BY Prof.Dr /Olfat Anwar

BY

Prof.Dr

Olfat Anwar

Special senses

There are five senses: sight (vision), hearing, smell, taste and touch

Each one of these 5 senses consists of organs with specialized cells that have receptors for specific stimuli. These cells have links to the brain.

Sight –I

The Eye, it lies in the orbital cavity of the skull where it is embedded in the orbital fat, but is separated from it by the facial sheath of the eye ball.

The eye ball is formed of three layers :

1. The Fibrous coat: It is the outer layer of the eye ball which is formed by cornea and sclera.

a- Cornea:

It is transparent and constitutes the anterior 1/6 of the fibrous coat.

It is responsible for the refraction of light entering the eye ball.

b- Sclera:

It constitutes the posterior 5/6 of the fibrous coat.

It is formed of dense white fibrous tissue.

2. The vascular and pigmented coat: Choroid:

It is the largest part of the middle coat, lying between the sclera and retina.

It is highly pigmented and rich in blood vessels so it nourishes the retina.

Ciliary body:

It is continuous with the periphery of the iris.

It lies on the inner aspect of the sclera and is continuous with the anterior margin of the choroid.

Iris:

It is a circular diaphragm behind the cornea.

It presents a central hole called the pupil.

It contains the constrictor and dilator pupillae muscles .The colour of the iris varies in different individuals due to presence of pigments.

The space between the iris and cornea is called the anterior chamber.

The space between the iris, ciliary body and lens is called posterior chamber.

The two chambers communicate through the pupil.

3. The Nervous coat : It is the inner layer of the eye ball, mainly formed by the retina (sensitive to light).

The axons of the ganglion cells of the retina form the optic nerve fibres.

These fibres leave the eyeball at the optic disc to form the optic nerve which pierces the sclera and the choroid about 3mm to the medial side of the posterior pole of the eye ball.

Three structures : he Refracting media:T Lens: -1

It is biconvex, transparent, and elastic and lies between the iris and vitreous body. It focuses light on the retina.

The suspensory ligament of the lens fixes the lens in position and connects it to the ciliary muscle.

Therefore the curvature of the lens is affected by the contraction of the ciliary muscle and the degree of tension of the suspensory ligament.

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Aqueous humour -2 : It is the fluid filling the posterior and anterior chambers of the eye.

It is formed by diffusion from the capillaries of the ciliary processes into the posterior chamber.

The fluid then circulates through the pupil into the anterior chamber.

It regulates and maintains the intraocular tension; also it carries nutrition for the non vascular cornea and lens.

:body Vitrous -3 It is a transparent, colourless, gel-like substance which fills the concavity of the retina.

It occupies about 4/5 of the eyeball and lies behind the lens.

It holds the lens and the retina in their places

Hearing -II The ear is the organ of hearing and equilibrium. It is

formed of external, middle and internal ears

:. External ear1

It consists of auricle and external auditory meatus that ends with the tympanic membrane (ear drum).

The Auricle: It is a thin plate of elastic cartilage covered by skin.

It collects the sound waves.

The External auditory meatus: Its lateral third is cartilaginous, while the medial two-thirds are bony.

It transmits the sound waves to the tympanic membrane.

Tympanic membrane: It's an oval semitransparent membrane which separates the external ear from the middle ear.

The membrane is obliquely situated in the bottom of the external auditory meatus

. Middle ear:2

It is a narrow space known as tympanic cavity lines with mucous membrane and containing air reaching it from the pharynx by the Eustachian tube.

It contains the ossicles (malleus, incus and stapes); auditory muscles and tympanic plexus.

. Inner ear:3 It is formed by the bony labyrinth

containing the membranous labyrinth.

- Bony labyrinth: is formed of cochlea, vestibule and 3 semicircular canals.

- Membranous labyrinth: is formed of 4 parts, cochlear duct, saccule, utricle and semicircular canals It is filled with fluid called endolymph and bathed in the perilymph of the bony labyrinth. The cochlea contains sensory organs for hearing while semicircular canals, utricle and saccule contain sensory organs for equilibrium.

Taste –III The receptors for taste, called taste buds, are

situated chiefly in the tongue, but they are also located in the roof of the mouth and near the pharynx.

They are able to detect four basic tastes: salty, sweet, bitter, and sour.

The taste buds close to the tip of the tongue are sensitive to sweet tastes,

In the back of the tongue are sensitive to bitter tastes.

The taste buds on top and on the sides of the tongue are sensitive to salty and sour tastes.

At the base of each taste bud there is a nerve that sends the sensations to the brain.

The sense of taste functions are in coordination with the sense of smell.

Smell –IV The nose is the organ responsible for the sense of smell.

The olfactory bipolar cells are placed in the mucosa of the roof of the nose and upper part of the nasal cavity.

The smells themselves consist of vapours of various substances.

The smell receptors interact with the molecules of these vapours and transmit the sensations to the brain.

Touch –V The sense of touch is distributed

throughout the body.

Nerve endings in the skin and other parts of the body transmit sensations to the brain.

Four kinds of touch sensations can be identified; cold, heat, contact, and pain.

Hairs on the skin magnify the sensitivity and act as an early warning system for the body.

The fingertips and the sexual organs have the greatest concentration of nerve endings.

Skin

Thank You