January 6 Lecture - January 6, 2009 Chapter 17: Special...

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January 6, 2009 Chapter 17: Special Senses The human body has five special senses: - Equilibrium - Gustation (Taste) - Hearing (Auditory) - Smell (Olfaction) - Vision Vision - Apparatus consists of o a number of refractive media within the eyeball o The retina o Optic nerves o Visual centres located within cerebral hemisphere (the occipital lobe) - Muscles are also essential for normal eye functioning o Some of these are intrinsic (found inside the eyeball) which are used to help focus the image o Extrinsic muscles attach the outer the layers of the eyeball which are involved in moving or rotating the eye - Accessory structures that are not involved in producing images but still associated with the eye (usually involved in protection of the eye) o Eyelids o Lacrimal apparatus Accessory Structures - Eyeball rests within orbital cavity formed by bony structure that is part of the skull o Bony portion is lined by a bulbar fascia , which extends around the surface of the eye - Between the bulbar fascia and the bony portion of the orbit is a substantial amount of fat known as the bulbar fat pad which acts as a cushion, padding the eye if the eye is suddenly jarred - Frontal portion of the eye is partially covered by the palpebra superior (upper eyelid) and palpebra inferior (lower eyelid) o Both act as a curtain in front of the eye o Work in conjunction with the eyelashes o Protect and prevent entrance of foreign materials from entering the eye, also regulate amount of eye that is exposed to light - Corners of the eye (where the upper and lower lids join), known as a canthus o Medial canthus is the inner corner of the eye Can also be referred to as medial commissures o Lateral canthus is the outer corner of the eye Can also be referred to as lateral commisures - Margin of eye lid contains eye lashes
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o At the base of each eye lid is a small sebaceous gland, which function to produce an oily substance known as sebum, which is released out onto the free margin of the eyelid It is estimated that sebaceous glands produce about 1 mL of sebum per day Sebum waterproofs the surface of the eye and helps prevent the tissues of the eyeball from dehydrating Sebum also reduces the possibility of microbes entering the eye Sebum also facilitates the distribution of lacrimal fluid over the eye Oily substance mixes with the lacrimal fluid and allows it to be swept over the front surface of the eye If the sebaceous gland becomes infected, the lower margin of the eye may become reddish in colour – known as a sty (an inflammation of small sebaceous glands) - Upper and lower eyelid are lined by a mucous membrane known as the conjunctiva o Usually not visible o Can be seen if blood vessels become dilated in the eye
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January 6 Lecture - January 6, 2009 Chapter 17: Special...

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