#10 - Orbit

Side One - Superior Approach

  1. On one side (use the side on which the cavernous sinus was dissected), remove the thin roof of the orbit (N9) with a chisel. It is the floor of the anterior cranial fossa lateral to cribriform plate (N9). The frontal sinus may extend over the roof of the orbit. It is quite variable in size. If it covers the roof of the orbit, two thin layers of bone must be removed to enter the orbit.
  2. Immediately deep to the bone is periorbita (899/N79), a thin layer of periosteum. This periosteum, which separates easily from bone, is continuous with the dura mater and with the periosteum covering the superficial surfaces of the bones making up the orbital margins.
  3. The frontal nerve (911/N82), a branch of the ophthalmic division of the fifth cranial nerve, can be seen through the periorbita. Make a cut though the periorbita to one side of the nerve and reflect laterally being careful not to injure the nerve. These flaps can be removed. The contents of the orbit are now seen embedded in orbital fat. Removing the fat by teasing it away from nerves, muscles and blood vessels is the best way to proceed.
  4. Follow the frontal nerve anteriorly noting that it divides into supraorbital and supratrochlear branches (N82).
  5. Close to the lateral wall, isolate the lacrimal nerve (N82) and follow it to the lacrimal gland (N82) that is located in a bony depression in the superior lateral part of the orbital cavity close to the eyelid. It is difficult to remove enough bone to see the gland.
  6. Medially, locate the trochlear nerve (N82) entering the posterior part of the superior edge of the superior oblique muscle (909/N82). Attempt to establish continuity with the trochlear nerve found in the wall of the cavernous sinus and follow it as it courses superior to the oculomotor nerve, through the superior orbital fissure, and above the common tendinous ring around the optic nerve and the levator palpebrae superioris muscle.
  7. Study the superior oblique muscle following it anteriorly as far as possible. Its tendon passes through a cartilaginous trochlea (pulley) (N82) before inserting on the eyeball. Display of the trochlea depends upon considerable anterior bone removal; it may not be seen. If exposed, the trochlea has most likely been dislodged from its bony attachment.
  8. The levator palpebrae superioris (909/N82) muscle lies just below the frontal nerve. Cut the muscle at its midpoint (distal to its nerve) and reflect the ends anteriorly and posteriorly.
  9. The superior rectus muscle (N82) will now be exposed. Cut this muscle in the same manner as the levator. Entering the posterior segments of the superior rectus and the levator palpebrae look for the superior division of the oculomotor nerve (N82). The nerve is difficult to find at this stage of dissection. It will become more evident subsequently.
  10. Superior ophthalmic veins (912/N81) in the fat lateral to the superior rectus may now be evident. At times they are large and obscure the field. If that is the case, they may be removed. These veins pass through the superior orbital fissure and drain into the cavernous sinus.
  11. Locate the nasociliary nerve (911/82) as it crosses above the optic nerve from lateral to medial and follow it posteriorly to the ophthalmic division of the trigeminal (N81), then anteriorly to its point of disappearance from the orbital cavity into the nasal cavity as the anterior and posterior ethmoidal nerves (N82). Just above the optic nerve long ciliary branches (912/N82) from the nasociliary nerve to the eye ball should be evident. They are tiny threads. The posterior ethmoidal branch (N82) is usually difficult to find.
  12. Locate the origin of the ophthalmic artery (912/N81) from the internal carotid close to the optic foramen and note its inferior position to the optic nerve in the optic canal. Identify the artery in the orbit. In most instances it courses first lateral to the optic nerve and then crosses the optic nerve superiorly in company with the nasociliary nerve. In some individuals (15%), the artery crosses below the optic nerve. The central retinal artery (N81), the first branch of the ophthalmic, enters the optic nerve to provide the only blood supply to the retina. It is too small to demonstrate. Since the optic nerve is covered with three meningeal layers, the central retinal artery and its accompanying vein must traverse these layers. The vessels are therefore subject to cerebrospinal fluid pressure. The other branches of the ophthalmic (N81, notice the spelling, it is a Greek word), lacrimal, ciliary, ethmoidal etc. are identical to the accompanying nerves.
  13. To follow the abducens nerve (N82) from the cavernous sinus through the superior orbital fissure to the lateral rectus muscle (N82) it is advisable to chisel away as much of the lesser wing of the sphenoid as possible lateral to the optic foramen. This will expose the tendinous ring (909/N79), providing origin for the four rectus muscles. The lateral rectus’ two attachments (N79) to the ring have a gap between them, through which the abducens nerve, the oculomotor nerve, and the nasociliary nerve pass. Carefully sever the superior attachment of the lateral rectus so the abducens and oculomotor nerve (N82) can be followed all the way from the cavernous sinus to their terminations.
  14. Once the space between the tendinous attachments of the lateral rectus has been opened, the ciliary ganglion (912/N82) can be identified. The size of a pin head, it should lie just lateral to the optic nerve with communications from the nasociliary nerve sensory root (N82), motor root (N82) from inferior division of oculomotor nerve and fine direct branches, short ciliary nerves (N82), continuing to the eyeball. If the ophthalmic artery crosses below the optic nerve it tends to interfere with ability to display the ganglion clearly.
  15. The inferior division of the oculomotor nerve (N82) can now be followed to the inferior rectus and medial rectus muscles (N82). It may not be possible with the eyeball intact to see its branch to the inferior oblique muscle (N80).

Side Two - Anterior Approach

  1. On the side opposite the dissection of the orbit from above, carefully reflect the palpebral portion of the orbicularis oculi muscle (N22) and note the medial palpebral ligament (901/N77). Remove enough muscle to display the tarsal plates (N77) in both eyelids. Tarsal glands are located in the plates. Note that attempts to push a probe into the orbit above the upper tarsal plate or below the lower plate are rebuffed by the orbital septum (N77), a continuation of the periorbita. The attachment of the levator palpebrae superioris (N77) to the upper tarsal plate can also be seen.
  2. Expose the lacrimal sac (901/N78) with blunt dissection. The sac lies in the groove in the lacrimal bone behind the medial palpebral ligament. It is difficult to expose. Cut into it and probe the nasolacrimal duct (N78).
  3. Cut the orbital septum close to the margins of the orbit and locate the lacrimal gland (N78). Raise the eyeball and identify the inferior oblique muscle (N80). Provided the eyeball has not been removed because of the deceased’s desire to donate the cornea, remove this eyeball from the front by cutting the muscles attached (leave ½ inch stubs of the muscles attached to the eyeball) and the optic nerve. This maneuver is difficult because there is little room between eyeball and orbit. Push eyeball aside vigorously in order to widen the space. Identify the muscle remnants attached and the bulbar fascia (909/N79).
  4. Cut the eyeball (905/N83) that has been removed along its equator into anterior and posterior halves. Identify as many of the following structures as possible.
    1. Optic papilla (N83) - entrance of optic nerve.
    2. Retina (N83) - the retina is usually detached and rather jelly like.
    3. Retinal arteries (N83) - these are the arteries examined with the ophthalmoscope.
    4. Choroid vascular layer (N83) - has much pigment.
    5. Sclera (N83).
    6. Lens (N83) - adherent to the ciliary body by suspensory ligaments. Remove.
    7. Ciliary body (N85) - Striations of ciliary muscle may be seen.
    8. Iris (N83).
    11. Cornea (N83).

Notice that the sclera and cornea are continuous; that the choroid, ciliary body, and iris are continuous; that the retina changes at the ora serrata (N83) from visual to nonvisual in character and lines the posterior surface of the ciliary body and iris.