Anatomy & Embryology

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Ocular anatomy and embryology from the BCSC

What is the only extraocular muscle that has a single blood supply?

Lateral rectus (single vessel derived from lacrimal artery)

What is the blood supply to the extraocular muscles?

Each muscle (except LR) is fed by 2 anterior ciliary arteries.

What muscles are innervated by the superior division of CNIII? By the inferior division?

Superior CNIII: Levator, SR
Inferior CNIII: MR, IO, IR

What is the ratio of nerve fibers to muscle fibers in the extraocular muscles?

1:3 - 1:5. Very high, allowing for precise control.

Are the eye muscles made up of slow, tonic fibers or fast, twitch fibers?

Both. Slow, tonic fibers do smooth pursuit, and fast, twitch fibers do saccades.

What is the normal LENGTH of the palpebral fissure?

27-30 mm

What is the normal width of the palpebral fissure?

8-11 mm

What is the only extraocular muscle without a tendon?

Inferior oblique

What is normal levator excursion?

15 mm

What is the average increase in palpebral fissure from the frontalis?

2 mm

What are the different layers of the eyelid from anterior to posterior?

1. Skin (epidermis, dermis)
2. Eyelid margin
3. Subcutaneous tissue
4. Orbicularis
5. Septum
6. Levator
7. Muller's muscle
8. Tarsus
9. Conjunctiva

What lid structure is responsible for the eyelid fold?

Insertion of the levator aponeurosis

What are the two portion of the orbicularis muscle?

1. Orbital (voluntary)
2. Palpebral (voluntary and involuntary)
--palpebral further divided into preseptal and pretarsal portions

How far above the eyelid margin is the marginal artery located?

3.0-3.5 mm

What structure divides the main lacrimal gland into its palpebral and orbital components?

The levator palpebrae superioris

The orbital septum is an extension of what?

Periosteum

What is the origin of the levator palpebrae superioris?

Lesser wing of the sphenoid

What is the tarsus made of?

Connective tissue (not cartilage)

What is the normal diameter of the lacrimal punctum?

0.3 mm

What percentage of full term neonates are born with an occluded NLD?

30%

What is the blood supply to the intracanalicular optic nerve?

Ophthalmic artery

What is the blood supply to the intracranial optic nerve?

Ophthalmic artery, ICA

Extramacular fibers from the inferonasal retina cross where in the chiasm?

Anteriorly, at the Wilbrand knee

Macular fibers from the nasal retina cross where in the chiasm?

Posteriorly

Describe the course of visual information from retina to brain.

Optic nerve, optic chiasm, optic tract, lateral geniculate body, optic radiations, visual cortex in occipetal lobe.

What is the only cranial nerve that completely decussates?

CN IV

Name the four nuclei that give rise to CN V.

1. Mesencephalic nucleus
2. Main sensory nucleus
3. Spinal nucleus and tract
4. Motor nucleus located in pons

Bilateral, concentric sensory loss centered about the mouth ("onionskin") is caused by a lesion in the...

Trigeminal sensory nucleus in brainstem

What is the function of homeobox genes?

Control spatial arrangement of tissues

What are the first cells of the retina to become clearly differentiated?

Ganglion cells

Foveal remodeling occurs until what age?

4 years

When and where does myelination of the optic nerve begin? End?

Begins at optic chiasm at 7 mths gestation
Ends at lamina cribrosa at 1 mth old

What goes through the superior orbital fissure?

CN III, IV, VI, V1, sympathetics, and superior ophthalmic vein

Where do tears drain?

Inferior meatus

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