Common Defects of Eye
Myopia or Near-SightednessPeople suffering from this defect hold the object very close to the eyes than a normal person would hold. This defect is caused by the lengthening of the eyeball from the front to the back or also may be due to the increased convexity of the lens of the eye. The result is that the image is formed in front of the retina. This defect can be corrected by using appropriate concave lenses.
Hypermetropia or Far-SightednessThis defect is caused by conditions just the opposite of the conditions that cause myopia. This can be corrected by wearing appropriate convex lenses. The eyeballs of the people suffering from this defect are unduly short and the image is formed far behind the retina.
AstigmatismIn this defect the lens and the cornea have different curvatures along different meridians of the eye. It can be corrected by using cylindrical glasses.
Presbiopia or Oldage SightThis is due to the loss of flexibility of the lens because of which the individual feels difficulty in focusing on nearer objects. This defect starts appearing from the age of about forty to forty-six years and is corrected by the use of convex lenses.
CataractIn this defect the lens becomes opaque due to several causes. The correction of this defect involves surgical removal of the lens and replacement with a convex lens.
Ear (Hearing and Balance)
The ear consists of three parts:
- External ear,
- Middle ear,
- Internal ear.
- External Ear
It consists of a skin covered cartilaginous organ called the pinna enclosing a canal called the external auditory meatus. The pinna collects sound waves and directs them to the tubular external auditory meatus. Some animals have the capacity to move the external ears in the direction of the sound. In the human such muscles are vestigial and the capacity to move the pinna is not there. The external auditory meatus bears ceruminus glands. They secrete a waxy substance which lubricates and protects the tympanum or ear drum. The external auditory meatus passes through the tubular portion of the tympanic bone.
- Middle Ear
It consists of an irregular tympanic cavity, filled with air. In between the tympanic cavity and the external auditory meatus lies a thin membrane called ear drum or tympanic membrane or tympanum. The tympanic cavity communicates with the pharynx by a tube called eustachian tube. It is normally kept closed. It maintains an equal pressure on either side of the ear drum and thus protects the tympanum from bursting on account of any sudden increase of external pressure on it due to some explosion or loud noise, etc. The middle ear communicates with the internal ear by two apertures in the outer wall of the periotic bone. These are fenestra ovalis or oval window and the fenestra rotunda or round window. Both these openings are covered by membranes. There are three small bones called ear ossicles in the tympanic cavity, namely malleus, incus and stapes. The malleus is a hammer shaped bone and it is attached at one end to the tympanum. The incus is long and anvil shaped. It is attached at one end to the malleus and at the other end to the stapes. The stapes is stirrup shaped. Its base is attached to the membrane in the fenestra evalis. The malleus is connected to the tympanic membrane by a muscle called tensor tympani. It adjusts the tension of the tympanic membrane. It tightens the tympanic membrane in case of gentle voice and loosens it in case of loud voice.
Sectional Diagram of the Human Ear
- Internal Ear
It consists of a membranous labyrinth. It fits into a bony labyrinth in the periotic bone which has the same shape as that of the membranous labyrinth. The space between membranous labyrinth and bony labyrinth is filled with a fluid called perilymph. The membranous labyrinth is also filled with fluid called endolymph. In the endolymph float several calcareous bodies called ear stones or otoliths. Internally the membranous labyrinth consists of vestibule, three semicircular canals, and the cochlear duct. The vestibule consists of an upper chamber called the utriculus and lower chamber called the sacculus. The utriculus and the sacculus are connected to each other by a small sacculo-utriculus canal. Three semicircular canals open into the utriculus. They lie at right angles to each other. The semicircular canals are concerned with the sense of position of the body (balancing) and not with hearing.
The Structure of Internal Ear
From the sacculus arises a long tubular part, the cochlea, which is coiled like a conch shell and is embedded in a bone of the skull. It carries a system of canals and spaces which are filled with lymph like fluids. There are very sensitive membranes running across it. Branches from the auditory nerve enter these membranes and this part is concerned with the sense of hearing.
Mechanism of Hearing
The pinna collects the sound waves which travel through the external auditory meatus and set vibrations in the tympanum (ear drum). These vibrations are carried by the ear ossicles to the perilymph through the fenestra ovalis. The sound vibrations finally reach the cochlear portion of the internal ear through the endolymph. The fluid in the cochlea is set in motion and the auditory cells are stimulated. The movement of the fluid depends on the frequency of the sound waves. The stimulus is carried to the brain through the auditory nerve. The auditory cells in different parts of the membrane of the cochlea are sensitive to different pitches of sound. The cells at the base of the cochlea are stimulated by sound of high frequency while those in the upper part are stimulated by sounds of low frequency.
The utriculus and the three semicircular canals are concerned with maintaining the equilibrium of the body.