Basic Anatomy of Eye😊

Basic Anatomy of Eye

Canal of Petit

• Small annular space in the eye.

• Located between posterior zonules and anterior hyaloid membrane.

• Also called Retrozonular space.

General Features

• Shape: Aspherical (Oblate spheroid)

• Volume: 6 mL

• Location: Orbit
• Orbit Walls: 4 walls
• Thinnest wall: 
• Medial
• Weakest wall: 
• Floor
• Most common orbital fracture: 
• Blow out fracture

Fracture floor of Orbit

• Known as Blow-out fracture.

• Blunt Trauma to Orbit

• Occurs due to tennis ball injury (trauma direct to orbit)

• Orbit contents may herniate into maxillary sinus.

• Floor is more susceptible than medial wall.

• Radiological sign: Teardrop sign.

• Infraorbital nerve is involved.

• Other symptoms
• Enophthalmos (sunken eye)
• Diplopia on upward gaze
• Loss of sensitivity over the cheek

Cone of Muscle & Annulus of Zinn

• Annulus of Zinn:
• Common tendinous ring.
• Origin of Extraocular Muscles (EOM).

• Orbit Shape:
• Apex: Conical
• Base: Quadrangular
• Medial and lateral walls are present.

Routes of Local Anaesthesia

Site

• Through the lower eyelid at the junction of middle 1/3 and lateral 1/3

Steps

Eye related anasthesia and N supply

Route	Target Space	Complications
Retrobulbar	Intraconal	More complications
Peribulbar	Extraconal	Fewer complications

• SET CEI

Nasociliary Nerve

• Courses along medial wall of the orbit.

• Branches
• Posterior ethmoidal nerve
• Anterior ethmoidal nerve
• Infratrochlear nerve

Innervation

• Lacrimal sac

• Inner canthus

• Lateral aspect of nose

Anterior Ethmoidal Nerve Block

• Branch of nasociliary nerve.

• Blocks distal external nasal branch.

• Clinical Use
• Provides pain relief during procedures involving:
• Lacrimal sac
• Inner canthus
• Lateral nose
• Infratrochlear Nerve Block Used in
• Dacryocystorhinostomy (DCR)

Infraorbital nerve block :

Sphenopalatine ganglion block :

• Anaesthesia of internal nose.
• Via nose
• Via greater palatine foramen (Medial to 3rd molar)

NOTE:

• Biopsy above VC → anesthetize ILN at thyrohyoid membrane

Layers of the Eyeball

Outermost Layer:

1. Cornea:
• Transparent
• Forms anterior 1/6th of eyeball.
• Convex, refracts light.

2. Sclera:
• Opaque (white).
• Forms posterior 5/6th of eyeball.
• Covered by conjunctiva.

3. Limbus:
• Corneoscleral junction.
• Contains stem cells.
• Specific marker: ABCG2
• Universal marker: CD34
• Mnemonic: cornea → anterior 1/6th → 1+6 = 7 → ABCdefG → ABCG2

Middle Layer: Uvea (Vascular)

1. Iris:
• Sphincter pupillae: Responsible for miosis (constriction).
• Dilator pupillae: Responsible for mydriasis (dilatation).

2. Ciliary body:
• Pars plicata (Anterior): Ciliary projections, secrete aqueous humor.
• plicata - secrete
• Pars plana (Posterior): Relatively avascular, entry point to fundus.
• Plana - plane
• Plane of entry
• No blood supply

3. Choroid:
• Posteriormost layer.
• Increased vascularity.

Inner Layer: Retina (Neurosensory)

• Converts sensory impulses into neural signals.

Cross-section of Eyeball

• Pars plana: 
• Site of entry into the vitreous.

• Optic disc: 
• Blind spot (no photoreceptors).

• Fovea:
• Maximum cones.
• Center of visual field.
• Provides best image.
• Foveal fixation develops by 3-4 months.

A scan Ultrasonography

• Determine Axial length of eyeball

Limbal dermoid:

• Benign, congenital tumor.

• Composed of choriostomatous tissue.

• May include bone, cilia, hair follicles.