Human Anatomy Lesson 18
Orbit, Eye, Ear
In this lesson, you will be learning details about the orbit, eye, and ear. We covered the bones that make up the orbit in lesson 14, and we will review those briefly here. In addition, you know some of the nerves that enter the orbit, since we studied the cranial nerves that enter the orbit through the superior orbital fissure. In this lesson, you will learn the fate of those nerves once they enter the orbit. We will spend a good portion of the rest of the lesson on the contents of the orbit, which include the eyeball, muscles that move the eyeball, and the arteries, veins, and nerves that supply the orbit. In the second part of the lesson, we will briefly consider the gross anatomy of the ear cavity.
This lesson really blurs the lines between anatomy and physiology, since we talk so much about how the structure of the eyeball affects vision. This is a very good review of clinical anatomy and physiology of the visual system.
Way more than you ever wanted to know about the middle ear. That said, there are so many clinical problems that people have in this region that a book like this is necessary.
Learning Objectives - By the end of this lesson, you should be able to ...
- List the bones that are in each wall of the orbit
- List and briefly describe the superficial structures of the orbit, including the tarsal plates, fascia and ligaments, and lacrimal structures
- List and describe the origins, insertions, innervation, and actions of the muscles that move the eyeball
- Trace the arterial supply and venous drainage of the eyeball and orbit
- List and describe the nerves that provide somatic motor, somatic sensory, and autonomic innervation of the eyeball and structures in the orbit
- List and briefly describe the main structures in the external, middle, and internal ear
- Describe how hearing and balance work
Bones of the Orbit
As noted above, we covered the bones of the orbit previously - here are some additional details about which bones make up which parts of the orbit:
- Roof: largely made up of the frontal bone, with a minor contribution from the lesser wing of the sphenoid bone. The roof of the orbit is also the floor of the anterior cranial fossa.
- Medial wall: made up of the maxilla, lacrimal, ethmoid, and sphenoid bones.
- Floor: largely made up of the maxilla and zygomatic bones, with minor contribution from the perpendicular plate of the palatine bone. The floor of the orbit is the roof of the maxillary sinus.
- Lateral wall: made up of the zygomatic bone and the greater wing of the sphenoid.
The main content in the orbit is, of course, the eyeball. However, the eyeball does not fill the entire orbit, and there is plenty of space for other structures. Most of the rest of the orbit is filled with fat and muscles, arteries, veins, and nerves, all of which we will cover in this lesson. The orbit also contains the following structures, which we will review briefly in this section:
- Eyelids: made up of superior and inferior tarsi (sing.=tarsus), which are the ligaments that make up the structure of the eyelids. The posterior surface of the tarsi are covered by conjunctiva, which reflect onto the sclera of the eyeball. The tendon for the levator palpebrae superioris muscle inserts onto the superior tarsus, and acts to raise the eyelid. Fibers of the orbicularis oculi muscle attach onto the tarsi and act to close the eyelids. Sensory innervation to the eyelids is via the supra-orbital, supratrochlear, infratrochlear, and lacrimal branches of CN V1, in addition to the infra-orbital branch of CN V2. Motor innervation is via branches of CN VII (for orbicularis oculi) and CN III (for levator palpebrae superioris).
- Lacrimal apparatus: including all the structures that produce tears and fluid that coat the surface of the eyeball - the lacrimal gland, duct, lake, etc.
Openings Into the Orbit
- Optic canal: passage between the middle cranial fossa and orbit through which the optic nerve and ophthalmic artery run.
- Superior orbital fissure: passage between the orbit and middle cranial fossa through which run the following structures: superior and inferior branches of CN III (oculomotor nerve); CN VI (abducent nerve); lacrimal, frontal, and nasociliary branches of CN V1 (ophthalmic nerve); and the superior orbital vein.
- Inferior orbital fissure: passage between the pterygopalatine fossa and orbit at its posterior end and temporal and infratemporal fossae and orbit posterolaterally, through which runs CN V2 (maxillary nerve), infra-orbital vessels, and a vein communicating with the pterygoid plexus.
- Infraorbital foramen/groove: foramen and groove in the anterior aspect of the orbit through which the infra-orbital nerve and a branch of CN V2 (maxillary nerve) run.
Tissues Surrounding the Eyeball
- Periorbita: tissue continuous with the dura mater that lines the orbit, and contributes to the tendinous ring at the rear wall of the orbit.
- Fascial sheath: tissue that encloses most of the eyeball and attaches to the sclera near the cornea. The extraocular muscles that move the eyeball attach to the fascial sheath.
- Suspensory ligament: sling-like specialized lower part of the fascial sheath that suspends the eyeball in the middle of the orbit.
- Check ligaments: ligaments of the medial and lateral rectus muscles that attach to the walls of the bony orbit, and are continuous with the fascial sheath and suspensory ligament.
Muscles that Move the Eyeball
The muscles that move the eyeball are called the extraocular muscles.
- Superior rectus muscle: runs from the common tendinous ring to the anterosuperior aspect of the eyeball. Innervated by CN III. Elevates, AD-ducts, and medially rotates the eyeball.
- Inferior rectus muscle: runs from the common tendinous ring to the anteroinferior aspect of the eyeball. Innervated by CN III. Depresses, AD-ducts, and laterally rotates the eyeball.
- Medial rectus muscle: runs from the common tendinous ring to the anteromedial aspect of the eyeball. Innervated by CN III. AD-ducts the eyeball.
- Lateral rectus muscle: runs from the common tendinous ring to the anterolateral aspect of the eyeball. Innervated by CN VI. AB-ducts the eyeball.
- Superior oblique muscle: runs from the body of the sphenoid superior and medial to the optic canal to the posterosuperior surface of the eyeball. Innervated by CN IV. Depresses, AB-ducts, and medially rotates the eyeball. The tendon of the superior oblique muscle runs under the tendon and part of the belly of the superior rectus muscle.
- Inferior oblique muscle: runs from the medial floor of the orbit posterior to the orbital rim to the lateral side of the eyeball. The inferior oblique muscle runs over the tendon of the inferior rectus muscle.
Arteries in the Orbit
The arterial supply of the orbit and eyeball is via the ophthalmic artery, which is a branch of the internal carotid artery. To remember their distribution, it may be useful to understand that they can be split into lateral (lacrimal), central (central retinal, long and short posterior ciliary, supra-orbital) and medial (posterior and anterior ethmoidal, dorsal nasal, supratrochlear) groups of arteries.
- Lacrimal artery: the lateral-most branch of the ophthalmic artery; runs along the lateral wall of the orbit to supply the lacrimal gland and other lacrimal structures.
- Central retinal artery: runs in the optic nerve; supplies blood to the optic nerve and retina.
- Long and short posterior ciliary arteries: in the center of the orbit, enter the eyeball directly.
- Supra-orbital artery: runs on the medial side of the eyeball and just medial to the optic nerve, and leaves the front of the orbit. Supplies the orbit, forehead, and scalp.
- Posterior and anterior ethmoidal arteries: branch off the ophthalmic artery to run medially into the ethmoid, where they supply ethmoid air cells.
- Dorsal nasal and supratrochlear arteries: two medial terminal branches of the ophthalmic artery that supply blood to the upper nose and forehead.
Veins in the Orbit
The distribution of veins in the orbit is considerably simpler than the distribution of arteries - there are really just two main branches. They meet up with each other and with the infra-orbital vein to drain into the cavernous sinus:
- Superior ophthalmic vein: forms from the supraorbital and angular veins.
- Inferior ophthalmic vein: offshoot of the angular vein.
Innervation of the Eyeball and Orbit
There are several cranial nerves which innervate structures in the orbit, including CN III, CN IV, CN VI, and branches of CN V1. We will look at these cranial nerves and their branches in more detail below:
- Oculomotor nerve (CN III): innervates levator palpebrae superioris, superior rectus, medial rectus, inferior rectus, and inferior oblique muscles. This nerve also supplies a branch to ciliary ganglion, which carries preganglionic parasympathetic fibers to the ciliary ganglion. These preganglionic fibers synapse with postganglionic fibers that innervate the sphincter pupillae and ciliary muscles of the eye.
- Trochlear nerve (CN IV): innervates the superior oblique muscle. It may help you remember this muscle if you remember that the pulley through which the tendon of the superior oblique muscle runs is called a trochlea.
- Abducent nerve (CN VI): runs just lateral to the common tendinous ring, innervates the lateral rectus muscle.
- Lacrimal nerve: this small branch of the ophthalmic nerve (CN V1) runs along the superior border of the lateral rectus muscle, and supplies the lacrimal gland, conjunctiva, and lateral part of the upper eyelid.
- Frontal nerve: this large branch of the ophthalmic nerve (CN V1) runs outside the tendinous ring between levator palpebrae superioris and periorbita in the roof of the orbit, and divides into the supra-orbital nerve and supratrochlear nerve, which provide sensory innervation to the face anterior to the orbit.
- Nasociliary nerve: this branch of the ophthalmic nerve (CN V1) runs inside the tendinous ring in a medial direction inferior to the superior rectus muscle. It gives off a communicating branch with ciliary ganglion, then the following branches: long ciliary nerves, posterior ethmoidal nerve, infratrochlear nerve, and anterior ethmoidal nerve.
Parts of the Ear
The ear can be split into the external ear, middle ear, and internal ear.
- External ear: made up of the auricle and pinnae of the ear and the external acoustic meatus, which ends in the tympanic membrane. The tympanic membrane is the boundary between the external and middle ear. Most of the external ear is surrounded by cartilage, fat, and other tissue, but the last 1/3 is encased in temporal bone, forming a real canal.
- Middle ear: one wall of the middle ear is made up of the tympanic membrane, and the opposite wall is formed from petrous temporal bone on the opposite side of the inner ear. The epitympanic recess rises above the level of the external acoustic meatus in the petrous temporal bone, and it houses the main parts of the malleus and incus, two of the three bones in the middle ear. The other bone, the stapes, is positioned lower in the middle ear, and the base of the stapes fits into the oval window, which is adjacent to the vestibule of the inner ear. The round window is in the inferior part of the middle ear, and it is adjacent to the cochlea. The inferior part of the middle ear is open to the pharyngotympanic tube, which opens into the pharynx. This tube used to be called the eustachian tube, and many of you may still know it by this name.
- Inner ear: the inner ear houses the structures related to balance and hearing. The semicircular canals, utricle, and saccule are related to balance, and the cochlea and cochlear duct are related to hearing. The vestibulocochlear nerve (CN VIII) splits into two nerves, the vestibular nerve which runs to the organs of balance and the cochlear nerve which runs to the organs of hearing. The organs of balance and hearing are connected to one another, since the cochlear duct terminates in the saccule.
Accessory Muscles of Hearing
Two muscles aid in hearing:
- Tensor tympani: runs from the cartilaginous part of the pharyngotympanic tube and the greater wing of the sphenoid to the handle of the malleus, and pulls the malleus to tense the tympanic membrane and dampen sound hitting the membrane. It is innervated by CN V3.
- Stapedius: runs from the "pyramidal eminence" inside the middle ear to the neck of the stapes, and pulls the stapes posteriorly to prevent excessive oscillation. It is innervated by CN VII.
Hearing and Balance
Hearing: Sound waves enter the external ear through the external acoustic meatus and hit the tympanic membrane, which vibrates, thereby moving the handle of the malleus. Loud noises produce excessive vibration of the tympanic membrane, and in these cases the tensor tympani muscle fires, pulling on the handle of the malleus and tensing the tympanic membrane to control such excessive vibrations. The malleus and incus are connected to one another in a pivot-like configuration, so movements of the malleus move the incus. The incus is connected to the stapes, and movements of the incus cause the stapes to pound into the oval window, which is a membranous structure that opens into the vestibule of the inner ear. Excessive vibration of the stapes at the oval window is controlled by the stapedius muscle, which attaches to the neck of the stapes. Note that sound waves moving through the external ear have been transformed into mechanical vibrations in the middle ear. These vibrations move through the cochlear duct and "wind" through the cochlea to the center, at which point they move in the opposite direction, or "unwind," until they reach the round window, which works to dissipate energy into the tympanic cavity of the middle ear. Mechanical vibrations are turned into electrical signals in the cochlea, and these signals pass through the cochlear nerve and back into the brain for processing.
Balance: the three semicircular canals function as a gyroscope to control pitch, roll, and yaw. Lymph flows through the semicircular canals and the utricle and saccule, and is very sensitive to the movements of the body through space. Branches of the vestibular nerve attach to the ampullae at the end of each semicircular canal, and transmit proprioceptive information back to the brain.
This is our last lesson on the head - next we turn to the neck, and after that we move on to the upper limb.
- Human Anatomy Lesson 19
Lesson #19 covers the neck
- Human Anatomy Lesson 20
Lesson #20 covers the pharynx and larynx
- Human Anatomy Lesson 21
Lesson #21 covers the shoulder, posterior scapular region, and axilla
- Human Anatomy Lesson 22
Lesson #22 covers the arm
- Human Anatomy Lesson 23
Lesson #23 covers the forearm, wrist, and hand