Lab 6 The Orbit A. Bony Landmarks 1. Refer to the skull and identify: a. Maxillary bone b. Zygomatic bone c. Frontal bone d. Lacrimal bone e. Ethmoid bone f. Sphenoid bone g. Palatine bone h. Optic canal i. Superior orbital fissure j. Inferior orbital fissure k. Infraorbital groove l. Cribriform plate B. Orbit dissection, superior approach The eyeball occupies the anterior half of the bony orbit and the posterior half is largely filled with fat and the extraocular muscles. 1. With a chisel or bone rongeurs, carefully remove the roof of the orbit as far anteriorly as possible, but leave the superior orbital margin intact. The tough membrane just inferior to the bone is the periorbita which envelops the orbital contents. Expose the frontal air sinus and the anterior and posterior ethmoid air cells (sinus) medially. Note the mucous lining of the sinuses. Posteriorly, remove the lesser wing of the sphenoid bone and anterior clinoid process to expose the superior orbital fissure and optic canal. 2. Nerves and Muscles. First, inject the eyeball with water to partially fill the collapsed globe. Now incise the periorbita and remove it. Locate the stump of the trochlear nerve and follow it anteriorly to the superior border of the superior oblique muscle which it innervates. Trace the large frontal nerve (N. V1) anteriorly and note its division into the supratrochlear and supraorbital nerves. 2. Next find the delicate lacrimal nerve (N. V1) and trace it anterolaterally to the lacrimal gland. A branch of the oculomotor nerve (N.III) passes into the muscle on its underside. Examine the superior oblique muscle and anteriorly note its trochlea (pulley). Laterally, identify the lateral rectus muscle and look for its abducent nerve (N. VI) on its medial side. Find the nasociliary nerve (N. V1) and try to identify its long ciliary branches. Another small branch of the nasociliary is the anterior ethmoidal nerve which passes through the ethmoidal sinus and ends on the nose as the external nasal nerve (Do not worry about finding this branch.) 3. Finally, locate the oculomotor nerve and note that it divides into superior and inferior branches. Attempt to find the very small ciliary ganglion which lies about 1 cm anterior to the apex of the orbit just lateral to the optic nerve. Short ciliary nerves connect it to the eyeball. Understand the importance of this parasympathetic ganglion. Gently displace the globe to one side or the other and try to identify the medial and inferior rectus and inferior oblique muscles. 4. Vessels. Find the superior ophthalmic vein and note that it anastomoses with tributaries of the facial vein at the medial angle of the eye. Posteriorly, the vein drains into the cavernous sinus. Inferiorly, small veins (not visible) connect the inferior ophthalmic vein with veins in the infratemporal fossa (pterygoid plexus of veins). Identify the ophthalmic artery. It arises from the internal carotid artery and runs anteriorly into the orbit inferior and lateral to the optic nerve.
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