The rest of the orbital cavity is filled with orbital fat, muscles of the eyeball, the eyeball, the lacrimal gland, ophthalmic artery and its branches, and the ophthalmic vein and its tributaries.
We will start examining the contents of the orbit from above down.
|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:
|In this layer, we see three muscles:
posterior ethmoid (pe)
|By reflecting the superior rectus and the superior oblique, we can now examine the eyeball itself and adjacent items:
|Reflection of the eyeball and optic nerves brings us to the lowest items in the orbit:
|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.|
|In the adjacent figures, we point out the muscles that perform the various movements of the eye (The 3 axes are in black):
|The remaining muscle of the orbit, levator palpebrae superioris, raises the eyelid.|
|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?
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.
|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.|
|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:
The diagram shows a cut through the external, middle and inner ears.
Contents of the middle ear cavity:
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
|Bones of orbit
Foramina of orbit
Contents of orbit
|Floor of orbit
levator palpebrae superioris
|Cranial Cavity||Parotid & Infratemporal Regions|
|Copyright©1999 by Wesley Norman, PhD, DSc|