The Orbit

The orbit can be studied from the front or from the top. In most dissection laboratories, the roof of the orbit is broken away and the orbital structures are cleaned and studied. First lets take a look at the bones and foramina of the orbit since these may be referred to later during the lesson.

Bones of the Orbit Viewed From the Front

In the images below, you are looking at the left orbit and the medial side of the orbit is to the left.
Bones
  1. frontal
  2. zygomatic
  3. maxilla
  4. lacrimal
  5. ethmoid
  6. greater & lesser wing of sphenoid
  7. palatine
Orbital foramina
  1. optic
  2. superior orbital fissure
  3. inferior orbital fissure
  4. posterior ethmoid
  5. anterior ethmoid
Structures that pass through the foramina are:

Contents of the Right Orbit from Top to Bottom
When the frontal bone is chipped away and the orbit is entered, the first thing you will see is a connective tissue structure enclosing the eyeball and its muscles, vessels and nerves, the periorbita. This is a glistening structure.
After the periorbita is stripped away, you will then encounter a large amount of fat filling the structures in the orbit. Once this fat is cleaned away, you will find the following structures:
  • trochlear nerve (T)
  • frontal nerve (F)
    supraorbital nerve (SO)
    supra trochlear nerve (ST)
  • lacrimal nerve (L)
  • lacrimal gland (Lac)
  • levator palpebrae superioris (LPS)
After these structures are identified, the levator palpebrae superioris is reflected forward and the next layer of items is revealed.
In this layer, we see three muscles:
  • superior oblique (SO)
  • superior rectus (SR)
  • lateral rectus (LR)
and the nasociliary nerve (nc) anterior ethmoid (ae)
posterior ethmoid (pe)
By reflecting the superior rectus and the superior oblique, we can now examine the eyeball itself and adjacent items:
  • optic nerve (ON)
  • short ciliary nerves (sc)
  • medial rectus muscle (MR)
  • ophthalmic artery (red)
  • superior ophthalmic vein (blue)
  • eyeball
Reflection of the eyeball and optic nerves brings us to the lowest items in the orbit:
  • ophthalmic artery (OA)
  • ophthalmic vein (OV)
  • ciliary ganglion (cg)
  • inferior division of oculomotor nerve (ION) (with cg attached)
  • inferior rectus muscle (IR)
  • inferior oblique muscle (IO)
Finally, we reach the orbital floor which is made up of the zygomatic and maxillary bones. A sulcus can be seen in the maxillary bone and this holds the infraorbital nerve (ION). The zygomatic bone has a zygomatic foramen which accepts the zygomatic branch of the infraorbital nerve (Z). The infraorbital nerve is a branch of the maxillary  division (V2) of the trigeminal nerve. It enters the orbit through the inferior orbital fissure.
Once the structures in the orbit are learned, you should then try to understand how the muscles act on the eye to move it. This knowledge is necessary in clinical situations when you examine the eye during a full physical examination or after head injuries. In order to fully understand the movements of the eyeball you should realize that the eyeball moves around three axes: (1) vertical, (2) horizontal and (3) anteroposterior. Movements around the vertical axis are abduction and adduction, around the horizontal axis, elevation and depression and around the anteroposterior axis, medial and lateral rotation. This last movement usually requires the use of special equipment to see it, so we won't consider it at this time.

Movements of the Right Eye as Viewed from Above

In the adjacent figures, we point out the muscles that perform the various movements of the eye (The 3 axes are in black):
  • Elevation is movement of the eye around the horizontal axis so that it is looking upward. This movement is performed by a pair of muscles:
  • superior rectus (SR)
  • inferior oblique (IO)
  • Depression is movement of the eye around the horizontal axis so that it is looking downward. This movement if performed by a pair of muscles:
  • inferior rectus (IR)
  • superior oblique (SO)
  • adduction is movement of the eye around the vertical axis so that it is looking toward the nose. There is only one muscle that originates this movement.
  • medial rectus (MR)
  • abduction is movement of the eye around the vertical axis so that it is looking laterally. The muscles that perform this action are:
    • lateral rectus (LR)
    • superior oblique (SO)
    • inferior oblique (IO)
    • superior rectus (SR)
    • inferior rectus (IR)
The remaining muscle of the orbit, levator palpebrae superioris, raises the eyelid.

Table of  Muscles of the Orbit and Their Nerve Supply and Actions

Muscle Action Nerve Supply
levator palpebrae superioris raises the eyelid upper division of oculomotor nerve (III)
superior rectus elevates and abducts eyeball upper division of oculomotor nerve (III)
superior oblique depresses and abducts eyeball trochlear nerve (IV)
medial rectus adducts the eyeball lower division of oculomotor nerve (III)
lateral rectus abducts the eyeball abducens nerve (VI)
inferior rectus depresses and abducts eyeball lower division of oculomotor nerve (III)
inferior oblique elevates and abducts eyeball lower division of oculomotor nerve III)
A question to consider:  If you look into a person's eye and one of them is in a down and out position, which of the cranial nerve is probably injured? Check out the pupil. Is it dilated or constricted? Consider that the lower division of the oculomotor nerve (III) carries parasympathetic nerve fibers to the ciliary ganglion where they synapse on second neurons that travel as the short ciliary nerves to constrictor muscles of the pupil. When these are active, the pupil will constrict.
Another question to consider:  If you are performing an eye examination and are asking a person to follow you finger from the nose laterally to his/her left and the persons left eye follows your finger fine but the right eye stops and looks straight forward, which muscle might not be functioning?

Examination of Individual Muscles

What if you want to try to isolate the movement of  only one of the muscles to see if it is functioning properly? In this case, take the superior oblique as an example, have the person move their eye until the AP axis of the eyeball is parallel to the direction of pull of the muscle, in this case superior oblique. To make the AP axis parallel to the oblique part of the muscle, have the person look medially.  Now ask the person to look down. If this can be done, the muscle is functioning properly and its nerve supply must be intact.
Try the inferior oblique. Again have the person look  medially first, then up. This will test the inferior oblique and if it works properly, shows that the inferior division of the oculomotor nerve is intact. The point here is that there are two ways to look at the functionality of the muscles that move the eye: (1) have the individual follow your finger in all directions. This tests groups of muscles and their intactness; (2) check the individual muscle for functionality.

Arteries of the Orbit

The only artery supplying structures in the orbit is the ophthalmic artery which is a branch of the internal carotid just before that artery enters into the formation of the Circle of Willis. It enters the orbit with the Optic nerve through the optic foramen. It gives rise to the very important branch, the central artery of the retina.

The Ear

Dissection of the ear is difficult in the lab and usually takes too much time in most anatomy courses, but, you should have some understanding of what makes up the ear.

The ear is usually broken up into three parts:

  • External ear
    • auricle
    • external auditory meatus
  • Middle ear (or tympanum, tympanic cavity)
    • tympanic membrane
    • tympanic cavity & antrum
    • pharyngotympanic tube (or auditory tube, or Eustachian tube)
  • Internal (inner) ear
    • osseous labyrinth
    • membranous labrynth


External Ear
  1. concha
  2. crus helix
  3. helix
  4. scaphoid fossa
  5. antihelix
  6. antitragus
  7. tragus
lobule not labeled
Middle ear

The diagram shows a cut through the external, middle and inner ears.
The middle ear cavity is like a small box with lateral, medial, anterior and posterior walls, a floor and a roof. This box is located inside the temporal bone. Items to identify are:

  • mec-middle ear cavity
  • ptb-temporal bone,
  • eam-external auditory meatus,
  • ec-ear cartilage,
  • tm-tympanic membrane.
    Medial wall
  • promontory with its tympanic plexus of nerves
    Lateral wall
  • tympanic membrane (tm)
    Anterior wall
  • pharyngotympanic tube (at)
  • sulcus for internal carotid artery
    Posterior wall
  • opening into mastoid air cells
    Roof
  • tegmen tympani of petrous temporal bone ptb
    Floor
  • sulcus for internal jugular vein (not shown)

Contents of the middle ear cavity:

  • malleus (m)
  • incus (i)
  • stapes (s)


Internal Ear

The internal or inner ear is made up of an osseous part (ol) which encloses a membranous part (ml), in blue on the image. The membranous part includes the vestibule and semicircular canals for equilibrium and the cochlea for sound.

If visualizing the middle ear cavity in three dimensions is difficult, print out the following image and make the proper folds. The associated structures are listed and numbered.
Middle ear cavity
  1. canal for tensor tympani muscle
  2. auditory tube
  3. internal carotid artery
  4. facial nerve
  5. geniculate ganglion
  6. greater petrosal nerve
  7. lesser petrosal nerve
  8. promontory formed by 1st turn of cochlea
  9. tympanic plexus
  10. round window
  11. lateral semicircular canal
  12. facial canal and facial nerve
  13. oval window for stapes
  14. tympanic nerve from IX
  15. jugular vein
  16. opening to mastoid air cells (aditus ad antrum)
  17. tendon of stapedius muscle
  18. malleus
  19. tympanic membrane
  20. chorda tympani
  21. stylomastoid foramen

List of items to identify in this lesson

Bones of orbit

frontal
zygomatic
greater wing of sphenoid
maxillary
lacrimal
ethmoid (orbital plate)
palatine
lesser wing of sphenoid

Foramina of orbit

optic foramen
anterior ethmoidal foramen
posterior ethmoidal foramen
superior orbital fissure
inferior orbital fissure
zygomatic foramen

Contents of orbit

optic nerve
eyeball
ophthalmic artery
central artery of retina
superior ophthalmic vein
ophthalmic division of V
frontal
supraorbital
supratrochlear
lacrimal
nasociliary
anterior ethmoidal
posterior ethmoidal (very small)
infratrochlear
trochlear nerve
abducens nerve
oculomotor
upper division
lower division
ciliary ganglion

 

Floor of orbit

infraorbital nerve
zygomatic nerve

Muscles

levator palpebrae superioris
superior rectus
superior oblique
medial rectus
lateral rectus
inferior rectus
inferior oblique

External Ear

concha
crus helix
helix
scaphoid fossa
antihelix
antitragus
tragus
lobule not labeled

Middle ear

malleus
incus
stapes

 

Cranial Cavity   Parotid & Infratemporal Regions


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