The orbit

Bony Orbit

The orbit is a pyramidal shape, with its tip going into the skull. The tip (or apex) contains vital neurovascular structures involved in sight and movement of the eyeball. It is the route of communication between intra-cranial cavity and orbit.

If we break the bony orbit down into sections, we can understand exactly how intricate its anatomy really is:

Medial Wall:

This extends from the lacrimal crest of the maxilla, to the apex of the orbit. It is made up of the maxilla, lacrimal, ethmoid and sphenoid bones. Its margin with the roof is demarcated by the fronto-ethmoid suture.

There is an extremely thin wall called the ‘Lamina papyracea’ which is the plate of bone of the ethmoid sinus. 

The lacrimal fossa (where the lacrimal sac sits), lies anteriorly on this wall. The naso-lacrimal canal then leads from the maxilla to the inferior meatus of the nasal cavity.

Some key foramina in this wall are:

• The optic canal - this has the body of the sphenoid bone joining it posteriorly. 
• The anterior and posterior ethmoid foramina. 

Floor:

The floor is made up of the zygomatic, maxilla and palatine bones. It slopes upwards towards the apex and merges with the medial wall at the maxilla-ethmoid suture.

The floor is separated from the lateral wall by the inferior orbital fissure.

The orbital floor is a very thin structure, especially by the infra-orbital groove. This becomes the infra-orbital canal and houses the infra-orbital nerve, which exits from the infra-orbital foramen. 

Lateral Wall:

This is mainly formed by the zygomatic bone and greater wing of sphenoid. It is separated from the roof of the orbit by the superior orbital fissure and separated from the floor by the inferior orbital fissure (this communicates with the infra-temporal fossa and pterygopalatine fossa) .  

The lateral wall separates the orbit from the infra-temporal and middle cranial fossae.

The Zygomaticotemporal and Zygomaticofacial nerves/vessels pass through their respective foramina in lateral wall – these are extremely small. 

Roof:

This is made up almost entirely by the frontal and some of the sphenoid bone.

Just behind the rim is the a depression for the trochlea(/trochlear) - this is a ring of cartilage that the superior oblique tendon passes through.

The supra-orbital notch lies at the upper margin of the roof.

Superior Orbital Fissure:

The superior orbital fissure is an important part of the orbit as many key nerves pass through it.

It lies between the greater and lesser wings of the sphenoid bone and is wider at the bottom than it is at the top.

In the orbit, there is a fibrous ring, called the Annalus of Zinn, which divides the superior orbital fissure into spaces. This tendinous ring is also the origin to the 4 extra-ocular rectus muscles.

The superior orbital fissure transmits the following:

• Maxillary nerve (Va)
  • Lacrimal
  • Frontal
  • Nasociliary
• Trochlear nerve (IV)
• Superior division of oculomotor (III)
• Inferior division of oculomotor (III)
• Abducens (VI)

There are some mnemonics to remember the order the nerves pass through from top to bottom. We will let you find those out as some variations can be rather rude!

Peri-orbital Structures

Surface Anatomy:

This consists of the:

• Eyelids – muscular flaps which protect the eye. 
• Palpebral fissure – gaps between eyelid when eye is open.
• Lacrimal papilla – prominence in medial end of the eye, it’s opening is called the punctum. 
• Medial canthus – another word for the medial end of the eye, this can sometimes be covered by an epicanthal fold.

Eyelids:

The layers of the eyelid from most superficial to the deepest are as follows:

1. Epidermis
2. Dermis
3. Subcutaneous tissue
4. Fibres of the Orbicularis oculi muscle
5. Tarsal plates
6. Tarsal glands
7. Conjunctiva

The orbital septum is a connective tissue layer that attaches to the margins of the bony orbit. The thickened areas of this are called the inferior and superior tarsal plates. 

These plates are attached to the lacrimal bone medially, by the medial palpebral ligament, and laterally to the zygomatic bone, by the lateral palpebral ligament. 

These plates form the main structure of the eyelids and sit below the orbicularis oculi muscle. 

The upper eyelid is primarily controlled by the levator palpebrae superioris.

Conjunctiva:

This consists of non-keratinised stratified squamous epithelium over a thin layer of fibrous tissue. It creates a clear membrane of the sclera of the eye for protection.

It can be split into the palpebral conjunctiva (on the inner aspects of the eyelids) and the bulbar conjunctiva (from the eyelids onto the surface of the eyeball).

The conjunctiva and upper eyelid are innervated by branches of the ophthalmic nerve.

The lower eyelid is innervated by the infraorbital branch of the maxillary nerve. 

Lacrimal Gland:

These are paired exocrine glands which sit in the lacrimal fossa of the lateral wall of the orbit (this is different to that of the lacrimal sac in the medial wall of the orbit).

Inflammation of the gland is called 'dacryoadenitis'.

• This produces tears which flow into the lacrimal ducts.
• From here, they flow into the superior/inferior lacrimal caniculi and into the lacrimal sac.
• Tears then drain through the naso-lacrimal duct and into inferior meatus of the nose.

Tears consist of a salt solution, mucus and lysosyme (for protection of the eye).

Innervation of the Orbit

Motor to Extra-Ocular muscles

Oculomotor (III):

This is split into superior and inferior divisions which pass along lateral wall of Cavernous Sinus.

These branches then enter via the medial part of superior orbital fissure.

• Superior branch: superior rectus and levator palpebrae superioris.
• Inferior: medial/inferior rectus and inferior oblique muscles + a ciliary preganglionic branch.

Trochlear (IV)

This enters via the superior orbital fissure, outside of the tendinous ring and innervates the superior oblique muscle.

Abducens (VI)

This enters the orbit via the superior orbital fissure, inside the tendinous ring and innervates the lateral rectus muscle.

A handy way of remembering which nerve innervates its respective muscle is this made up formula: LR6-SO4-R3. Ie. Lateral Rectus (VI), Superior Oblique (IV), Rectus Muscles (III).

Sensory

Ophthalmic (Va)

• Lacrimal branch
  • Enters via superior orbital fissure, outside tendon ring
  • Passes along the lateral wall of orbit and pierces the septum.
  • Supplies lateral upper eyelid and conjunctiva. 
  • Carries parasympathetic secretomotor nerves from the facial nerve via the pterygopalatine ganglion to the lacrimal gland. 
• Frontal branch
  • Enters via superior orbital fissure, outside tendon ring
  • Divides into: medial supra-trochlear nerve + lateral supraorbital nerve
  • Medial supra-trochlear nerve supplies skin of forehead
  • Supra-orbital does mucosa of frontal sinus
• Nasociliary branch
  • Enters via superior orbital fissure, inside tendon ring
  • Splits into the following:
    • Ciliary ganglion branch
    • 2 long ciliary nerves
      • Supply the cornea
      • Sympathetic post-ganglionic supply to dilator pupillae muscle
    • Posterior ethmoidal nerve
      • Supplies the ethmoidal air cells and sphenoid sinus
    • Anterior ethmoidal nerve
      • Supplies the anterior and middle ethmoidal cells + external nasal nerve for skin over nose
    • Infra-trochlear nerve
      • Supplies the lacrimal sac, conjunctiva, upper eyelid and side of nose.

For more information on the Maxillary (Vb) [branches pass through but do not supply the contents of the orbit] and Optic (II) nerves, read on and also see our 'Cranial Nerves' lessons.

Ciliary Ganglion

This is involved in autonomic innervation. It lies lateral to the optic nerve and sits behind the eyeball.

It is responsible for miosis (constriction of pupil) and accommodation.

• The pre-ganglionic parasympathetic fibres travel with the oculomotor nerve (III) and relay in the ganglion.
• Post-ganglionic fibres then pass to the sphincter of the iris and ciliary body. They do this by hitch-hiking with branches of the ophthalmic nerve.

Blood Supply

Arterial (1)

• The ophthalmic artery is a branch of the internal carotid artery. 
• It enters the orbit via the optic canal. 
• It branches into the following:
  • Lacrimal and Nasociliary branches to follow the respective nerves to their terminal structures
  • Muscular branches to extra-ocular muscles 
  • Ciliary arteries to eyeball
  • Central artery of retina

Venous (1)

• This leaves via the superior and inferior ophthalmic veins
• They pass through the superior orbital fissure to the cavernous sinus.
• The superior vein communicates with facial vein.
• The inferior vein communicates with pterygoid plexus via the inferior orbital fissure.

Lymph

Drain from orbit to parotid nodes, then to deep cervical chain.

Clinical Relevance

You know when your eye twitches and it's really annoying. That twitch that only lasts a couple of seconds is your levator palpebral superioris muscle. Eye-twitching is usually related to stress and fatigue, so make sure you take regular breaks between topics/lessons!

The inferior retinal vein communicating with the pterygoid plexus is relevant for the spread of infection. Remember, some maxillary tooth infections can spread through tissues spaces and affect the eye - so you have to be aware of what could happen if it is severe. Additionally, conditions that affect the eye may present when taking histories or examinations and you must be able to apply this and refer if concerned.

Orbital fractures can be significant and lead to blindness. For more detail on the types and their management, see the relevant OMFS Lesson on mid-facial fractures.

Anatomy of the Eye

Structure

Fibrous Layer - superficial, anterior cornea and posterior sclera

Cornea:

• This is a transparent cover over the iris and helps to focus light onto retina.
• It is made up of non-keratinised stratified squamous epithelium
• The middle cornea contains collagen and fibroblasts.
• The inner cornea consists of simple stratified squamous epithelium.

Sclera:

• This is the outermost layer. It is fibrous and is white until it becomes transparent at the anterior by the cornea.
• It consists of dense connective tissue of collagen and fibroblasts.
• Its function is to give shape + rigidity to the eyeball.
• It is also where the extra-ocular muscles insert.
• At the sclera/cornea junction, is a structure called the scleral venous sinus (otherwise known as the canal of Schlemm). This is where the aqueous humour drains.

Vascular Layer = choroid, ciliary body and iris

Choroid:

• This lines most of inner surface of sclera.
• It contains melanocytes and hence is a pigmented vascular layer between sclera and retina.
  • The melanin absorbs stray light rays to prevent scattering within the eyeball.
• It starts from the region of the optic nerve and extends anteriorly, where it is continuous with the ciliary body near the oro serrata (anterior margin of retina).

Ciliary Body:

• This is located between the choroid and the iris.
• It extends from the ora serrata to the junction of the sclera and cornea.
• It also has melanin produced by melanocytes.
• It is ring-shaped and has a series of transparent fibres, called the suspensory ligaments of the lens (zonular fibres)
• Within the body are ciliary processes (folds that secrete aqueous humor) and smooth ciliary muscle (changes shape of lens by changing length of zonular fibres).

Iris:

• This is a thin disc-like structure with the pupil in its centre.
• It is attached to the ciliary processes.
• It separates the aqueous humor into the anterior and posterior chambers between iris and lens.
• It contains melanocytes and circular (sphincter) and radial (dilator) smooth muscle fibres.
• The sphincter and dilator pupillae muscles change the pupils’ shape in response to light.

Pupil

• This is primarily a hole in the iris.
• As the iris controls its size, it is dependent on autonomic reflexes in response to light.
  • Parasympathetic fibres of the oculomotor stimulate circular (sphincter pupillae) muscle fibres to contract and constrict pupil to manage amount of light entering.
  • Sympathetic will stimulate radial (dilator pupillae) muscle fibres to dilate pupils and allow more light in. 

Retina

The is the innermost coat of eye. It is thin and highly vascular. Due to its dense vascularity, it can be examined for changes in diseases like diabetes, cataracts, hypertension etc. 

It has a pigmented and neural layer:

• Pigmented- melanin to absorb scattered rays.
• Neural – outgrowth of the brain: process visual stimuli before sending impulses via the optic nerve. This layer can be broken down into:
  • Photoreceptor layer (rods and cones) **
  • Bipolar cell layer
  • Ganglion cell layer (inner and outer synaptic layers)

There are 3 main areas on the posterior portion of the retina:

• Optic Disc
  • This is the area where the optic nerve enters retina.
  • It can be known as the 'blind spot' as it does not contain any rods or cones.
  • The retina’s central artery enters eye through optic disc and divides into superior and inferior branches.
• Macula Lutea
  • This lies laterally to the optic disc.
  • It is a depressed, yellow area that contains the fovea centralis in its centre.
• Fovea Centralis
  • Only contains cones

Rods vs Cones:

• Fun fact, your eye contains about 6 million cones and 120 million rods (1).
• Cones have same internal structure with a protein called photopsin. They have a fast recovery time after being stimulated.
• Cones produce colour vision – approximately split as follows: red 60%, green 30%, blue 10% (2).

• Rods are found in the periphery of the eyeball.
• Rods contain optic discs - these are membrane bound structures containing proteins.
• Rods are more sensitive to light, so when they detect it, it tends to be your back and white vision. They have a slow recovery time after stimulation.

• How they work in brief (read on for more detail):
  • The photoreceptors are connected to neurones which have axons which go into the optic nerve.
  • Light enters the eye and hits the fovea - this causes a higher resolution as neurones are not obstructing and light can directly hit the cones.

Lens

• This lies posterior to the iris.
• It is a transparent biconcave structure responsible for focussing the light.
• It contains crystallins, which layer up to create the refractive quality of lens.
• It is connected to ciliary body by suspensory ligaments and is avascular.

Anterior Cavity

• The anterior chamber lies between the cornea and iris.
• The posterior chamber lies between the iris and lens.
• Both contain aqueous humor, which nourishes the lens and cornea.
• Another fun fact: Aqueous humour is replaced about every 90 minutes (2).

Posterior Cavity

• This is the vitreous chamber. It is a jelly-like substance.
  • It gives the retina an even surface for processing stimuli.
  • It contains water, collagen and hyaluronic acid which is not replaced throughout life.  
  • It also has phagocytic cells to remove debris. If debris builds up, these can be referred to as 'floaters'.
  • The Hyaloid canal in middle used to hold hyaloid artery in embryo.

• Aqueous humour and the vitreous body create an intra-ocular pressure of around 16mmHg (2).

Muscles

The Eyeball lies in the orbit surrounded by a fascial sheath, which separates it from the orbital muscles and fat. The Extra-ocular muscles have to pierce this sheath before inserting into sclera.

The eye is suspended in the orbit by ‘check ligaments’ and the ‘suspensory ligament of lockwood’.

The rectus muscles originate from a ring of tendon and attach to sclera. 

Levator palpebrae superioris contains some smooth muscle supplied by the superior cervical ganglion – if damaged, this can cause ptosis eg in Horners syndrome.

Optic Nerve Anatomy

However for more detail on the optic nerve and how it works, continue reading:

• The axons from ganglionic cells of the retina make up the optic nerve and come together at the optic disc.
• They leave the eye and travel as the optic nerve posteriorly and medially through the orbit.
• Posteriorly, the optic nerve passes through the optic foramen to enter the cranial cavity.
• 2 optic nerves from each eye then meet at the optic chiasm. This lies superior to the hypophyseal fossa.
  • Here, some axons cross to the opposite side, and others do not. The 'nasal half' of each retina crosses over.
• The optic chiasm then gives rise to optic tracts, which terminate in the lateral geniculate nucleus of thalamus.
• This then gives rise to optic radiations, which terminate in primary visual areas of occipital lobes in cerebral cortex.

Physiology of Sight

Electromagnetic spectrum is the range of different electromagnetic radiation, varying by wavelengths. 

A light stimulus consists of electromagnetic waves from 400nm (violet)-700nm (red). Objects appear to be the colour that they reflect. 

Introduction

The stimulus of light is an electromagnetic wave. These are detected by photoreceptors.

The retina is the membrane that lines the back of the eye and contains rods and cones. These rods and cones convert light into a neural impulse.

In short, the photoreceptors detect light and stimulate the outer synaptic layer bipolar cells. These then stimulate the inner synaptic layer ganglionic cells, which send an impulse down the optic nerve.

Image Formation

Light is refracted (bent) by the cornea and lens before entering the eye to the retina. Most light refraction occurs at the cornea, then at the lens.

Ideally the rays should land on the central fovea. However if the rays do not head towards this: accommodation occurs by ciliary muscle control. 

When the images hit the retina, they are upside down – but the brain processes this. 

Humans have binocular vision – both eyes focus on one object. This is by light rays hitting the same point on the retina in both eyes. Convergence means this still happens with closer objects as the extra-ocular muscles rotate the eye for this to occur. 

Physiology

Phototransduction

The phototransduction cascade turns rod on/off.

Rods and cones are categorised by their receptor end shape. Their receptors contain photopigments (these are pigments in their plasma membranes). These photopigments undergo rapid turnover.

• In cones, these have folds.
• In rods, these have pinches called discs.

Physiology of Phototransduction:

1. Light is absorbed by a photopigment (protein which undergoes conformational change when absorbing light wavelengths). This conformational change initiates a receptor potential. 
2. The photopigment in rods is called rhodopsin (cones have their own for blue/red/green).
   • Rhodopsin consists of an opsin glycoprotein and a Vit A derivative called retinal (derived from carotene, which you can get from carrots!).
3. In darkness- retinal is bent and this form is called cis-retinal.
4. When light is absorbed, this provides energy for the conformational change and it straightens to trans-retinal.
5. The trans-retinal then separates from the opsin as a result of some chemical processes (see further reading). This is called bleaching. 
6. An enzyme, retinal isomerase, then changes trans-retinal back to cis-retinal. It then rebinds to an opsin molecule, forming another photopigment ready for the process to start again. This is called regeneration.

Creation of action potential:

1. In darkness, ligand gated sodium ion channels are open and sodium ions flow into the photoreceptor cell. The ligand that opens these channels is cGMP (cyclic guanosine monophosphate).
2. This partly depolarises the photoreceptor membrane (-30mV in comparison to a normal neurone of -70mV) (2). 
3. The neurotransmitter glutamic acid is therefore released continuously, but can be inhibitory to the bipolar cells. 
4. When light hits a photoreceptor, the cis-retinal changes to trans-retinal. 
5. A subunit, in this process, breaks off and binds to cGMP phosphodiesterase on another part of the disc. This phosphodiesterase breaks down cGMP to GMP. 
6. This breakdown causes the sodium channels to shut. Therefore, the cell becomes hyperpolarised to -70mV (2). The glutamic acid release stops. 
7. This hyper-polarisation stimulates the ganglion cells to form action potentials in their axons. 

Therefore, rods become inactive when light hits their photoreceptors. This turns the bipolar cells on, which in turn, switches on the retinal ganglion cell. These then create the action potential which enters the optic nerve and travels to the brain.