1. Introduction to Anatomy of the Eye and its Adnexa

Let us imagine we are traveling with a ray of light into the eye. The first structure

we will encounter is the cornea (Fig 2).This is a transparent structure which allows

light to enter inside the eye. It fits into the surrounding white opaque tissue; the

sclera like a watchglass and together they constitute the outer coat of the eye.

This outer coat protects the eye from injury and invading micro-organisms (Fig 3).

Light passing from the cornea enters a space full of clear fluid known as the

anterior chamber (Fig 4) , this space is bounded by the cornea anteriorly and by

the iris posteriorly. The iris is the part which gives the eye its color.

The Cornea is made up of 5 layers:

The epithelium, Bowman's layer, the

corneal stroma, Descemet's membrane

and the endothelium

The stroma of the cornea is made up of

collagen fibres called lamellae. The

corneal lamellae are arranged in a

specific order which helps to achieve

corneal transparency; other factors for

corneal transparency include corneal

avascularity, and the presence of active

endothelial pump which makes the

cornea dry and not misty.

Fig 1: A Cross section of the human eye.

Fig 2: The Cornea illuminated by a slit

beam.

Fig 3: The White sclera seen

underneath the transparent

conjunctiva.

The iris (Fig 5)is made up of muscles, blood vessels and pigmented cells. It has a

central circular opening through which light travels , known as the pupil (Fig 5).

The pupil is a circular opening in the iris that allows transmission of light. It

controls the amount of light reaching the retina by dilating and constricting.

The detailed anatomy of the innervation of the pupil will be discussed in chapter 9

(Connection of the eye to the brain).

The iris contains two types of smooth

muscles:

The circular constrictor pupillae and

the longitudinal dilator pupillae

The constrictor pupillae is under

parasympathetic control and on

contraction the pupil narrows

The Dilator pupillae is under

sympathetic control and on

contraction the pupil dilates

Fig 5: The iris and Pupil

Fig 4: The Anterior Chamber.

The iris is continuous posteriorly with the ciliary body (fig 6) and the choroid.

(Fig 7) These 3 structures are made up of blood vessels and contain pigment and

are collectively known as the uveal tract or the middle or vascular coat of the

eye. Its main function is to provide nutrition and support for the overlying and

underlying structures.

This middle coat of the eye would be the first structure to encounter if we were to

peel away the sclera as seen in fig 8.

The ciliary body is made up of two

parts: The pars plicata and the pars

plana.

The pars plicata is covered with

epithelium that secretes aqueous

humour and gives attachment to the

suspensory ligament or zonule that

suspends the crystalline lens in place.

It contains a smooth muscle known

as the ciliary muscle whose

contraction results in relaxation of

the zonule and bulging of the lens,

thus changing its curvature and

power. This is known as

accommodation. Fig 8: The choroid and its blood vessels with

the sclera peeled away.

Fig 7: The Choroid and its blood vessels. Fig 6: A diagram of the ciliary body (pars

plana and [pars plicata).

After light passes through the pupil it meets a transparent object suspended from

the ciliary body known as the crystalline lens (Fig 9). The lens is an important

refractive medium of the eye. The space between the crystalline lens and the iris is

known as the posterior chamber (Fig 10). The transparent fluid that fills the

anterior chamber is known as the aqueous humor and is formed by the ciliary

body and secreted into the posterior chamber, behind the iris, it then passes through

the pupil to fill the anterior chamber and be drained through the angle (Fig 10)

between the cornea and sclera.

Aqueous formation, circulation and drainage through the angle, in addition to the

structures of the angle will be discussed in more detail in chapter 5 (Aqueous

humour, IOP and glaucoma).

Fig 9: A diagramatic representation of the

crystalline lens. Fig 10: Passage of aqueous humour from the

ciliary body in the posterior chamber, through

the pupil into the anterior chamber.

Once light has been refracted by the lens it meets the transparent gel like structure

that fills the space behind the lens. This gel like structure is known as the vitreous

body (Fig 11) and (Fig 12). The vitreous is formed of 99% water in addition to

special proteins called glycosaminoglycans. No clear function is known to the

vitreous apart from transmitting light to the retina. However, it may have a shock

absorbing role due to its gel-like state and it may also act as a reservoir for certain

nutrients or chemical mediators.

The Lens:

The lens is a biconvex avascular structure present behind the

pupil. It contributes about +20 D to the refractive power of the eye.

This power can be increased during accommodation, which can be

achieved by ciliary muscle contraction resulting in relaxation of the

zonule and allowing the elastic capsule to change the shape of the

lens in order to become more convex. This allows seeing near objects

clearly.

The Lens is also a dynamic structure that is continuously growing with

human growth. The capsule which surrounds the lens is elastic and

has an epithelial layer on the inner surface of its anterior portion from

which lens proteins are formed. As it grows, the lens proteins become

more aggregated in the center and lose fluid. This hardening or

sclerosis leads to a distinction of the lens proteins into the harder

central nucleus and the softer peripheral cortex. Due to this sclerosis,

the lens becomes less pliable by age and consequently loses its ability

to increase its power by changing its shape (Accommodation).

Having traversed the vitreous body, light will reach its final destination by falling

upon the light sensitive neurosensory structure known as the retina (Fig 13). The

retina is the inner coat of the eye. The retina is a very intricate structure which is

closely connected anatomically and embryologically to the brain, and will be

discussed in more detail in chapter 8 (The Retina: Functions and diseases).

The retina performs the essential task of converting light to electrical impulses.

These impulses are then transmitted via the optic nerve (Fig 14) to the brain. The

optic nerve can be actually viewed as being a tract of the central nervous system

rather than a true nerve. The occipital cortex is the part of the brain responsible for

vision i.e. giving meaning to those electrical impulses. The route from the retina to

that part is called the visual pathway (Fig 15).

The detailed anatomy of the optic nerve and visual pathway will be discussed in

chapter 9 (Connection of the eye to the brain).

Fig 11: A diagramatic representation of the

vitreous body.

Fig 12: The vitreous body as seen after

peeling the sclera, choroid and retina away.

Fig 13: Fundus photograph showing the

appearance of a normal retina.

Due to the importance of the retina in the process of vision, all the aforementioned

ocular structures can be looked at as serving the purpose of allowing light to reach

the retina and to provide the retina with protection and nutrition.

Additional protection is provided by the ocular adnexa and orbit. The orbit (Fig

16) is a bony socket in-which the eye is placed. Within the orbit the eye is

surrounded by fat,muscles, nerves and vessels ,in addition to the lacrimal gland

(Fig 17) that secretes the main ( watery) part of the tears.The tears are then drained

through the lacrimal drainage system (Fig 17, See details in chapter 3)).

Fig 14: The Optic nerve seen passing from the

back of the eye into the cranial cavity.

Fig 15: The visual pathway.

Fig 16: The Bony orbit. Fig 17: The lacrimal gland and lacrimal drainage

system.

Protecting the surface of the eye are the eyelids (Fig 18). The eyelids and the outer

surface of the eye are covered with a transparent membrane known as the

conjunctiva (Fig 19). It lubricates the ocular surface and contributes to tear

formation aswell. The anatomy of the orbit, eyelids, lacrimal gland and lacrimal

drainage system will be discussed in further details in chapter 4 (Diseases of the

protective structures of the eye).

Fig 18: A cross-section in the eyelids. Fig 19: A cross-section in the conjunctiva of

the upper and lower lids in red.

The conjunctiva is a membrane that

lines the inner surface of the eyelids

and the outer surface of the globe. It

is formed of three parts. The

palpebral part (related to the lid),

the bulbar part (related to the globe)

and the fornix, which is the pouch

connecting both these parts. The

fornix of the lower lid is the site

where eye-drops are instilled.

The conjunctiva plays a protective

role, in addition to its important

contribution to tear formation.

The muscles in the orbit known as the Extraocular muscles (Fig 20& 21) are

responsible for moving the eyes in different directions in asmooth and coordinated

way.

The anatomy of the extra-ocular muscles and their function will be discussed in

further details in chapter 7 (Ocular motility in health and disease).

Fig 20: The extra-ocular muscles in

both eyes.

Fig 21: The extra-ocular muscles in

relation to the orbit.