Eye anatomy

Bones of the orbit

The bony orbit is made out of seven bones, which include the maxilla, zygomatic bone, frontal bone, ethmoid bone, lacrimal bone, sphenoid bone and palatine bone. The orbit appears as a pyramid, with its base opening anteriorly onto the face, while the apex is pointed posteromedially. The orbit hosts the eyeballs, extraocular muscles, optic nerve, lacrimal apparatus, fat tissue, fascia and vessels that supply these structures.

The walls of the orbit contain several fissures, openings and fossae which are important for housing the orbital structures, and for neurovascular communication of the orbital content with the central nervous system. Some of the most important openings are the superior orbital fissure and its downstairs neighbour, the inferior orbital fissure. More about them and all the other landmarks of the orbital bones can be found in the following article and quizzes.

Eyelid anatomy

The eyelids are soft tissue structures that cover and protect the anterior surface of the eyeball. The anatomy of the eyelid may seem complex, but if we dissolve its multi-layered structure it is actually quite simple: 

• Skin
• Subcutaneous tissue
• Muscle - orbital part of the orbicularis oculi muscle
• Orbital septum - extensions of the periosteum from the orbital margin, that extends through both eyelids and supports them; 
• Tarsus - plates of the dense connective tissue present in both eyelids. Superior tarsus is associated with muscles that raise the upper eyelid.
• Conjunctiva - thin membrane that covers the posterior surface of the eyelid and reflects onto the anterior surface of the eyeball. 

Lacrimal gland

The lacrimal gland is a part of the lacrimal apparatus, which besides the gland consists of its numerous ducts, the lacrimal canaliculi, lacrimal sac and nasolacrimal duct. Everyone who has cried at least once, especially because of sleep deprivation that medical college brings, probably thinks that the lacrimal gland is placed in the medial part of the orbit since the tears run from there. But actually, the gland is placed in the lacrimal fossa at the upper part of the superior orbital wall, and its duct opens at the medial part of the orbit causing the tears to run down our nose when we think about how many anatomy pages we have left and it’s already 5 AM.

So, the function of the lacrimal gland is the production of tears, and it is regulated by the lacrimal nerve, a branch of the ophthalmic nerve (CN V1). Everything about the anatomy of the lacrimal gland and lacrimal apparatus can be found in these learning materials. We made them in a fun and approaching way, so we promise you won't cry: 

Eye muscles

There are two groups of eye muscles:

• Extraocular muscles that move the eyeballs within the orbit
• Intrinsic ocular muscles which are within the eyeball itself and control how the eyes accommodate.

Six extraocular muscles move the eye: superior rectus, inferior rectus, medial rectus, lateral rectus, superior oblique and inferior oblique muscles; and one other, levator palpebrae superioris, opens the eyelid. 
 

Don’t understand how all these muscles work? You can find out everything about them in the following learning materials.

Eyeball

The eyeball is the main structure within the orbit because it enables us to see. Its importance reflects on its size, making it the largest structure in the orbit. It is something like a little camera inside our head, with all the necessary parts for capturing an image. It is round with an anteriorly convex bulge. The most superficial layer of the convexity is the cornea, which refracts and focuses the details of the image. Posterior to the cornea are the anterior chamber, lens, vitreous chamber and the retina.

You have probably noticed that there is something in the eye called ‘the lens’, which is also present inside our digital cameras. It has the same function as a digital lens, but in the eyeballs it helps in focusing the light onto the retina.

The wall of the eyeball has three layers: an outer (fibrous) layer, a middle (vascular) layer and inner layer.

Let us have a closer look at the individual layers of the eyeball and their components.