Grade 12 Zoology Note

Sense organ

                                                 Fig.Structure of human eye

Eye structure:

The human eye is a complex optic instrument. Its main goal is to “transfer” the correct image to the optic nerve.


It is a transparent coat covering the front part of the eye. It has no blood vessels, but its refraction is great. It is part of the eye optic. Cornea borders sclera which is a non-transparent eye coat.

Anterior chamber:

 It is a space between cornea and iris. It is filled with intra-ocular fluid.

Iris looks like a circle with an opening in the middle (pupil). Iris consist of muscles that change pupil size by constricting and relaxing. IT is a part of the eye choroid. Iris is responsible for the colour of the eyes (if it is blue this means it contains few pigment cell, if brown – a lot). Its function is same as of aperture in a camera – to adjust light flow.

Pupil is an aperture in iris. Its size usually depends on the illumination level. The more light the smaller the pupil.

Crystalline lens:

 Itis the eye “natural lens'. It is transparent, elastic – can change its shape, focusing in almost instantly, therefore one can see well both near and far. It is located in a capsule and is withheld by ciliary zonule. The crystalline lens like cornea is a part of the eye optic.

Vitreous body:

It is a gel-like transparent substance located in the posterior part of the eye. The vitreous body supports the sphere of the eye ball and is part of the intraocular metabolism. It is a part of the optic system.


It consists of photoreceptors (light sencing) and nerve (ganglion) cells. There are two types of receptor (transducer) cells in retina: cones and rods. These cells producing rhodopsin enzyme transform light energy (photons) into electric energy of neural tissue, i.e. photochemical reaction takes place.

Rods have high light sensitivity and allow seeing in poor light, they are also responsible for periphery vision. Cones adversely need plenty of light for functioning but allow to distinguish small details (responsible for direct vision) and ensure colour appreciation. Most cones are located in macula which is responsible for the sharpest vision. Retina adjoins choroid but not too snug in some areas. It is here that it may detach under various retina diseases.


It is the non-transparent outer coat of the eye bulb and in the frontal part of the eye it verges into the transparent cornea. 6 eye moving muscles are attached to it. It contains a few nerve terminals and vessels.


It inlays the back part of sclera, it adjoins retina and is closely linked to it. Choroid is responsible for blood supply of intraocular structures. And with retina disorders it is usually involved in the pathology process. Choroid has no nerve terminals therefore when there is a trouble there, there is no pain which usually alarms about a problem.

Optic nerve: 

 It transfers signals from nerve terminals to the brain.

The orbit is the bony cavity that contains the eyeball, muscles, nerves, and blood vessels, as well as the structures that produce and drain tears. Each orbit is a pear-shaped structure that is formed by several bones.

The outer covering of the eyeball consists of a relatively tough, white layer called the sclera (or white of the eye). Near the front of the eye, in the area protected by the eyelids, the sclera is covered by a thin, transparent membrane (conjunctiva), which runs to the edge of the cornea. The conjunctiva also covers the moist back surface of the eyelids and eyeballs.

Light enters the eye through the cornea, the clear, curved layer in front of the iris and pupil. The cornea serves as a protective covering for the front of the eye and also helps focus light on the retina at the back of the eye. After passing through the cornea, light travels through the pupil (the black dot in the middle of the eye). The iris—the circular, colored area of the eye that surrounds the pupil—controls the amount of light that enters the eye. The pupil dilates (enlarges) and constricts (shrinks) like the aperture of a camera lens as the amount of light in the immediate surroundings changes. The iris allows more light into the eye when the environment is dark and allows less light into the eye when the environment is bright. The size of the pupil is controlled by the action of the pupillary sphincter muscle and dilator muscle.

Behind the iris sits the lens. By changing its shape, the lens focuses light onto the retina. Through the action of small muscles (called the ciliary muscles), the lens becomes thicker to focus on nearby objects and thinner to focus on distant objects.

The retina contains the cells that sense light (photoreceptors) and the blood vessels that nourish them. The most sensitive part of the retina is a small area called the macula, which has millions of tightly packed photoreceptors (the type called cones). The high density of cones in the macula makes the visual image detailed, just as a high-resolution digital camera has more megapixels. Each photoreceptor is linked to a nerve fiber. The nerve fibers from the photoreceptors are bundled together to form the optic nerve. The optic disk, the first part of the optic nerve, is at the back of the eye. The photoreceptors in the retina convert the image into electrical signals, which are carried to the brain by the optic nerve.

There are two main types of photoreceptors: cones and rods. Cones are responsible for sharp, detailed central vision and color vision and are clustered mainly in the macula. The rods are responsible for night and peripheral (side) vision. Rods are more numerous than cones and much more sensitive to light, but they do not register color or contribute to detailed central vision as the cones do. Rods are grouped mainly in the peripheral areas of the retina.

The eyeball is divided into two sections, each of which is filled with fluid. The front section (anterior segment) extends from the inside of the cornea to the front surface of the lens. It is filled with a fluid called the aqueous humor, which nourishes the internal structures. The back section (posterior segment) extends from the back surface of the lens to the retina. It contains a jellylike fluid called the vitreous humor. The pressure generated by these fluids fills out the eyeball and helps maintain its shape.

The anterior segment is divided into two chambers. The front (anterior) chamber extends from the cornea to the iris. The back (posterior) chamber extends from the iris to the lens. Normally, the aqueous humor is produced in the posterior chamber, flows slowly through the pupil into the anterior chamber, and then drains out of the eyeball through outflow channels located where the iris meets the cornea.

Basic eye function:

1.optic system projecting an image;

2.system that perceives and “encodes” the received information for the brain; supporting “servicing” system.



Human Ear:

The human ear is divided into three sections, the outer, middle and inner ear and plays an important role in hearing. The outer ear consists of the pinna (auricle) that leads into the external auditory canal. It collects sound waves from a wide area and funnels the sound into the external ear passage. On the inside surface of the outer ear is the tympanic membrane (eardrum). It is stretched across the end of the auditory canal separating the outer ear from the middle ear.

The middle ear consists of small bones called ossicles. They are the malleus (hammer), the incus (anvil) and the stapes (stirrup). They transfer sound waves to the inner ear. Located covering an opening into the inner ear is called the oval window. Below is another membrane called the round window that stretches across the opening and adjoins the cochlea in the inner ear.

The inner ear comprises a coiled structure called the cochlea. The snail-like spiral coiled tube contains the receptors for sound and the vestibular apparatus that is associated with a sense of balance. The cochlear duct contains the organ of Corti, which contains auditory receptor cells. The auditory nerve transmits sound vibrations to the brain. 





                            Fig. Structure of human ear

Anatomically, the ear can be looked at in three parts:

1.Outer ear:

Pinna and auditory canal down to the level of the tympanic membrane

2.Middle ear:

It contains the malleus, incus and stapes bones - known as the ossicles

3.Inner ear:

 It contains the membranous and bony labyrinths, and the cochlea

Outer Ear:

The pinna consists of the auricular cartilage, and skin which allows for flexibility and elasticity. The auricular cartilage is flared distally into a flattened cone shape, covered by skin on both sides - more tightly on the medial or concave side than the convex side. The hair covering on the convex side is usually similar to the rest of the body but the hair covering on the concave or medial aspect is variable. There are also irregularities on the concave surface with ridges and prominences, the medial and lateral crus of the helix on the medial aspect of the opening of the external ear canal, opposite the rectangular tragus on the lateral aspect. Proximally, the auricular cartilage is rolled into a funnel shape, known as the concha. Foreign bodies can become lodged in the external auditory meatus, but glands are present that produce wax which can trap these.Muscles around the base of the ear that are attached to the skull allow movement of the pinna, so the ear can be directed to the source of sound:


Middle Ear:

1.The middle ear consists of the tympanic cavity, the auditory ossicles and the eustachian tube. The boundary between the middle and inner ear is the oval window.

2.The auditory ossicles are attached to the wall of the tympanic cavity by many ligaments and mucosal folds.

3.The tympanic cavity is located within the petrous temporal bone, and can be divided into dorsal, middle and ventral parts:

Dorsal: or epitympanum, conatining 2 of the auditory ossicles – the malleus and incus

Middle: or mesotympanum bounded by the tympanic membrane laterally and containing the third auditory ossicle, stapes, attached to the oval window. It opens rostrally into the nasopharynx via the eustachian tube

Ventral: or hypotympanum, or fundic cavity, which is the largest compartment. It is housed by the tympanic bulla which is a thin-walled, bulbous expansion of the temporal bone.

Inner Ear:

1.The inner ear contains the membranous labyrinth, which is surrounded by the bony labyrinth.

2.The membranous labyrinth is an interconnected group of fluid-filled membranous sacs. The fluid is endolymph.

3.It is the movement of the endolymph that stimulates the sensory cells within the membranous wall.


The membranous labyrinth consists of:

Vestibular labyrinth:

It contains the receptor organ involved with balance, containing the saccule, utricle and the semicircular ducts. The saccule and utricle contain sensory maculae within their walls, and there's a sensory crista within the ampullae of the semicircular ducts. The maculae and ampullae sense and conduct impulses concerned with balance via the vestibular nerve. The three semicircular ducts arise from the utricle, and the cochlear duct arises from the saccule.

Cochlear labyrinth:

It contains the organ involved with hearing. It consists of the organ of Corti, within the cochlear duct. The cochlear duct is fluid-filled, the fluid being endolymph. The organ of Corti contains the receptor cells for hearing.

Ductus reuniens: this is the duct through which the above two labyrinths communicate

The bony labyrinth consists of:


A chamber in the centre of the bony labyrinth, which communicates with both the cochlea and the semicircular canals. The oval and the round windows are both located in the lateral wall of the vestibule.

Semicircular canals:

It contain the semicircular ducts, which have arisen from the utricle of the vestibular labyrinth. There are three semicircular canals, corresponding to the three dimensions in which you can move, so they are almost at right angles with each other. Each duct has two crura (leg-like parts). One crus of each duct has an ampulla, which is an expansion of the duct. Movement of endolymph stimulates receptor cells within the ampullae.


It forms a spiral around a central hollow core of bone, called the modiolus,which contains the cochlear nerve. The spiral lamina projects into the spiral canal, partially bisecting the lumen into two parts, which are called the scala tympani and the scala vestibuli. The scala media (the cochlear duct) is in between these two parts.

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