Anatomy of the Eye. Visual system

In this note, we are going to discuss the anatomy and function of the eye, in particular its accessory structures.

The human eye is a sensory organ essential for vision, composed of various anatomical structures (the eyeball and its accessory structures) that function together to capture and process light, providing our visual perception.

Accessory structures

Let’s start with the accessory (or external) structures of the eye. These include the eyelids, the superficial epithelium of the eye, and the lacrimal apparatus. These structures are associated with protecting, lubricating, and supporting of the eye.

Eyelids and Eyelashes

The eyelids are movable skin folds that cover and protect the anterior surface of the eyeball.

They help to keep the eyes moist by spreading tears over the surface and remove debris through blinking.

The palpebral fissure is the gap between the upper and lower eyelids.

The upper and lower eyelids are connected at two angles: the medial angle (or medial canthus)

and the lateral angle (or lateral canthus).

The eyelashes, located along the margins of the eyelids, help to filter dust and other foreign particles by triggering the blinking reflex.

The eyelids have a multi-layered structure, and each layer is called a lamella. There are three lamellae:

Anterior lamella. It is a highly vascularized layer, consisting of skin and the underlying orbicularis oculi muscle.

The skin and subcutaneous tissue contain the ciliary (glands of Moll) and sebaceous glands (glands of Zeis)

Middle lamella. It consists of the so-called orbital septum and adipose tissue, providing structural support

Posterior lamella. It is an inner portion of the eyelid, lined with mucosa. It consists of the tarsal plates (superior or inferior),

the conjunctiva,

the superior eyelid muscle (levator palpebrae superioris)

and superior or inferior tarsal muscle.

The tarsal plates are the structures made of dense connective tissue.

They contain the eyelash follicles, associated with large sebaceous glands, known as tarsal glands (or Meibomian glands). These glands secrete a lipid-rich substance that prevents the eyelids from sticking together. At the medial corner of the eye, glands embedded within the so-called lacrimal caruncle, produce thick secretions that can accumulate along the eye’s edge, forming gritty deposits, especially after sleep.

The conjunctiva

is a layer of epithelium covering the inner surface of the eyelids (palpebral part)

and the eyeball (bulbar or ocular part).

The spot where the conjunctiva of the eyelid connects with the conjunctiva of the eye is called the fornix.

Lubrication is provided by mucous cells within the epithelium and various accessory glands, preventing friction and drying of the conjunctival surfaces.

Lacrimal Apparatus

The lacrimal apparatus is essential for the production, distribution, and drainage of tears, which serve various functions.

Primarily, tears provide crucial protection for the eyes. They also act as a lubricant, facilitating smooth eye movement. Furthermore, they are responsible for maintaining a thin, transparent layer on the surface of the conjunctiva. In addition to these practical functions, tears also play an important emotional role.

The lacrimal apparatus of each eye consists of a lacrimal gland,

superior and inferior lacrimal canaliculi,

a lacrimal sac,

and a nasolacrimal duct.

The lacrimal gland

is located in the lacrimal fossa at the superior and lateral part of the orbital roof.

The gland is divided into two sections: a smaller palpebral portion

and a larger orbital portion.

The fornix, mentioned previously, receives 10–12 ducts from the lacrimal gland.

The tears mix with products of accessory glands and the oily secretions of the tarsal glands and sebaceous glands, forming a tear film that covers the ocular surface.

Tears are swept across the eye surface by blinking, accumulating at the medial angle within the so-called lacrimal lake.