Basic Anatomy & Physiology of the Eye
In this chapter the basic anatomical structure of the eye with relevant clinical aspects are discussed.
The eye serves like a camera, capturing the scenes before it. The eyes are placed safe in a socket in the skull and are protected by the eyelids. Our eyes are more efficient than a film in a camera, capturing the scenes and sending it to the brain.
The eye is spherical in shape. There are 6 extraocular muscles present outside of the eye which help in the movement of eyes in various directions. The extraocular muscles are supplied by nerves from the brain. The eyeball is connected to the brain through a nerve called optic nerve (Fig. 2.1).
Basic Anatomy of Eye
Eyelids
There are two eyelids, the upper eyelid, and lower eyelid which is made of skin and soft tissues. The tarsal plate helps in giving the structure to the eyelid. The eyelids protect the eyes from external environmental pollution.
There are muscles connected to the eye which are called extra ocular muscles, and are present outside the eye. They are the superior rectus, medial rectus, the lateral rectus, the inferior rectus, the superior oblique and inferior oblique.
Conjunctiva
This is a thin white membrane with blood vessels covering the eye ball.
Cornea
The cornea is a transparent watch glass like structure, spherical in shape present in front of the eye. It is like a window to the eye. Cornea measures about 11.5mm in horizontal length. There are five layers in cornea.
Anterior chamber
The space between the cornea and iris is called the anterior chamber filled with aqueous humour, which is secreted by a structure called ciliary body. The depth of anterior chamber is 2.5mm. When the aqueous pressure in the eye increases the condition is called glaucoma.
Iris & pupil
Behind the cornea there is a brown circular diaphragm like structure called the iris. It consists of 2 types of muscles, the circular muscles and radial muscles. The central opening of the iris is called the pupil. Normal size of pupil 2-3mm. Depending on the intensity of light, the size of the pupil decreases or increases in size.
Lens
Behind the iris is situated, a transparent, structure called the lens. The nutrition to the lens is supplied by the aqueous humour. The shape of the lens is altered to see object at near and distance. Light rays passes through the lens and falls on the retina. When the lens looses it transparency it becomes an opaque structure, through which light cannot pass. This condition is called as cataract.
Vitreous gel
There is a colorless, transparent gel like substance behind the lens. The vitreous gel is like the white of an egg, giving shape to the eye.
The coats of the eye
There are 3 coats of the eye.
Sclera
The outer coat is the sclera, which is covered by conjunctiva, a thin white membranous tissue.
The extraocular muscles are inserted to the sclera.
Choroid
The choroid layer is middle coat situated between sclera and retina. The choroids tissue is dark brown in colour due to vascularity. It supplies nutrition to the retina, vitreous and other sensitive structures of the eye. It also prevents the scattering of light.
Retina
The retina is the inner most structure of the eye. The rays coming from objects fall on the retina. The retina is made of ten layers of neuronal tissues. Retina is basically transparent, cellophane - like tissue. The optic disc is the head of the optic nerve entering the eye. Arteries and veins course through the retina. Macula is the most visually scientific part of retina. It is pink in colour with a central depression called the optic disc cup. In diseases like glaucoma where the pressure in the eye is raised this cup is enlarged.
Function of eye
The ability to see is an amazing process, made possible by the parts of the eye working in conjunction with one another and with the brain. Light enters the eye through the transparent cornea, gets refracted and then passes through the pupil to reach the lens.
The pupil acts like the shutter of a camera. In bright light, it becomes smaller, thus restricting the amount of light entering the eye. In less bright illumination and in darkness, the pupil becomes larger, thus allowing adequate light to enter the eye.
After light enters the pupil, it passes through the crystalline lens, where it undergoes a second phase of refraction. The refracted light then passes through the vitreous humor to reach the retina. Here the light impulses are converted to electrical impulses and then fed into the optic nerve, which carries them to the visual centre in the brain, through a complex bundle of interconnected nerve channels. The brain processes these impulses to create the visual image we perceive.
Colour vision, the power to discriminate between different colours, is the result of light from objects being processed in certain special receptors in the retina and the brain. Any colour can be broken down into the three primary colours-red, green and blue and perceived as a combination of the three. Retina contains 3 corresponding types of cells (cones) which respond to these three colours. A defect in colour vision is called colour blindness; this can be partial or total.
When we look at a particular object, a lot of other objects surrounding it are also perceived. Hence our vision is not a small circle that we focus on but a field in which we see multiple objects. The visual field has two parts: the central visual field which is the area immediate to the object we are looking at and a peripheral visual field which includes the rest of the area surrounding the central field.
The eyeballs are constantly in motion-up and down, and either side. This is facilitated by a group of six muscles, whose movements are synchronized by interconnections in the brain to produce conjugate movement. For example, when we look to the right side, the right eyeball moves outward whereas the left eyeball moves inward. An imbalance in this harmony results in a disfigurement called squint or cross-eye.
Systemic Diseases and the Eye
The eyes are part of the body and diseases which affect the body may affect the eye also. The common systemic disorders affecting the eyes are hypertension, diabetes, neurological disorders, endocrine disorders, Connective tissue disorders etc.
Hence ophthalmologist plays an important role in helping the physician, Endocrinologist and Neurologist in examining fundus of these patients whenever they are referred.
Key points to remember
- The eyeball is made of three coats, the outer protective scleral, middle vascular choroid, inner neuronal retina.
- The cornea is a transparent watch glass like structure. It is like a window to the eye.
- The retina is made of neuronal tissues. The most visually sensitive part of the retina is macula.
- Lens is important for the focusing mechanism of eye. The shape of the lens alters to see objects at near and distance.
- The common systemic disorders affecting the eyes are hypertension, diabetes, neurological disorders, endocrine disorders, connective tissue disorders etc.
Student exercise
Write short answers
- What are the important systemic diseases affecting the eye?
- Which part of the eye secretes the aqueous humor?
- What is the function of retina?
- Which is the most sensitive part of the retina?
- What is the normal size of pupil?
