B.  POSTERIOR COMPARTMENT

=  RETROBULBAR SPACE = cone consisting of extraorbital muscles + envelope of fascia divides retrobulbar space into

(a)  intraconal space

(b)  extraconal space

GLOBE

Ocular globe diameter:  22–25 mm

Wall:   composed of 3 layers:

(a)  fibrous outermost layer

Function:   maintaining shape + pressure of globe

1.  Sclera = collagenous tissue layer continuous anteriorly with cornea posteriorly with dura mater

2.  Cornea

(b)  Uvea = pigmented vascular middle layer

[uvea, Latin = grape]

3.  Iris

4.  Ciliary body (anteriorly)

5.  Choroid (posteriorly) = most vascular structure of globe

Attachment:   tethered to sclera by arteries + veins

◊  Most frequent site of intraocular metastases!

(c) innermost sensory layer

5.  Retina = light-sensitive (sensory) inner layer

Attachment:   firm at anterior margin (= ora serrata) and posteriorly at optic disc

√  various layers of globe are NOT discernable at imaging, especially CT

√  sclera may be separated from choroid at high-res MRI

√  individually visualized in ocular (choroidal / retinal) detachments

Contents:   (a)   anterior segment containing

1.   Aqueous humor subdivided by iris into:

›  anterior chamber

›  canal of Schlemm (see below)

›  posterior chamber

  (b)  posterior segment containing

2.   Vitreous humor

Potential ocular spaces:

(a)  between retina + choroid → retinal detachment

(b)  suprachoroidal space between choroid + sclera → choroidal detachment

(c)  between vitreous + posterior hyaloid membrane → hyaloid detachment

Canal of Schlemm

[Friedrich Schlemm (1795–1858), German anatomist, sentenced for grave robbing, eventually professor at the University of Berlin]

=  SCLERAL VENOUS SINUS [misnomer – not a blood vessel]

=  circular ringlike endothelium-lined tube lymphatic tube resembling a lymphatic vessel

Location:  adjacent to outer angle of anterior chamber in pectinatum iridis (= periphery of cornea) between cornea + iris

Function:  collects aqueous humor from anterior chamber and delivers it into the venous system

OPTIC NERVE SHEATH

=  extension of dura mater

Content:

1.   Optic nerve

2.   Ophthalmic artery

3.   Small veins

OPTIC NERVE (CN II)

Histo:   CN II is an extension of brain = retinal ganglion cell axons myelinated by oligodendrocytes + enveloped within meninges

Segments:

A.  Retinal segment leaves ocular globe through lamina cribrosa sclerae

B.  Orbital segment travels in center of fat-filled orbit

√  surrounded by dural sheath containing CSF

C.  Canalicular segment lies in optic canal below ophthalmic artery; frequently overlooked on radiologic images

D.  Cisternal segment in suprasellar cistern leading to optic chiasm

√  anterior cerebral a. passes over superior aspect of nerve

OCULOMOTOR NERVE (CN III)

Exit:   from brainstem anteriorly between posterior cerebral (PCA) + superior cerebellar arteries

Segments:

(1)  cisternal segment courses through prepontine cistern

(2)  cavernous sinus segment along cephalad portion of the lateral dural wall

(3)  orbital segment through the superior orbital fissure

(4)  division into superior + inferior branches

Function:

(a)  motor fibers to

›  levator palpebrae muscle

›  all extraocular muscles except lateral rectus and superior oblique mm.

(b)  parasympathetic fibers to

›  internal eye muscles (pupillary sphincter and ciliary muscles) → constriction of pupil via Edinger-Westphal nucleus

Location:  periphery of CN III → subject to compression by extrinsic masses

Disturbed function:

(1)  “pupil-sparing” oculomotor palsy = loss of motor function in extraocular muscles with relative sparing of pupillary parasympathetic innervation ← compromised blood flow to central microvessels

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