Contents of the Orbit Flashcards
Which bone/s form the supraorbital margin?
Frontal bone
What is the role of the supraorbital notches?
Previously foraminae, frontal nerves pass here
Which bone/s form the infraorbital margin?
Zygomatic bone laterally
Maxilla medially
Which bone/s form the roof of the orbit?
Frontal bone
Less wing of sphenoid
Which bone/s form the floor of the orbit?
Zygomatic
Maxilla
Palantine
Which bone/s form the lateral wall of the orbit?
Zygomatic
Greater wing of sphenoid
Which bone/s form the medial wall of the orbit?
Maxilla
Lacrimal bone
Ethmoid
Body of sphenoid
On which wall of the orbit do fractures typically occur?
Medial wall (especially at the junction of the lacrimal and ethmoid bones)
What structures form the superior orbital fissure?
Greater and lesser wing of sphenoid
What is the role of the sclera? What % of the eyeball does it make up?
Maintains the shape of the globe by resisting internal and external forces, and provides attachments for EOMs
Makes up 5/6th of the eyeball
Describe the pattern of collagen in the sclera
Laid down in “whirls” for added strength
Where is the cornea and what is its role?
Makes up the anterior 1/6th of the eyeball
Principal refracting component (air-cornea interface is responsible for 60% of focussing)
List the 5 layers of the cornea
Epithelium (mucous membrane) Bowman's layer Stroma (collagen, continuous with the sclera) Descemet's membrane Endothelium
What characteristics of the cornea make it optimal for vision?
Avascular
Transparent
What is special about the endothelium of the cornea and what is its role?
Not like blood vessels (don’t dilate)
Specialised to control water balance in the cornea
How is collagen laid down in the cornea?
In fibrils, uniform in diameter and evenly spaced
Run parallel to each other in bundles (lamellae), with 200-300 lamellae in the stroma
Adjacent lamellae lie at angles to each other
List 3 factors which influence scleral opacity
Composition of the stroma
Hydration
Size and distribution of collagen
When may injury to the cornea result in a corneal scar?
When the injury penetrates the stroma (repair of collagen may not exactly reconstruct the pattern of collagen deposition to maintain transparency)
What is the anterior chamber angle?
The junction between the iris and the cornea where the aqueous humour drains
What is one cause of glaucoma?
Defect in the drainage of aqueous humour via the anterior chamber angle, leading to increased pressure in the eye
List the 4 key structures in the anterior chamber angle
Cornea
Trabecular meshwork
Canal of Schlemm
Ciliary body
How does the aqueous humour drain from the eye?
Drains through the trabecular meshwork via the canal of Schlemm into the venous outflow
What is the uvea?
The middle coat of the eye, which provides the eye’s vascular supply
What are the structures in the uvea, front of the eye to back?
Iris
Ciliary body
Choroid
What are the 3 functions of the ciliary body and what parts are responsible for each?
Formation of aqueous humour (via ciliary epithelium) Tethers lens (via ciliary processes) Performs accommodation (via ciliary muscle)
Where is the aqueous humour produced?
By the 2 layers of ciliary epithelium lining the ciliary processes
What is the role of the aqueous humour?
Maintains the health of the lens and cornea, and generates intraocular pressure
What structures are responsible for accommodation of the lens?
Zonules (anchored by the ciliary processes and attaching to the lens)
Ciliary muscles
What innervates the ciliary muscle? What is the result of contraction?
PNS (carried by CNIII)
Contraction of the muscle results in a loss of tension on the zonules; the lens “bulges” and this allows close-up objects to be better visualised
What is the result of relaxation of the ciliary muscle?
Zonules are pulled taut, the lens becomes thin and stretched out, and long distance objects are better visualised
What is presbyopia and what is the underlying mechanism?
Loss of accommodation with age
Caused by a reduction in the flexibility in the lens capsule and zonules
What are the 2 muscles of the iris? What are their respective functions and what is their innervation?
Sphincter pupillae: constricts pupil, innervated by PNS
Dilator pupillae: dilates pupil, innervated by SNS
Describe the structure and function of the choroid
3 layers of blood vessels including the choriocapillaris, which underlies the retina and supplies it with nutrients
List the 5 important structures of the retina
Optic nerve/disc Fovea/foveola Macular Posterior pole Orra serrata
What is the fovea?
Area of highest density of cones (no rods) providing high visual acuity
hat forms the optic nerve?
Axons of ganglion cells exiting the retina
What is the lamina cribosa?
Band of dense sieve-like collagen from the sclera (~1/3 of fibres) through which axons are transmitted
What is the pathological relevance of the lamina cribosa?
Can cause or exacerbate loss of ganglion cells in disease e.g. glaucoma
Which major blood vessels supply the orbit?
Tributaries of the opthalmic artery
Central retinal artery (CRA)
Which artery is visualised in fundoscopy?
CRA
List the 3 classifications of ciliary arteries supplying the orbit and describe their course
Anterior ciliary artery: feeds anterior structures once they get to the conjunctiva
Long posterior ciliary artery: pierces the globe, travels in the choroid to the front of the eye (supplies iris, ciliary body)
Short posterior ciliary artery: pierces the globe, travels in the choroid but stops at the optic nerve
Describe the dual blood supply of the retina
CRA: to inner retina
Posterior ciliary artery: to outer retina (i.e. photoreceptors)
What are the 2 muscles controlling the eyelid and what are their respective roles and innervations?
Orbicularis oculi: depresses upper lid, innervated by CNVII
Levator palpebrae superiorosis: elevates upper lid, innervated by CNIII
What structures are contained within the lacrimal apparatus?
Lacrimal glands and ducts
Nasolacrimal sac and duct
