Structure of Eye
Our eye contains eye lids,
eye lashes, eyebrows and lachrymal glands. A thin layer, called conjunctiva
covers the front portion of the eye. The eye ball is located in the eye socket.
Only 1/6 portion of the eye ball is visible to us.
Eye has three main layers. They are sclerotic layer or sclera, choroid layer
and retina. The outer most thick, tough, fibrous, non-elastic and white
coloured layer is sclera. The sclera bulges and forms cornea. The end of sclera
connects to the optic nerve. The second layer is choroid layer. This layer is
black in color and contains a lot of blood vessels. It encloses the eye except
the part pupil. The part formed by the choroid layer around the pupil is iris.
Radial and circular muscles are present in the iris. Biconvex Lens is present
immediately behind the pupil is attached to the ciliary muscles and suspensory
ligaments.
The lens divides the inner eye ball as aqueous chamber and Vitreous chamber.
Aqueous chamber is filled with water like fluid whereas vitreous chamber is
filled with jelly like fluid. J
Retina contains the cells,
called rods and-cones. The area of no vision, called blind spot and the area of
the best vision, called yellow spot are present in the retina The yellow spot
is also called Macula or Fovea.
Cells and tissues in the Eye
Making the fine distinctions
necessary for color vision is the job of the nearly seven million cones containing
the pigment iodopsin that come into play in brighter light. Each cone is
specialized to detect the light waves we sense either as blue, red, or yellow
and the array of colors formed by their combinations. Thus the yellow field,
the bright red morning sun, the blue sky and all other colors in nature are
sensed.
The cones concentrate most in
the very center of the retina in a small region called the fovea, which gives
us our sharpest Vision. With movements of our eyeballs, we use the fovea to
scan whatever interests us visually, the features of a face or, perhaps, a
flower.
There are other types of
cells in the retina that do not respond directly to light. These handle the job
of collecting impulses from many photoreceptors (rods and cones) and shuttling
them on to the nerve cells. Presence of some other receptor cells sensitive to
edges and boundaries of objects and those that respond to light and shadow and
motion in the retina have also been reported recently.:
Bundled together, the nerve
cells make up the optic nerve, which transport visual information from the eye
to the brain.
Diseases and defects of the
eye:
The main diseases and defects
of the eye are - Night
blindness, Xeropthalmia, myopia
(near sightedness), Hypermetropia (far
sightedness), glaucoma, cataract and colour
blindness. Some persons may have eye
defects by birth due to various reasons.
Structure of Ear
Extern ear:
It is the visible part of the
ear on either side of our head. It is a flap like structure, called the Pinna.
It leads to the ear canal. The pinna is crumpled and made up of cartilage.
Middle ear:
Middle ear plays an important
role in amplifying the vibrations received on the tympanum membrane. The chain
of three bones, Malleus, Incus and Stapes helps to do the same. Oval window is
a membrane, covered ending of the middle ear which opens into the inner ear through
round window.
Internal ear or Inner ear:
Internal ear consist of bony
labyrinth enclosing the membranous labyrinth. The membranous labyrinth consists
of Vestibule, three semicircular canals and cochlea. The anterior part of the
vestibule is Sacculus and the posterior part is Utriculus. Nerve fibers from
them form vestibular nerve.
The semicircular canals are
connected to the vestibule and filled with endolymph. Vestibule and semilunar
circles together form vestibular apparatus. It maintains the equilibrium of the
body, pertaining to the posture and balance of the body.
Cochlea is a spiral shaped
structure. It has three parallel tubes called scala vestibuli, scala media and
scala tympani.
The first two are separated
by the vestibular membrane. The second and third are separated by the basilar
membrane. Scala vestibuli and scala tympani are filled with Perilymph. Scala
media is filled with endolymph. It contains organ of carti and tiny cells
called primary sensory cells.
Cochlear nerve fibres form
cochlear nerve. The vestibu1a and cochlear nerves join together and form
auditory nerve
The Hearing/Auditory Sensation
External ear collects the
sound waves. They enter into the auditorymeatus. Then they strike the tympanum.
The vibrations from the tympanum reach the malleus, incus and stapes. They
magnify the intensity of the sound vibrations. The stapes transmits the
vibrations to the membrane of oval window. Then they transmit to the cochlea.
The bacillary membrane is moved then the vibrations reach to the organ of
carti. The impulses are sent to the brain through the auditory nerve. The
hearing can be done according to the responses given by the brain.
Functions of the ear:
To collect and transform
vibrations produced by sound to nerve impulses to be carried to the brain for
processing.
To maintain balance or
equilibrium
Structure of Nose
Our external nose has two nostrils. They lead to
the nasal cavity. Nasal septum divides the nasal cavity into two halves. The
nasal cavity is lined with mucous membrane and small hairs. Olfactory receptors
are present in the mucous membrane.
Smell and our Nose
Smell serves a protective
function by Sensing the odour of possibly dangerous food or, for
some animals, the scent of a predator. We humans seem to use the sense of
smell primarily in conjunction with taste to locate and identify foods, avoid
spoiled foods etc. Human beings use the sense of smell in much limited manner
as compared to other animals.
Tongue
Structure of the tongue
Our tongue is made up of
voluntary muscles. It contains about 10 thousand taste buds. The taste buds are
located in the walls of the papillae.
The taste receptor cells,
located in the Taste buds on the top and side of the tongue, sample flavors
from food and drink as they pass by on the way to the stomach. These taste
receptors cluster in small mucous membrane projections called papillae. Each is
especially sensitive to molecules of a particular shape.
Our skin is the sense organ
for touch. It contains cutaneous receptors for touch. The skin consists of two
main layers, called Epidermis and dermis.
Epidermis is the layer for
protection. It has sweat pores and small hairs. It contains three layers. They
are outer stratum corneum or cornified layer containing dead cells, middle
granular layer containing living cells and inner malpighian layer containing
the cells dividing constantly. Dermis lies below the epidermis. It is made up
of elastic connective tissue. it contains sweat glands, sebaceous glands, hair
follicles, blood vessels and fats.
Skin and touch
Skin is the outer most
covering of our Body. It regulates the body temperature and eliminates certain
waste material through sweat. It is the sense organ of touch. The sense of
touch is done by the cutaneous receptors. It is the largest organ of all. It provides
the first level of protection to the Body.
The color of the skin is due
to the presence of the pigment. called “Melanin”. This pigment gets
stimulation, when exposed to sun light. The skin becomes dark to protect other
layers of the skin from harmful effects of light. Skin is sensitive to touch,
temperature and pressure. It contains the separate receptors such as tactile
receptors for touch, pacinian corpuscles for pressure temperature etc .
‘Some of the diseases,
affecting the skin are.
• Viral diseases such as
measles, chicken pox etc.
• Bacterial diseases such as
leprosy
• Leucoderma, the disease due
to the deficiency of melanin.
• Pellagra the disease due to
the deficiency of vitamins.
• Fungal diseases such as
ring worm.