Eye Anatomy & Physiology

  The sense of vision is based on a step-wise process in which rays of light are focused on the Retina, where it is converted in to an electrical signal and is conveyed through the visual pathways to the brains occipital cortex. This signal is interpreted as a visual message by the brain. If any part of the system is affected by a disease, vision may be impaired.

As an Ocularist one should know the parts of the eye, its surrounding tissues, and its connections in the brain. As each part is described, you will know its structure (anatomy), function (physiology) and how it can become diseased (pathology).

  Ocular adnexa is the tissue surrounding the eye, it protects and preserves the normal function of the eyes. The Ocular Adnexa include:

§ Eyelids and Conjunctiva

§ Lacrimal apparatus

§ Orbit

§ Extraocular muscles

The eyelids are moving folds of soft tissue that protect the outer portion of the eyeball from injury, exclude light and lubricate the front surface of the eye.

The blinking action of the upper lid spreads lubricating tear film over the front surface of the eye.

The elliptical opening between the upper and the lower lids is called the “Palpebral Fissure”. The normal size of the palpebral fissure is about 15 mm when the lids are open.

The inner junction of the lids is called the “Medial Canthus” and the outer junction is called the “Lateral Canthus”. The medial canthus contains folds of fleshy tissue. The deeper one is called the “Plica Semilunaris” (semi-lunar fold) and the more visible one is the “Caruncle” . The medial lid margins of the upper and lower lids contain small openings to the tear drainage system that is called the “Puncta”.

The anterior edge of the lid margins contains the hair follicles for the eyelashes (cilia). The lashes serve to sweep airborne particles away from the eye during a blink. Oil secreting glands called “Meibomian Glands” are situated on the posterior edge of the lid margin. Just in front of these glands is the grey line, which divides the inner (close to the eye), and outer portions of the lid margin.

Acute inflammation of a lash follicle is called a “Stye” (hordeolum externum). It is a reddened, sore lump near the lid margin. Further from the lid margin, chronic inflammation may involve a meibomian gland producing a chalazion.

Blepharitis is the term applied to diffuse inflammation of the lid margin, which is usually the result of an overgrowth of the normal bacterial population of the lid. Patients with Blepharitis have reddened, crusted lid margins usually covering the entire extent of the lid.

The eyelids are composed of an outer layer of skin, an inner layer of palpebral conjunctiva and a layer of fibrous tissue and muscle between the two. The fibrous layer, called the “Tarsal Plate”, gives the lid its firmness. The orbicularis oculi is circular muscle that, upon contraction, results in forced eyelid closure, as in winking. The upper lid is raised by the levator plpebral superioris muscle, which attaches to the upper tarsal plate. The muscle is part of a group of extraocular muscels those are controlled by the third cranial (oculomotor) nerve.

Instead of lying in its normal position against the eyeball, the lid margin may turn away from or towards the globe. This turning out of the margin is called “Ectropion” and may lead to drying and irritation of the exposed cornea and conjunctiva.

An inward turning of the lid margin is called “Entropion”, in this condition, eye lashes may rub against the cornea and cause irritation and tearing, Inward turning of eye lashes is known as “Trichiasis”.

Loss of function of the levator muscle results in “Ptosis” a droopy upper eyelid that no longer elevates normally.

Diseases which affect the skin elsewhere on the body, may affect the skin of the lids. Careful examination may reveal dermatitis, cysts or tumours (especially basal cell carcinomas).

The conjunctiva is the translucent mucous membrane that lines the inner surface of the lids (the palpebral portion of the conjunctiva).

The bulbar conjunctiva ends at the limbus. The junction of the bulbar conjunctiva and the palpebral conjunctiva is called the “Fornix” or “Cul-de-Sac”.

The under surface of the upper eyelid is named as "Lid wiper" as it spreads the tear film over ocular surface with every blink

The conjunctiva may become inflamed, due to bacterial or viral infection or because of an allergic reaction, is called “Conjunctivitis”, it is sometimes called “Red-Eye”, produces enlargement of surface blood vessels, causing the normally white part of the eye, the “Sclera”, to appear red. Sometimes, a blood vessel may rapture spontaneously and allow blood to flow under the conjunctiva. This is referred to as a “Subconjunctival haemorrhage and usually resolves in a few weeks without any treatment. Most often this haemorrhage occurs without explanation or after violent s sneezing or coughing, but rarely it may be associated with high blood pressure or bleeding disorders. A pingueculum, a yellowish mass on the bulbar conjunctiva just nasal or temporal to the limbus, probably represents irritation from sunlight. Continued irritation, especially by exposure to intense sunlight, may lead to formation of a pterygium, a fleshy wedge of bulbar conjunctiva that grows from the canthus (usually medial) towards the cornea. It may cause some irritation, but is not harmful unless it grows over the central cornea and impairs vision. If this happens, it must be surgically removed.

  The lacrimal apparatus is composed of tear-producing glands and a tear drainage system. The lacrimal gland secretes the aqueous portion of tears and is located in the lateral segment of the upper lid just under the upper orbital rim. There are small, accessory lacrimal glands scattered throughout the upper fornix. The tear fluid is spread over the front surface of the eye when the upper lid closes during a blink. Tears then form a pool along both lid margins before passing through the upper and lower puncta (holes) into the canaliculi (little canals) of the drainage system. The upper and lower canaliculi merge into the common canliculus near the medial canthus, and the tears then flow into the lacrimal sac. The sac empties into the nasal cavity by means of the nasolacrimal duct. Where hot air being breathed out evaporates these tears.

The tear film is composed of three layers. The meibomian glands secrete the outer, oily layer, which helps prevent evaporation of the tears. The middle, aqueous (water) layer, secreted by the lacrimal gland, contains the oxygen and nutrients that nourish the cornea. The inner, mucinous layer is secreted by the goblet cells of the conjunctiva. The mucinous layer helps to maintain an even spread of tears over the cornea. The lacrimal system produces up to 1 ml (about 1/4th teaspoon) of tears during the waking hours, about 50% of which are lost to evaporation. No tears are produced during sleep. The tear-producing lacrimal gland is generally free of disease, but occasionally inflammation or tumours may occur in it. On the other hand, inflammation of the tear-collecting lacrimal sac, dacryocystits is relatively common. It usually occurs as a result of obstruction of the nasolacrimal duct. In infants, this obstruction is commonly the result of a congenital narrowing of the nasolacrimal duct where it opens into the nasal cavity. This often opens with time, if it does not it may require probing or other treatment. The signs of dacryocystitis are tenderness and swelling below the inner canthus, discharge or excessive tearing (epiphora). In the adult, obstruction of the tear outflow system occurs as a result of chronic lacrimal sac infection, facial trauma or tumours. If the blockage is severe, it may be necessary to bypass the nasolacrimal duct by surgically fashioning an opening between the sac and the nose (is called”Dacryocystorhinostomy”). Older persons and persons certain systemic diseases, may be subject to dry eyes (keratitis sicca). When one or more of the tear film components is insufficient, the tears cannot function properly and cornea becomes irritated. Patients usually treated with artificial tears.

  The orbit is the cavity in the skull, which houses the eyeball. Its walls consist of seven bones, and it contains the eyeball, the extraocular muscles, blood vessels and nerves cushioned by a great deal of fat.

Four of the muscles form the muscle cone within the orbit through which pass the optic nerve and most of the blood vessels and nerves to the eye. The muscle cone originates in the posterior orbit as a circle of muscles called “Annulus of Zinn”.

Since the back of the orbit narrows, any increase in the orbital volume will cause the eyeball to protrude; this condition is called “Proptosis or Exophthalmos”. Proptosis is often seen in association with Graves’ disease, a condition of unknown cause that affects both the thyroid gland and the soft tissues around the eyes. Other causes include tumours, inflammation (including infection) or haemorrhage.

Diffuse infection of orbital tissue or orbital cellulitis, appears as grossly swollen lids, a red eye and sometimes proptosis. When the tissue bulk in the orbit is greater than normal, the eye muscles may not be free to move normally. If the eyes go out of alignment (strabismus), the patient may complain of double vision (diplopia). Misalignment of the eyes also results when one (or more) of the eye muscles become restricted in its movement because it has become inelastic from scarring or trapped by broken orbital bones. Muscle scarring occurs after any long-term inflammation, but the most common cause is Graves’ disease.

  The eyeball is moved by the action of six extraocular muscles. The medial rectus pulls the eye towards the nose in adduction and inserts on the globe 5.5 mm from the medial limbus. The lateral rectus moves the eye away from the nose in abduction and inserts about 7 mm from the lateral limbus. The eye is elevated primarily by the superior rectus (insertion at 8 mm) and depressed primarily by the inferior rectus (insertion at 6 mm). The oblique muscles insert behind the equator of the globe and assist in elevation (inferior oblique) and depression (superior oblique).

The superior oblique also intorts the eye and the inferior extorts the eye. To understand intorsion and extorsion, imagine that the eye rotates on a pole piercing it from front to back. Intorsion means clockwise rotation of the right eye, counter-clockwise rotation of the left eye; extortion is rotation in the opposite direction.

The function of the six extraocular muscles are evaluated by asking the patient to move his eyes from straight ahead gaze, i.e. primary position in to upward gaze, downward gaze and into the six cardinal positions of gaze, which are right gaze, left gaze, up and right gaze, down and right gaze,  up and left gaze, down and left gaze. The movement of the two eyes into this diagnostic gaze positions are called “Versions”.