Vision

"Accessory structures of the eyes include the eyebrows, eyelids, eyelashes, the lacrimal apparatus (which produces and drains tears) and extrinsic eye muscles (which move the eyes).

ACCESSORY – extra

EYEBROWS (pl) – the lines of short hairs above each eye

EYELIDS (pl)– the pieces of skin that close over each eye

EYELASHES (pl) – short hairs that grow along the edges of eyes

LACRIMAL - relating to tears

TO DRAIN – to remove liquid

EXTRINSIC – coming from outside

The eyebrows and eyelids help protect the eyeballs from foreign objects, perspiration and direct rays of the sun. The upper and lower eyelids shade the eyes during sleep, protect the eyes from excessive light and spread lubricating secretions over the eyeballs by blinking.

Six extrinsic muscles cooperate to move each eyeball right, left, up and down and diagonally. Neurons in the brain stem and cerebellum coordinate and synchronize the movements of the eyes.

AN EYEBALL – the whole eye (including the part that cannot usually be seen)

PERSPIRATION - sweat

TO SHADE – to prevent direct light from shining on something

BLINKING - the act of opening and closing eyes rapidly

THE BRAIN STEM – the central trunk of the brain connected to the spinal cord

CEREBELLUM – a large region at the back of the brain responsible for a motor control

The lacrimal apparatus is a group of glands, ducts, canals and sacs that produce and drain tears. Each gland is about the size and shape of an almond. They secrete tears through the lacrimal ducts toward the nose to allow them to drain into the nasal cavity. Sometimes the glands produce excessive tears that may spill over the eyelids and fill the nasal cavity with fluid. This is how crying produces a running nose.

Tears are a watery solution containing salts, mucus and a bacteria-killing enzyme called lysoenzyme.

A GLAND – an organ that produces secretions

A DUCT – a pipe that carries liquid

A SAC – a bag

AN ALMOND – an oval nut

TO SECRETE - to produce liquid

A RUNNING NOSE - a nose with too much watery substance

MUCUS – a thick, protective liquid

The adult eyeball is divided into three layers:

1) fibrous tunic (sclera and cornea)

2) vascular tunic (choroid, ciliary body and iris)

3) retina

A TUNIC - a covering layer

FIBROUS – made of fibres

VASCULAR – made of vessels

1) The fibrous tunic is the outer coat of the eyeball and it consists of an anterior cornea and a posterior sclera.

The cornea covers the coloured iris and because it is curved it helps focus light rays onto the retina.

The sclera is a coat of dense connective tissue that covers the entire eyeball except the cornea.

ANTERIOR – towards the front

POSTERIOR – towards the back

CORNEA – the transparent outer covering of the eye

SCLERA – the white outer covering of the eye

2) The vascular tunic is the middle layer of the eyeball and it is composed of the choroid, ciliary body and iris.

The choroid is a thin membrane that lines the sclera and contains many blood vessels that help nourish the retina. The choroid also contains melanocytes that produce the pigment melanin (hence the dark brown colour of the layer). Melanin absorbs stray light rays and prevents reflection and scattering of light within the eyeball.

The choroid becomes the ciliary body and the cilliary muscle alters the shape of the lens for viewing objects up close or at a distance. The lens focus light rays onto the retina.

The iris (coloured circle) contains the smooth muscle fibers that regulate the amount of the light passing through the lens. It has got a hole in the centre called pupil through which light enters the eyeball.

TO LINE – to cover the inside surface of sth

TO SCATTER – to (cause to) move far apart in different directions

PUPIL = the circular black area

3) Retina is the third inner coat of the eyeball that is the beginning of the visual pathway.

It has two layers: the neural layer and the pigmented layer.

Image formation on the retina involves refraction of light rays by the cornea and the lens which focus an inverted image on the central fovea of the retina.

Improper refraction may result from myopia (nearsightedness), hyperopia (farsightedness) or astigmatism (irregular curvature of the cornea or lens). The first step in vision is the absorption of the light rays by the photopigments, then nerve impulses arise and move along the optic nerve to the brain."