Anatomy of the Orbit Flashcards
List the bones that form the orbit?
- Frontal
- Sphenoid
- Lacrimal
- Maxillary
- Ethmoid
- Zygomatic
3 foramina at the back of the orbit?
- Optic foramina
- Superior orbital fissure
- inferior orbital fissure
Why are the medial wall and floor of the orbit weaker than the rim/other walls?
Because they are closely associated to paranasal air sinuses - only separated by a thin wall
The eyeball itself is about 24mm long and the orbit is about 40mm long. What occupies the majority of this extra space?
Function?
Orbital fat
Mainly functions as a cushion
What are the conjunctiva? Function?
The mucosal lining of the inner eyelid, also covers the sclera
Changes from skin to conjunctiva at margin of eyelid
Helps lubricate the eye by producing mucus and tears, also performs immune surveillance preventing microbes from invading the eye
What is the name of the muscle that raises the top eyelid? Origin and insertion?
levator palpebrae superioris
Originates on the lesser wing of the sphenoid bone, inserts on the tarsal plate of the eyelid
What gives structure to the eyelids?
What is it composed of?
The tarsal plate
Dense connective tissue
What is the name of the glands associated with the tarsal plate? What do they secrete?
Meibomian glands
Secrete meibum, an oily substance that prevents evaporation of the eyes tear film
Where are the conjuctiva located?
- Begins at the margin of the eyelid
- Continues along the eyelid and then loops back around to cover the sclera (area where it turns around called fornix)
- Covers the sclera, stops at cornea (doesn’t cover cornea)
How does conjunctivitis present?
When the conjunctiva become inflamed the blood vessels of the usually transparent layer over the eye dilate
So the eye appears red
Also: itching & watery eyes
What is the nerve that supplies parasympathetic innervation to the lacrimal gland?
Facial nerve (CN VII)
Where does the lacrimal gland drain into?
Lacrimal duct
The superior fornix of the conjunctiva
eye
Describe the passage of tears from the lacrimal gland
Lacrimal gland -> lacrimal duct -> surface of eyeball -> lacrimal punctum -> lacrimal canaliculi -> lacrimal sac -> nasolacrimal duct -> inferior meatus of nasal cavity -> absorbed in nasopharynx
How does blinking promote the movement of tears into the lacrimal sac?
Since the orbicularis oculi muscles are over the lacrimal sac, blinking opens the sac and creates a negative pressure situation - drawing fluid into the sac
What is the function of the intrinsic muscles of the eye? Of the extrinsic?
Intrinsic - Control pupil diameter and lens curvature
Extrinsic - move the eye
What is the muscle responsible for lens accommodation? Where is it located?
Innervation?
Ciliaris muscle
Located in the ciliary body
CN III - oculomotor n.
What are the muscles that control pupil diameter?
Constrictor pupillae and dilator pupillae
What are the 6 extrinsic muscles of the eye? Fibre orientation for each?
Straight fibres from posterior:
- Superior, Inferior, Medial and Lateral RECTUS
Obliquely attached:
- Superior and inferior oblique muscles
Describe the innervation of the extrinsic muscles of the eye
SO4 LR6 - rest 3
- Trochlear (IV) supples superior oblique
- Abducent (VI) supplies Lateral rectus
- All others: oculomotor n. (III)
What are the cranial nerves that supply the orbit?
How do these nerves enter the orbit?
- Optic (II)
- Oculomotor (III)
- Trochlear (IV)
- Abducent (VI)
Enter through the superior orbital fissure, Except for optic which goes through the optic foramen
- Trigeminal V1 & V2 (V1 SOF, V2 IOF)
Which branches of the trigeminal nerve supply the orbit?
Opthalmic and Maxillary
- V1 (opthalmic) through SOF
- V2 (maxillary) through IOF
Which artery supplies blood to the orbit? Which carotid is it a branch of?
- Opthalmic artery
Branch of internal carotid
Which artery supplies the inner layers of the retina? How does it get to the retina?
Which artery is it a branch of?
Central retinal artery - travels within the optic nerve to get to the retina (accompanied by vein)
Branch of opthalmic artery
What is the optic disc?
Head of the optic nerve on the retina, physiological blind spot
What investigation can let you see inside of the eye?
Opthalmoscopy / Fundoscopy
How is the fundus of the eye segmented?
Divided into 4 segments, named based on whether they’re closer to nasal or temporal sided. Centre of the segments is the optic disc
- Lower nasal and temporal quadrants
- Upper nasal and temporal quadrants
Which veins drain the eye and orbit?
What do they drain into?
Superior and inferior opthalmic veins
Drain into the cavernous sinus (right behind the orbit)
go through the SOF
What do the intracranial venous sinuses lie between?
What do the sinuses collectively drain into?
The two layers of the dura mater
Drain into the internal jugular veins on each side
