W21-L1: Contents of the Orbit Flashcards
What forms the supra-orbital margin?
The frontal bone
What forms the infro-orbital margin?
Zygomatic bone laterally and maxialla medially
Which bones form the roof of the orbit?
Frontal bone, lesser
wing of sphenoid
Which bones form the floor of the orbit?
Maxilla,
zygomatic, palatine
Which bones form the lateral wall of the orbit?
Zygomatic, greater
wing of sphenoid
Which bones form the medial wall of the orbit?
Maxilla, lacrimal bone, ethmoid, body of sphenoid
Which bone of the orbit is often fractured?
Lacrimal and ethmoid bones as they are very thin, so medial wall
What are the three layers of they eye?
Outermost: Sclera and cornea
Middle layer: Vascular (Uvea)
Inner layer: Neuronal
How much of the eyeball does the sclera cover?
5/6ths
What is the function of the sclera?
-Maintains the shape of the globe, offering resistance to
internal and external forces.
-Provides attachments for the EOMs
Why is the sclera so tough?
The collagen and the arrangement which is not parallel, instead its in whirls
What is the cornea?
Structure that is continuous with the sclera and covers the anterior 1/6th of the eye
What is the function of the cornea?
Principal refracting component of the eye (~60% of refraction)
Why is the cornea optimum for vision?
Avascular and transparent
How many layers are there in the cornea?
Five
What is the epithelium layer in the cornea?
Mucus membrane
What is the stroma composed of in the cornea?
Collagen that is neatly arranged so its transparent
What is the role of the endothelium in cornea?
Controls water balance in the eye
How is the collagen arranged in the cornea?
- Uniform in diameter and evenly spaced
- run parallel to each other in bundles called lamellae
- adjacent lamellae lie at angles to each other
What does the varying scleral opacity depend on?
- Composition of the stroma
- Hydration
- Size and distribution of the collagen
What happens if you lose only the epithelium in the cornea?
It heals with no scar
What happens if you get stroma injury in the cornea?
Loss of ordered collage and transparency, a scar if in the cornea
What is the anterior chamber angle?
- Junction between iris and cornea
- Where aqueous humour drains out of eye.
What are the key structures in the anterior chamber angle?
- Cornea
- Trabecular meshwork
- Canal of Schlemm
- Ciliary body
What makes up the uvea?
Iris, ciliary body and choroid
What are some important structures in the ciliary body?
Ciliary processes (ciliary eptihelium) which form aqueous humour and form an attachment for ligaments of the lens
Ciliary muscle which allows us to focus (accommodation)
What is the function of aqueous humour?
- Important for maintaining the health of the lens and cornea.
- Creates intraocular pressure.
Where does the aqueous humour move around the eye?
Passes through pupil, and drains via anterior chamber angle into the venous supply.
What are zonules?
ligaments that attach between cililary processes and lens.
What does accommodation involve?
- ciliary muscle
2. Zonules that attach to lens.
What is the most important fibres of the ciliary muscle and what innervates them?
Circularly fibres are the most important, they are innervated by parasympathetic fibres
What causes expansion of lens?
Lack of tension due to contraction of ciliary muscles
Presbyopia
Refers to the loss of accommodation with
age caused by reduction in flexibility in the lens capsule and zonules
What is the iris?
The aperture of the eye
What muscles are in the iris?
- Sphincter pupillae: constricts pupil: innervated by parasymp NS.
- Dilator pupillae: dilates pupil: innervated by Sympathetic NS
What is the choroid?
The three layers of the blood vessels underneath the retina that supplies it
Which is the most important layer of the choroid?
Choriocapillaris
What are the important parts of the retina?
- Optic nerve/optic disc
- Fovea/foveola
- Macular
- Posterior pole
- Orra serrata
What is the orra serrata?
Edge of the retina
What is special about the fovea?
High visual acuity, avascular, high density of cones and no rods
What is the Lamina cribosa?
- Band of dense connective tissue.
- Sieve-hole that transmit nerve fibres.
- Collagen fibres from sclera.
- 3-10 sheets of connective tissue.
Why is the Lamina cribosa important?
In disease can damage ganglions around it
The orbit blood supply is from where?
From the opthalamic artery which is the first branch of the internal carotid
What supplies the retina from the inner surface?
Central retinal artery, first branch of opthalamic, pierces optic nerve and fans out over the retina
What are the ciliary arteries?
- Long posterior ciliary artery
- Short posterior ciliary artery
- Anterior ciliary artery
What is the path of the posterior ciliary arteries?
-posterior cilliary arteries pierce the globe around the
optic nerve and travel in the choroid
-if they travel short and feed head of optic nerve they
are called short, if they travel round to the front of the eye they are long
What is the path of the anterior ciliary arteries?
does not pierce the globe, runs forward and supplies anterior structures
What is the blood supply to the retina?
Dual blood supply to retina:
- Central retinal artery supplies the inner retina
- Posterior ciliary artery supplies the outer retina (photoreceptors)
What are the eyelids?
transition zone between mucus and skin
What are the two muscles of the orbit?
- Orbicularis oculi
- Levator palpebrae superiosis
What is the function of Levator palpebrae superiosis and what innervates it?
Elevates upper lid, innervated by CN III
What is the function of Orbicularis Oculi and what innervates it?
Sphincter muscle that depresses upper lid, innervated by CN VII
