The lacrimal system and adnexa

Summary

The eyelids play an important role in protecting the globe and the adnexal structures are crucial in producing clear vision.

Function of the eyelids

💡 The skin covering the eyelids is the thinnest skin on the body, at around 0.5 mm thick and contains no subcutaneous fat.

The eyelids protect the globe and help to lubricate the eyes by spreading tear film over them every time we blink. They also contain meibomian glands which provide the lipid component of tear film, prevent the eyes from drying by blinking and contain puncta through which tears drain into the lacrimal system.

The inner parts of the eyelids are supported by tarsal plates (see diagram), which are fibrous layers which gives the eyelids its shape and provides a place for the eyelid muscles to attach.

There are two tarsal plates; one on the upper eyelid called the superior tarsus and one on the lower eyelid called the inferior tarsus.

At the medial end of each tarsus, the medial palpebral ligament is formed (see diagram). This ligament crosses over the lacrimal sac (which we will discuss later), to attach to the maxilla (which we will also discuss later).

The medial palpebral ligament is also sometimes called the tendo oculi and is around 4 mm in length and 2 mm in width.

The glands of the eyelashes

Image courtesy of Lecturio

The eyelashes contain two types of glands:

• The glands of Zeis

• The glands of Moll

The glands of Zeis are sebaceous (oil) glands and the glands of Moll are apocrine (sweat) glands. The secretions produced by these two glands protect the surface of the eyelid.

There is another type of gland called the meibomian glands. They are also sometimes known as the tarsal glands, since they are located in the tarsal plate, between the eyelashes and the bulbar conjunctiva.

There are approximately 20 to 30 meibomian glands on the lower lid and 40 to 50 on the upper lid.

The meibomian glands secrete a mixture of oil and fat onto the surface of the eye. The secretions produced by the meibomian glands functions to decrease the evaporation of the tear film.

Movement of the eyelids

The movements of the eyelids are controlled by two muscles which attach onto the superior tarsus.

The first muscle can be seen in the diagram above and is the levator palpebrae superioris. The levator palpebrae superioris opens the eyelid and is supplied by CN 3.

The other muscle surrounds the eyes under the skin of the eyelids on top of the tarsus and surrounding structures and is called the orbicularis oculi (see diagram). The orbicularis oculi closes the eyelid and is supplied by CN 7.

You can also see the edge of the medial palpebral ligament arising through the orbicularis oculi muscle.

The orbicularis oculi muscle has a palpebral part and an orbital part as shown in the diagram.

There is also another muscle worth mentioning, the Müller’s muscle, which is also sometimes referred to as the superior tarsal muscle. The Müller’s muscle arises from the deep surface of the levator palpebrae superioris muscle and inserts onto the superior tarsus (see diagram), hence the name.

Just like the levator palpebrae superioris, the Müller’s muscle also functions to open the eyelid. However, unlike the levator palpebrae superioris, the Müller’s muscle is a smooth muscle innervated by the sympathetic nervous system.

So, the two muscles that elevate or open the eyelid are the levator palpebrae superioris (skeletal muscle supplied by CN 3) and the Müller’s muscle (smooth muscle supplied by the sympathetic nervous system).

Clinical correlates - ptosis

💡 Damage to the nerves supplying either of the two eyelid elevator muscles can cause drooping of the upper eyelid. This is known as ptosis. Ptosis can also occur in old age as the part of the levator palpebrae superioris which inserts onto the tarsus (aponeurosis) weakens with age.

It is worth knowing that the levator palpebrae superioris is a stronger eyelid elevator than the Müller’s muscle. This means that a CN 3 nerve lesion affecting the levator palpebrae superioris may cause a complete ptosis, whereas a sympathetic lesion affecting the Müller’s muscle may cause just a partial ptosis (see diagram).

Clinical correlates - Bell's palsy

A lesion in CN 7 due to Bell’s Palsy, for example, may affect the orbicularis oculi muscle which closes the eyelid. This means that the patient won’t be able to close their eye.

The lacrimal system

The lacrimal system describes the pathway through which tears drain. Tears are produced by the lacrimal gland, located above each eyeball. The tears then drain through the tear drainage system.

The tear drainage system consists of:

• The lacrimal puncta (small openings in the medial margin of each lid)

• The lacrimal canaliculi

• The common canaliculi

• The valve of Rosenmuller

• The lacrimal sac

• The nasolacrimal duct

• The valve of Hasner

Finally, the tears drain into the inferior nasal meatus. Each blink helps to pump tears through this tear drainage system.

Function of tears

Each blink coats the surface of our eyes with tear film. The tear film is replenished with each blink as tears drain through the lacrimal drainage system. Tears serve several important functions:

They create a smooth surface on the eye

Tears smooth out the surface of our eyes so that light is refracted without distortion at the cornea, allowing us to see clearly.

They prevent dry eyes

Tears lubricate our eyes by coating the surface of our eyes, as well as protecting it from external irritants.

They supply oxygen and nutrients to the eyes

The cornea is avascular, so oxygen and nutrients are transported to the surface cells of the eyes via tears.

They prevent infection

Foreign bodies and debris that enter our eyes are washed away by tears. Also, tears contain lysozyme, lactoferrin, defensins and immunoglobulins, particularly IgA which are antibacterial in function.

The structure of tears

The make-up of the tears are important since they must not evaporate as soon as we have finished blinking, otherwise our eyes would dry out pretty quickly. This property of the tear film to be maintained for a while until the next blink is called tear stability.

The tear film is composed of three different layers from the outside in to ensure tear stability and to allow the different functions of tears: lipid layer, aqueous layer and mucin layer (see diagram below).

The lipid layer

The lipid layer is secreted by the Meibomian glands. As mentioned above, these glands secrete oil, which coats the surface of our eyes and keeps the water component of our tears from evaporating.

Disruption in the function of the Meibomian glands can result in the tear film evaporating too quickly. This may happen if the openings of the Meibomian glands become blocked or the composition of the oily substance changes. This may occur with age and can result in decreased tear stability, leading to dry eye syndrome.

The aqueous layer

This layer makes up the majority of the tear film. It performs the key functions of tears mentioned above.

The mucin layer

Mucin is secreted by the goblet cells on the surface of the eye, which helps distribute tears across the surface of the eye evenly. Mucin plays an important role in tear stability.

Sources

1. Mass, Mark B. Abelson, MD, CM, FRCSC, FARVO, George Ousler, Aron Shapiro and David Rimmer, Andover. The Form and Function of Meibomian Glands. https://www.reviewofophthalmology.com/article/the-form-and-function-of-meibomian-glands. Accessed 27 June 2022.