Orbit and Eye anatomy Flashcards
What bones form the supraorbital and infraorbital margins?
supra = frontal bone Infar = zygomatic bone laterally and maxilla medially
What bones form the roof of the Orbit?
frontal bone and lesser wing of sphenoid
What bones form the floor of the orbit?
Zygomatic (laterally) and Maxilla, with small contribution from the palatine bone
What bones form the Lateral wall of the orbit?
zygomatic & greater wing of sphenoid
What bones form the medial wall of the orbit?
Maxilla, lacrimal bone, ethmoid, body of sphenoid
Which bones are fragile and particularly prone to fractures?
lacrimal mostly and ethmoid
What are the 3 layers of the eye?
outer = Sclera + cornea middle = uvea Inner = retina
What are the features and functions of the sclera?
outer layer of the eye - forms 5/6th of the eyeball. It is Strong and white due to the disordered arrangement of collagen (whirls)
Functions: maintain the shape of the globe, resists external and internal forces, attachment for EOMs
What are the features and functions of the cornea?
Makes up the last (anterior) 1/6th of the outer layer of the eye and is continuous with the sclera. It is Avascular and transparent and is the principle refractory component of the eye.
Why is the cornea transparent and the sclera opaque?
Different collagen arrangment. Cornea has collagen fibrils of uniform diameter and spacing which run parallel to one another in bundles (lamellae). Adjacent lamellae run at angles to one another. In contrast collagen in the sclera is disorganised (whirls) with different diameter fibres.
What are the 5 layers of the cornea?
Epithelium (constantly replaced) Bowmans layer Stroma (collagen) Descemet's membrane Endothelium
What is the function of the endothelium of the cornea?
unique compared to other endotheliums -> is specialised to control fluid balance within the cornea -> do not get replace - we have a finite number and if we lose these cells cornea swells -> cannot see
What determines the opacity of the sclera?
composition of the stroma
hydration
size and distribution of collagen
What is the anterior chamber and angle?
junction between iris and cornea -> is the drainage point of aqueous humour out of the eye
What are the key structures in the anterior chamber angle?
cornea
trabecular meshwork
canal of schlemm
ciliary body
What structures make up the Uvea?
Ciliary Body, Iris, Choroid
What are the functions of the ciliary body?
Has processes (epithelium) that form aqueous humour and tethers lens, also is responsible for accomodation (via the ciliary muscle)
What is the role of the Aqueous humour?
important for maintaining the health of the lens and cornea, creates intraocular pressure
Where is aqueous humour drained?
pass through the trabecular meshwork and out of the eye through the canal of Schlemm at the anterior chamber angle into the venous supply
What is accommodation of the eye and what structures are involved
Adjustment of the Lens
Ciliary muscle
Zonules
What are the zonules?
ligaments that connect the lens to the ciliary processes
What controls contraction of the ciliary muscle?
parasympathetic nervous system
When the ciliary muscle is contracted - what happens to the zonules and lens? When does the Ciliary muscle contract?
zonules are lax allowed the lens to thicken = good for CLOSE vision (high refractory power)
What is Presbyopia? what is it caused by?
loss of accommodation with age due to a reduction in the flexibility of the lens capsule and zonules
What is the Iris?
part of the Uvea = coloured part. thin circular structure (diaphragm) around the pupil (central aperture) which is responsible for controlling the diameter/size of the pupil and thus the amount of light reaching the retina.
What are the two muscles associated with the iris?
sphincter pupillae
Dilator Pupillae
Sphincter pupillae is innervated by what?
parasympathetic fibres running with CNIII
Dilator pupillae is innervated by what?
Sympathetic fibres
What is the choroid? what is its function?
3 layers of blood vessels (most important is choriocapillaris) sitting underneath and supplying nutrients to the retina
What are the features of the innermost layer of the eye?
Retina ->
visual parts : Optic disc, Macula
non-visual parts: Ora Serrata, Lamina Cribrosa
What is the optic disc?
region where the optic nerve enters (posteriorly)
What is the Fovea?
centre of the mucula. Avascular region of high visual acuity- has a high density of cones and NO rods.
What is the lamina cribrosa?
band of connective tissue fibres from sclera that create a sieve like structure through which the axons of the optic nerve have to pass -> helps maintain the pressure gradient BUT if pressure increases can get displaced causing pinching of the pores containing the optic nerve fibres and blood vessels -> Glaucoma
What is the blood supply to the retina?
dual: from Central retinal artery (supplies inner retina), Posterior Cilial artery (supplies outer retina - photoreceptors)
What are the 2 muscle that act on the eyelid and what do they do?
Orbicularis Oculi (sphincter -> depresses eyelid) Levator Palpebrae superiosis -> elevates upper eyelid
What is Levator palpebrae superiosis innervated by?
CNIII - Oculomotor
What is Orbicularis Oculi innervated by?
CNVII - Facial
