KEY ANATOMICAL FACTS
The human eye is an organ which reacts to light for several purposes. The eye is not properly a sphere, rather it is a fused two-piece unit. The smaller, less curved unit (cornea) is linked to the larger unit called the sclera. The cornea and sclera are connected by a ring called the limbus. The iris and its black center, the pupil, are seen instead of the cornea due to the cornea’s transparency.
Dimensions differ among adults by only one or two millimeters. The vertical measure, generally less than the horizontal distance, is about 24 mm among adults, at birth about 16-17 mm. The eyeball grows rapidly, increasing to 22.5-23 mm (approx. 0.89 in) by the age of three years. From then to age 13, the eye attains its full size. The volume is 6.5 ml (0.4 cu. in.) and the weight is 7.5 g. (0.25 oz.)
The eye is made up of three coats, enclosing three transparent structures. The outermost layer is composed of the cornea and sclera. The middle layer consists of the choroid, ciliary body, and iris. The innermost is the retina, which gets its circulation from the vessels of the choroid as well as the retinal vessels.
Within these coats are the aqueous humor, the vitreous body, and the flexible lens. The aqueous humor is a clear fluid that is contained in two areas: the anterior chamber between the cornea and the iris and exposed area of the lens; and the posterior chamber, behind the iris and the rest. The lens is suspended to the ciliary body by the suspensory ligament, made up of fine transparent fibers. The vitreous body is a clear jelly that is much larger than the aqueous humor, and is bordered by the sclera, zonule, and lens.
Anatomy of the eyelid
The description above offers a superficial overview of the anatomy of the eyelid. If one were to look at the eyelid in a more detailed manner, a sagittal section taken across the eyelid will offer a clear view of the various structures that form it. The structures that are visualized depend on the plane at which the sections are taken.
As mentioned above, the tissues can be divided into planes by structures called the septum. The orbital septum differentiates the orbital tissue from the lid. Behind the septum are a number of different other structures, a knowledge of which is essential if surgery is to be performed. In particular, it is important to identify the anterior and posterior lamellae. In essence, the anterior lamella consists of the skin and the orbicularis oculi muscle while the posterior lamella consists of the conjunctiva and the tarsus. Below we detail the structures of the eyelid.
The upper eyelid starts at the eye and extends upwards, joined by the skin of the forehead. Similarly, the lower eyelid starts at the eye and extends to join the skin of the cheek. Upon close inspection, it is evident that the lower eyelid skin is looser than the upper eyelid skin, particularly as we age and skin laxity increases.
At the top of the upper eyelid is a fold in the skin called a skin crease or the superior palpebral sulcus. It lies roughly 8 to 11 mm above the margin of the upper eyelid and consists of fibers of the levator aponeurosis. Similarly, there exists another skin fold in the lower eyelid called the inferior palpebral sulcus. This skin fold is often more prominent in children and can become less prominent as one gets older. Anatomically, the inferior skin crease is seen around 3 to 5 mm below the outer aspect of the lid margin.
The inner aspect of the eyelid is called the inner canthal region. At this region runs a fold of skin called the nasojugal fold. From an anatomical point of view, this fold lies between the orbicularis oculi and the levator labii superioris. The nasojugal fold is that area of the inner aspect of the eye where tears flow, and is known as the tear trough. In addition, there is the malar fold which runs from the outer aspect towards the nasojugal fold.
When the eyes are open, the space between the upper and lower eyelids is typically described as ‘fusiform’. This space is also called the palpebral fissure, which measures between 28 to 30 mm wide and around 9 to 10 mm in height. If one were to examine the palpebral fissure, it would be evident that the highest point of the fissure lies at that point on the upper eyelid that corresponds to a point at the inner aspect of the pupil. Younger individuals have the upper eyelid slightly higher than older individuals.
There are two points at which the upper and lower eyelids meet. The one on the inner aspect is called the medial canthus while that at the outer aspect is called the lateral canthus. Both of these have a unique angle at which the upper and lower eyelids meet. When examined along a horizontal plane, the medial canthal angle is located around 2 mm lower than the lateral canthal angle in Caucasians; it is 3 mm lower in Asians. The nose lies around 15 mm on the inside of the medial canthus.
In a nutshell, the palpebral fissure consists of the medial and lateral canthus, the lacrimal papillae (part of the tear glands, also called lacrimal glands) and a small opening of the lacrimal glands through the lower eyelid at the medial canthus called the punctum lacrimale.
Skin and subcutaneous tissue
The eyelid is primarily made of skin. It is the thinnest skin in the body and is less than 1 mm thick. Within the skin are a number of glands called sebaceous glands that secrete an oily substance called sebum. These glands are in larger numbers at the nasal aspect of the eyelid. If one were to trace back the skin of the upper and lower eyelid, it would be clearly evident that once it joins the forehead or the cheek, the texture of the skin changes and becomes a lot thicker. The texture of skin is also different at the various folds described above. Below the skin is a layer of thin connective tissue called a subcutaneous tissue (sub = under, cutaneous = skin).
Underneath the skin, along with the subcutaneous tissue is a thin layer of fat, negligible in comparison to other parts of the body. Typically, subcutaneous tissue is absent at points where the skin is attached directly to underlying ligaments such as the medial and lateral palpable ligaments. The skin and subcutaneous tissue can be subject to certain clinical conditions such as dermatochalasis and blepharochalasis.
Orbicularis oculi muscle
The orbicularis oculi muscle plays an important part in the function of the eyelids and also in facial expressions. When it contracts and relaxes, the skin over the muscle tends to move as well. The orbicularis oculi muscle is attached to the skin through fibrous tissues that form what is called the superficial musculoaponeurotic system.
Broadly divided, the orbicularis oculi muscle consists of two main parts. The orbital part plays a role when the eyelids need to be tightly shut. It is further divided into pretarsal and preseptal segments. The other part is called the palpebral portion which helps in winking and blinking.
The orbital part of the orbicularis oculi muscle has a close relationship with other muscles responsible for facial expression. It originates from the inner margin of the orbit, further attaching to the upper and inner aspect of the orbital bone, the maxillary process that arises from the frontal bone, the lower and inner aspect of the orbital bone and the frontal aspect of the maxillary bone. The path taken by the muscle is typically described as ‘horseshoe shaped’. The muscle fibers mingle with the surrounding facial muscles such as the corrugator supercilii and the frontalis muscle.
Submuscular areolar tissue
Submuscular Areolar Tissue is a loose connective tissue that lies beneath the orbicularis oculi muscle. It can form an anatomical plane that divide the eyelid into a front (anterior) and back (posterior) portion. The fibers of the levator aponeurosis pass through this plane in the upper eyelid. A small portion of these fibers contribute towards the development of the upper eyelid crease. Similarly, in the lower eyelid, the fibers of the orbitomalar ligament passed through this plane. If this anatomical plane were to be tracked towards the eyebrow area, the retro-orbicularis oculi fat will be traversed. If the plane were to be tracked towards the cheek, the sub-orbicularis oculi fat would be traversed.
TARSI AND ORBITAL SEPTUM (TARSAL PLATES)
Dense fibrous tissue called tarsal plates help the eyelids maintain their shape and integrity. Each of these tarsi is around 1mm thick and 29 mm in length. There are 2 main types of tarsi – the superior tarsus and inferior tarsus. The superior tarsus is crescentic in shape and measures around 10mm vertically in its central aspect. It narrows outs as it traverses towards the nose and outer aspect of the eyelid. Its lower area is what forms the back of the eyelid that lies next to the conjunctiva of the eyeball. Similarly, the inferior tarsus lies in the lower eyelid, measures 3.5 – 5mm in height at its center, and also lies in contact with the conjunctiva. Each of the tarsi are attached to the margin of the orbits through the medial and lateral palpebral ligament. Within the tarsal plates are 25 tiny glands called mebomian glands. These glands are as tall as the tarsus, and they open at a point just in front of the lid margin where the conjunctiva meets the skin (mucocutaneous junction). If one were to look closely, they lie behind a grey line on the margin of the eyelid.
Medial palpebral ligament
The medial palpebral ligament is also called the medial canthal tendon (MCT), which is a band of fibrous tissue that holds the inner aspect of the tarsal plates in place. It is closely related to the orbicularis oculi muscle and the tear ducts. The MCT is composed of an anterior limb which is formed by a small part of the superficial aspect of the orbicular-is muscle that lies behind the tarsus. It traverses along a horizontal plane but is also attached to the frontal bone through a superior extension. The deeper part of the orbicularis muscle inserts into the back aspect of the lacrimal crest and the lacrimal sac fascia. The fascia of the lacrimal sac is therefore closely related to the various aspects of the MCT.
Lateral palpebral ligament
The lateral palpebral ligament is also called the lateral canthal tendon (LCT), and is a band of fibrous tissue that originates from the tarsus, travels outwards under the orbital septum and eventually inserts into the lateral orbital tubercle (this lies around 1.5mm behind the lateral orbital rim). The LCT is around 10.5mm long and 6.5 mm wide, and at its midpoint attaches around 10mm below the frontozygomatic suture. The orbital septum and the LCT are separated by a pocket of fat called the Eisler pocket. The LCT is attached to the outer part of the orbital rim through a superficial plane of fascia. This has been also called the superficial lateral canthal tendon and helps to keep the lateral canthus stable. When traced above and below, the LCT attaches to the lateral horn of the levator aponeurosis above while the lower aspect forms an arc where it attaches. During outer movement of the eye (abduction), the lateral canthal angle moves around 2mm as well, and this occurs due to the fibers that attach from the back of the lateral check ligament of the lateral rectus muscle.
There are a number of different fat pads that are present within and around the eyelid. One layer of fat called the pre-aponeurotic fat is found right behind the orbital septum and in front of the levator aponeurosis. Also within the upper eyelid are two more areas that contain fat pads that are centrally and medially (towards the nose) located. The medial fat pad is pale yellow in color and lies in front of the levator aponeurosis.
The central fat pad is broader and yellow in color. As it travels outwards, it wraps around the inner aspect of the lacrimal gland. The lacrimal gland can be clearly seen and differentiated from this fat by its pink color and lobulated structure. The lacrimal gland is positioned just posterior to the orbital margin but might possibly prolapse slightly making it more prominent when the eye is examined.
While the above described the fat pads within the upper eyelid, the lower eyelid fat pads are slightly different in structure. The inferior oblique muscle separates the central fat pad from the medial fat pad. There is a small amount of fat that lies in front of the inferior oblique muscle as well. The inferior oblique muscle originates from a small indentation in the lower border of the orbital floor, moving behind the orbital margin and at the upper aspect of the nasal lacrimal canal. It passes underneath the inferior rectus muscle and through the tenon capsule, ultimately located at the point close to the macula of the eye. This rather winding course of the inferior oblique muscle makes it vulnerable to injury around the eyelid and eye.
The eyelids are supplied by branches of the internal and external carotid arteries. The ophthalmic artery branches off the internal carotid artery and supplies blood to different parts of the eyelid. At the inner part of the upper eyelid, the ophthalmic artery splits into two and travels outwards. The branch that supplies the lower eyelid is a branch that arises from the superior marginal vessel (that supplies the upper eyelid). The superior and inferior marginal vessels that arise from the ophthalmic artery together form the marginal arcade.
The marginal arcade arteries are located at the front of the tarsus, 4 mm from the upper eyelid and 2 mm from the lower eyelid margin each. The superior marginal arcade gives rise to a peripheral arcade that runs in front of the Muller muscle, giving it a superficial plane and making it prone to injury during eyelid surgery. Another branch of the internal carotid artery is the lacrimal artery that passes through the orbital septum along each eyelid and ultimately joins the marginal arcade.
While the above described the branches of the internal carotid artery, the external carotid artery supplies the eyelids as branches of the facial artery, infraorbital artery and the superficial temporal artery.
The lymphatic drainage of the eyelid is rather extensive. The majority of the upper eyelid and the outer half of the lower eyelid drain into the pre-auricular lymph nodes, while a small part of the middle of the upper eyelid and the inner half of the lower eyelid drains into the submandibular lymph nodes.
There are numerous muscles around the eyeball that control of different movements. These muscles are called extraocular muscles. They include the medial rectus, lateral rectus, inferior and superior oblique and inferior and superior rectus muscles. These are responsible for the various directions of movement of the eyeball, including rotation of the eyeball. Within the eyelid, the levator palpebrae superioris is responsible for elevation of the upper eyelid.
The extraocular muscles are supplied by a variety of different cranial nerves. These include the oculomotor nerve, the trochlear nerve and abducens nerve.
The socket within which the eyeball is located is called the orbit. It is a pyramidal shaped fossa that is created by the fusion of different orbital bones. These bones originate from the different aspects of the skull such as the frontal bone, sphenoid bone, zygomatic bone and the palatine bones. The arrangement of these bones is such that the walls are parallel to each other. The orbit measures 4 cm in height, 3.5 cm in width and is around 5 cm in depth.
Within the orbit are a number of blood vessels and nerves. These pass through the bone through various openings called fissures. There are three main openings – the superior orbital fissure, the inferior orbital fissure and the optic canal. Through these openings, various cranial nerves pass through and supply the muscles and blood vessels in the orbit. The superior orbital fissure allows for the passage of the frontal nerve, lacrimal nerve, nasociliary nerve and the recurrent branch of the lacrimal artery along with the superior orbital and ophthalmic veins. The inferior orbital fissure allows for the passage of the infraorbital nerve, zygomatic nerve, infraorbital artery and vein and parasympathetic nerve supplying the lacrimal gland. Through the optic canal passes the optic nerve, central retinal vein and ophthalmic artery.
The lacrimal gland is responsible for tear production. It is divided by the levator aponeurosis into an orbital lobe and a palpebral lobe. It is supplied by the lacrimal nerve which is a branch of the ophthalmic division of the trigeminal nerve. The lacrimal gland secretes tears that are drained through a series of ducts. The lacrimal system consists of lacrimal papillae, canaliculi, lacrimal sac and naso-lacrimal duct.
The fascia that is present around the eyeball divides the orbit into a number of different connective tissue planes. Within each of these planes lie different structures. Having a knowledge of the structures helps the surgeon in locating them.