Human being contacts with his environment and controls his situation and actions through his five senses consisting of the senses of sight,hearing,taste,smell and touch.Each of these senses perceives through specific receptors the information from outside and following changing such information to neural messages,sends them to the brain.
Eye,the main organ of sight: The eyeball is located in a lipid space inside the orbit held in place by eye muscles.The suscles control its movements. The appendices of the eye include the eyelids , lacrimal glands,and muscles. The orbit is a sphere full of an aqueous liquid enclosed in a firm external cover called sclera.The anterior part of sclera, namely , cornea , is translucent and makes possible the entry of light into eyes . At the back of cornea,is situated the iris with a pore in the middle called pupil.The light, emitted from objects,enters the eye and passes through the pupil and lens and reaches the retina located at the back of the orbit. Light coming from the objects,after passing through cornea and following refraction by the lens,produces a smaller inverted picture on the retina.This picture is received by millions of neurons located on retina membrane and after being changed to neural messages is sent to the brain. Ear,the main organ of hearing:
Is an organ that receives sound vibrations and also is effective in maintaining ballance.Ear is composed of three main parts: 1.external ear including auricle ecoustic meatus,and tympanic membrane.2.middle ear composed of an air cavity containing three small bones called malleus,incus and stapes. These three bones are motioned consecutively and transfer the sound waves transmitted via ear drum to the malleus from oval window to the internal ear.3.internal ear is a cavity full of liquid located at the depth of the cranium.Part of it contains the cochlea which receives vibrations from the middle ear.These vibrations are transmitted to the ear liquid and perceived by sense cilia.These cilia are arranged in v-shaped bundles and once being bent and straightened by the liquid,transmit the neural messages to the brain.
Semicircular canals and scala vestibuli located in the internal are control the ballance of the body.These canals contain a liquid which vibrates once one moves. Such vibrations are perceived by sensory cilia and transmitted to the brain to issue necessary commands to ballance the body.Its worth saying that the eustachian tube connects the middle ear to a pore near the pharynx to equilibrate air pressure inside the middle ear cavity with that of the outside air.Otherwise hearing would be disturbed and will lead to earache.Usually during the process of yawning and swallowing, the eustachian tube open to help the ballancing process.
Tongue,the main organ of taste: Tongue is a muco-muscular organ that in addition to taste perception,participates in functions of chewing,swallowing and sound producing . On the mucous surface of the tongue,there are many small projections called papillae to which taste nerve endings terminates.Papillae are distributed as filiform,and conical elements on the whole surface of the tongue.Generally the sweet taste is sensed by tip of the tongue, sourness by the sides,bitterness with the end,saltiness with anterior and lateral parts of the tongue.
Nose,the main organ of smell: The upper part of the nasal cavity is the main olfactory section.In this part there are about 20 million smell receptor neurons each terminating to some cilia.During respiration,smell transmitting molecules enter the nasal cavity and are received by cilia which produce neural messages.Such messages are transferred via olfactory bulb and finally through olfactory nerve to the brain and in this way we are able to recognize more than 10000 differents types of smell.
Skin,the main organ of touch: Body skin weighing about 4 kg and a surface area of about 2 square meter is composed of 3 main sections: 1. the superficial part or epidermis ,supporting underneath layers. On the surface of this layer,there are flat and dead cells which are usually separated from it,through by washing and rubbing and continuonsly replaced by underneath layer. 2. the more internal layer called dermis which is full of sense endings which can reveal sensations of touch,pressure,pain,heat and cold ) at each square centimeter of skin there can be found up to 240 nerve endings called receptors (.This layer also,contains fat and sweat glands and a profusion of blood vessels. 3. hypodermis which attaches skin to other parts of the body and contains half of the body fat and protects the body against cold and mechanical blows.