal cavity. They continually release a dilute salt solution (tears) onto the anterior surface of the eyeball through several small ducts. Tears also contain mucus, antibodies, and lysozyme—an enzyme that destroys bacteria. They cleanse and protect the eye surface as they moisten and lubricate it.
• Extrinsic eye muscles


  Six muscles attached to the outer surface of each eye, producing gross eye movements and enabling the eyes to follow a moving object
• Extrinsic eye muscles


  • Lateral rectus
  • Medial rectus
  • Superior rectus
  • Inferior rectus
  • Inferior oblique
  • Superior oblique
• Eyeball


  A hollow sphere with a wall composed of three tunics, or layers, and its interior filled with fluids called humors that help to maintain its shape
• Fibrous layer


  • Outermost layer consisting of the protective sclera and the transparent cornea
• Sclera


  Thick white connective tissue seen anteriorly as the "white of the eye"
• Cornea


  The central anterior portion of the fibrous layer that is crystal clear, through which light enters the eye. It is well supplied with nerve endings, mostly pain fibers, and is the most exposed part of the eye, vulnerable to damage but with extraordinary ability to repair itself. It is the only tissue in the body that is transplanted from one person to another without the worry of rejection, as it has no blood vessels and is beyond the reach of the immune system.
• Vascular layer


  • Middle layer of the eyeball with three distinguishable regions: choroid, ciliary body, and iris
• Choroid


  The most posterior, blood-rich nutritive tunic that contains a dark pigment to prevent light from scattering inside the eye
• Ciliary body


  Attached to the lens by a suspensory ligament called the ciliary zonule
• Iris


  Has a rounded opening, the pupil, through which light passes. Circularly and radially arranged smooth muscle fibers form the iris, which acts like the diaphragm of a camera to regulate the amount of light entering the eye.
• Sensory layer


  The innermost layer of the eye, the delicate two-layered retina, which extends anteriorly only to the ciliary body
• Retina


  • The outer pigmented layer is composed of pigmented cells that absorb light and prevent it from scattering, and act as phagocytes to remove dead or damaged receptor cells and store vitamin A needed for vision. The transparent inner neural layer contains millions of receptor cells, the rods and cones, which are called photoreceptors because they respond to light.
• Rods


  Allow us to see in gray tones in dim light and provide our peripheral vision
• Cones


  Discriminatory receptors that allow us to see the details of our world in color under bright light conditions. There are three varieties of cones, and lack of all three cone types results in total color blindness, whereas lack of one cone type leads to partial color blindness.
• Lens


  A flexible biconvex crystal-like structure that focuses light entering the eye onto the retina. In youth, the lens is transparent and has the consistency of firm jelly, but as we age it becomes increasingly hard and cloudy, causing cataracts and vision to become hazy and distorted.
• Aqueous humor


  A clear watery fluid in the anterior segment of the eye, anterior to the lens, that helps maintain intraocular pressure and provides nutrients for the avascular lens and cornea.
• Vitreous humor


  A gel-like substance in the posterior segment of the eye, posterior to the lens, that helps prevent the eyeball from collapsing inward by reinforcing it internally.
• Drainage of aqueous humor is blocked


  Fluid backs up, pressure within the eye increases to dangerous levels, and the delicate retina and optic nerve are compressed, resulting in the condition of glaucoma which can lead to blindness unless detected early.
• Pathway of light through the eye

  1. Light passes from one substance to another, changing speed and bending (refracting) as it encounters the cornea, aqueous humor, lens, and vitreous humor

  2. The refractive power of the cornea and humors is constant, but the lens can change shape to focus light properly on the retina, a process called accommodation

  3. The image formed on the retina is a real image, reversed from left to right, upside down, and smaller than the object.
• Visual fields and visual pathways to the brain

  Axons carrying impulses from the retina leave the eye as the optic nerve, with fibers from the medial side of each eye crossing over at the optic chiasma. The optic tract fibers synapse with neurons in the thalamus, whose axons form the optic radiation running to the occipital lobe of the brain where visual interpretation occurs. Each side of the brain receives visual input from both eyes, providing binocular vision and depth perception.
• Hemianopia


  Loss of the same side of the visual field of both eyes, resulting from damage to the visual cortex on one side only, as occurs in some strokes.
• Eye reflexes


  1. The autonomic nervous system controls the internal eye muscles, including those of the ciliary body and iris, which alter lens curvature and pupil size

  2. The extrinsic eye muscles controlled by cranial nerves III, IV, and VI produce eye movements and convergence when viewing close objects

  3. The photopupillary reflex constricts the pupils in bright light, and the accommodation pupillary reflex constricts them when viewing close objects.
• Long periods of reading tire the eyes and often result in eyestrain, as it requires almost continuous work by both the internal and external eye muscles.
• External ear


  Composed of the auricle (or pinna) and the external acoustic meatus, a short, narrow chamber carved into the temporal bone of the skull, with ceruminous glands secreting earwax.
• Tympanic membrane (eardrum)


  Separates the external from the middle ear, vibrating when sound waves enter the auditory canal.
• Middle ear


  A small, air-filled, mucosa-lined cavity within the temporal bone, flanked laterally by the eardrum and medially by a bony wall with the oval and round windows.
• Pharyngotympanic (auditory) tube


  Runs obliquely downward to link the middle ear cavity with the throat, with continuous mucosae, normally flattened and closed but opening briefly during swallowing or yawning to equalize pressure on both sides of the eardrum.
• Unequal pressure on the two sides of the eardrum

  Causes the eardrum to bulge inward or outward, leading to hearing difficulty and sometimes ear aches.
• Auditory canal


  The canal that sound waves enter and eventually hit the tympanic (tympanum = drum) membrane, or eardrum, causing it to vibrate
• Tympanic (tympanum = drum) membrane


  The eardrum that separates the external from the middle ear
• Middle ear cavity


  • Small, air-filled, mucosa-lined cavity within the temporal bone
  • Flanked laterally by the eardrum and medially by a bony wall with two openings, the oval window and the inferior, membrane covered round window
• Pharyngotympanic (think throat-eardrum: pharynx-tympanic) tube


  Runs obliquely downward to link the middle ear cavity with the throat, and the mucosae lining the two regions are continuous
• Ossicles


  The three smallest bones in the body that transmit the vibratory motion of the eardrum to the fluids of the inner ear