2.4 Ocular Anatomy Flashcards
The Anatomy of the Orbit
The bony orbit has been described variously as a pyramid
whose apex is directed inwards and upwards,
as a cone and as a pear whose stem points towards the optic canal.
Its roof consists of the orbital plate of the frontal bone,
with the anterior cranial fossa above,
while its floor is formed by the zygoma and the maxilla,
with the maxillary sinus beneath.
Its medial wall is formed by parts of
the maxilla,
lacrimal bone,
ethmoid and sphenoid,
and beyond it lie the
ethmoid air cells and the
nasal cavity.
The zygoma and the greater wing of the sphenoid make up its lateral wall.
Contents of Orbit
The bony orbit contains the globe,
together with the muscles,
nerves and blood vessels that
subserve the normal functions of the eye.
Normal length + layers
The normal globe has an axial length of around 24 mm
(as measured in the anteroposterior diameter).
An eye longer than 26 mm is usually myopic.
Its outer layer comprises sclera and cornea;
the middle vascular layer contains the choroid,
the ciliary body and the iris;
and the innermost layer comprises neural tissue in the form
of the retina.
The movements of the globe
The movements of the globe are
controlled by the six extraocular striated muscles.
The four recti (lateral, medial, superior and inferior) originate
from the annulus of Zinn,
the tendinous ring which encircles the optic foramen,
and insert beyond the equator of the globe.
The lateral and medial recti have two heads.
The superior oblique muscle originates above and medial to the annulus,
curves round the trochlea
(which acts like a pulley),
before inserting behind the equator and beneath the
superior rectus.
The inferior oblique originates from the lacrimal bone and inserts
posterolaterally on the globe, having passed beneath the inferior rectus muscle.
Motor innervation:
the lateral rectus is supplied by the sixth cranial nerve, the abducens (VI),
and the superior oblique is supplied by the fourth, the trochlear (IV).
SO4\LR6
The remaining muscles are supplied by the third cranial nerve, the oculomotor (III).
(This also supplies levator palpebrae superioris, which elevates the eyelid).
Autonomic innervation
Autonomic innervation:
sympathetic innervation is by the long and short ciliary nerves
via the superior cervical ganglion.
Nerve impulses dilate the pupil via the dilators of the iris.
Parasympathetic innervation is by the short postganglionic ciliary
nerves via the ciliary ganglion.
The pre-ganglionic supply comes from the oculomotor nerve,
and its impulses constrict the pupil.
Sensory supply:
this is derived mainly from the ophthalmic branch of the fifth
cranial nerve, the trigeminal (V),
although branches of the maxillary division make
some contribution to lateral structures and to the nasolacrimal apparatus.
There are a large number of sensory nerves for such an anatomically confined area.
The examiner is unlikely to dwell on these in any detail but, in summary,
the innervation that may have relevance for ocular surgery can be outlined as follows.
The ophthalmic division V1 branches into the
frontal nerve, which then divide:
Frontal N Branches
- supratrochlear nerves (medial upper conjunctiva),
- the supraorbital nerve (upper conjunctiva)
- the long ciliary nerve (cornea, iris and ciliary muscle).
V1
V1 also forms the nasociliary nerve,
which in turn branches into the infratrochlear nerve
(inner canthus and lacrimal sac)
and the long sensory root to the ciliary ganglion
(thence to the cornea and iris).
The lacrimal branch of V1 supplies the rest of the conjunctiva.
Foramina
Optic canal
Foramina:
the orbit contains nine fissures and foramina,
of which three are particularly
important:
1. the optic foramen (canal),
2. the superior orbital fissure
3. inferior orbital fissures.
Optic canal
. The optic nerve and ophthalmic artery traverse the optic foramen.
Superior orbital fissure
Through this fissure run the oculomotor, trochlear and
abducens nerves to the extraocular muscles,
together with the frontal, nasociliary and
lacrimal nerves and the superior and inferior ophthalmic veins.
The oculomotor, abducens and nasociliary nerves traverse the lower part of the fissure and enter the muscular cone between the two heads of the lateral rectus.
The trochlear, frontal and lacrimal nerves remain outside the cone.
- Inferior orbital fissure
Through the inferior fissure run the zygomatic and infraorbital
nerves (branches of V2), the infraorbital artery and the inferior ophthalmic vein.
Ophthalmic Reflexes
- Corneal reflex
- Pupillary Reflex
- miosis
- mydriasis - Abnormal Sx
Corneal reflex:
This is the normal blink reflex but is used as part of brain stem death testing.
Stimulation should provoke both a direct and a consensual reflex.
The afferent pathway from the nasociliary branch of the ophthalmic division of
the trigeminal nerve (V1) leads to the trigeminal sensory nucleus in the medulla
oblongata.
Interneurons connect to the facial motor nucleus which mediates the
motor response (contraction of the orbicularis oculi muscles)
via the temporal and zygomatic branches of the facial (seventh cranial) nerve.