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EYE ANATOMY & PHYSIOLOGY Dr. Cesar Carrillo [email protected] Sight For All **Disclaimer** The images contained in this presenta;on are not my own, they can be found on the web Eye Anatomy and Physiology A thorough understanding of the anatomy and physiology of the eye, orbit, visual pathways, upper cranial nerves, and central pathways for the control of eye movements is a prerequisite for proper interpreta;on of diseases having ocular manifesta;ons. Furthermore, such anatomic knowledge is essen;al to the proper planning and safe execu;on of ocular and orbital surgery Eye Anatomy and Physiology Objec4ves: Brief overview of eye anatomy and relevant physiology — Embriology — The Orbit, Cranial Nerves, Blood supply and Venous drainage — The Ocular Adnexa — The Extraocular muscles — The Conjunc;va, Sclera and Cornea — The Uveal tract — The Lens — The Re;na, Vitreous and Op;c Nerve — The Visual Pathway Embryology The eye is derived from three of the primi;ve embryonic layers: — Surface ectoderm, including its deriva;ve the neural crest — Neural ectoderm — Mesoderm — Endoderm does not enter into the forma;on of the eye — Mesenchyme is the term for embryonic connec4ve 4ssue. Ocular and adnexal connec;ve ;ssues previously were thought to be derived from mesoderm, but it has now been shown that most of the mesenchyme of all of the head and neck region is derived from the cranial neural crest Embryology Development of the structures of the head and neck occurs between 3-8 weeks of gestation — Eye develops as an ectodermal diverticulum from the lateral aspect of the forebrain — Diverticulum grows out laterally from the side of the head, and becomes the optic vesicle Embryology — Proximal bit becomes the optic stalk — Week 3: organogenesis — Day 22 : optic groove appears Embryology — 3rd month: differentiation of precursors of rods and cones — 4th month: formation of retinal vasculature/lamina cribrosa/ physiologic cup of optic disc — 5th month: eyelid separation Embryology — 6th month: cones differentiate, nasolacrimal system becomes patent — 7th month: rods differentiate, myelination of optic nerve — After birth: macula develops Embryology — Surface ectoderm: lens, lacrimal gland, cornea epithelium, conjunc;va, adnexal glands, eyelid epidermis — Neural crest: corneal keratocytes, endothelium, trabecular meshwork, iris and choroid stroma, ciliary muscle, sclera fibroblast, vitreous, op;c nerve meninges, orbital car;lage and bone, orbital connec;ve ;ssue and nerves, EOM, eyelids subepidermis — Neural ectoderm: op;c vesicle and cup, re;na, RPE, ciliary epithelium pigmented and non-‐pigmented layers, iris posterior epithelium and muscles, op;c nerve fibers — Mesoderm: EOM, orbital & ocular vascular endothelium Embryology of specific structures STRUCTURE EMBRYONIC LAYER SIZE/STAGE Lids and Lacrimal Apparatus Mesenchyme 16mm/6 w – 5th m Sclera and Extra-‐ocular Muscles Mesenchyme/Mesoderm 20mm /7w – 4th m Anterior Segment Neural crest 12mm/5w – 6th m Lens Surface ectoderm 13mm/ 7th m Ciliary Body and Choroid Neural ectoderm/Mesenchyme 6mm/3 .5w – 3rd m Re;na Neural ectoderm 10mm/5w – 6m a_er birth Vitreous Neural crest/Mesenchyme 4.5mm/3mm – 4m Op;c Nerve Neural ectoderm 26mm/8w -‐ 3m a_er birth Blood Vessels Mesenchyme/Mesoderm 16mm/6w – 8m The Orbit — Pyramid of four walls that converge posteriorly — The medial walls are parallel and separated by the nose — The lateral and medial walls form an angle of 45 degrees, which results in a right angle between the two lateral walls — Volume of the adult orbit :30 mL — Eyeball occupies only about one-‐ fi_h of the space — Anterior limit of the orbital cavity: orbital septum The Orbital Walls — The roof : frontal bone Posteriorly, the lesser wing of the sphenoid bone containing the op;c canal, the lacrimal fossa in the anterior lateral aspect — The lateral wall: separated from the roof by the superior orbital fissure, which divides the lesser from the greater wing of the sphenoid bone. The anterior por;on of the lateral wall is formed by the orbital surface of the zygoma4c (malar) bone (strongest part of the bony orbit) The Orbital Walls — The orbital floor: separated from the lateral wall by the inferior orbital fissure. The orbital plate of the maxilla forms the large central area and is the region where blowout fractures most frequently occur. The zygoma4c bone laterally complete the inferior orbital rim. The orbital process of the pala4ne bone forms a small triangular area in the posterior floor The Orbital Walls — The medial wall : ethmoid bone; lacrimal bone; The body of the sphenoid ( most posterior aspect of the medial wall). The angular process of the frontal bone forms the upper part of the posterior lacrimal crest. — The anterior lacrimal crest is easily palpated through the lid and is composed of the frontal process of the maxilla. The lacrimal groove lies between the two crests and contains the lacrimal sac The Orbital Walls WALLS BONY STGRUCTURES FORMING THE WALLS Roof Orbital plate of frontal bone, and lesser wing of sphenoid Floor Orbital plate of maxilla, orbital surface of Zygoma;c, orbital process of Pala;ne Lateral wall Zygoma;c, and greater wing of Sphenoid Medial wall Frontal process of Maxilla, lacrimal bone, orbital plate of ethmoid, and body of sphenoid The Apex Region — Entry portal for all nerves and vessels to the eye and the site of origin of all extraocular muscles except the inferior oblique — The superior ophthalmic vein and the lacrimal, frontal, and trochlear nerves pass through the lateral por;on of the fissure that lies outside the annulus of Zinn. The Apex Region — The superior and inferior divisions of the oculomotor nerve and the abducens and nasociliary nerves pass through the medial por;on of the fissure within the annulus of Zinn — The op4c nerve and ophthalmic artery pass through the op;c canal, which also lies within the annulus of Zinn Cranial Nerves — CN II Op;c – Vision — CN III Oculomotor – 4 muscles – Sympathe;cs to pupil — CN IV Trochlear – Superior oblique — CN V Trigeminal – Ocular sensa;on — CN VI Abducens – Lateral rectus — CN VII Facial – Orbicularis oculi Oculomotor Nerve — Runs along the lateral wall of the cavernous sinus — Two nuclei: oculomotor -‐ EW — Two branches: • Superior: Superior rectus and levator palpebrae superioris • Inferior: medial rectus, inferior rectus, inferior oblique • A short thick branch is given to the lower part of the ciliary ganglion, and forms its short root (short ciliary nerves: sphinter pupillae and ciliaris muscle) Trochlear Nerve Motor nerve (general soma;c efferent for SOM) Unique cranial nerve: — Thinnest (less axons) — Greatest intracranial length — Only that exists from the dorsal aspect of the brainstem (nucleus lesion → affects contralateral eye) — Runs along the lateral wall of the cavernous sinus (with Caro;d artery, III & V nerves) — Enter SOF, travel over the levator — Ends in a tendon (Trochlea) Trigeminal Nerve — Face sensa;on and motor func;ons (bi;ng and chewing) — The largest CN — 3 mayor branches: 1. Ophthalmic V1 (sensory): Nasociliary, Lacrimal , & frontal 2. Maxillary V2(sensory) 3. Mandibular V3 (sensory and motor) Trigeminal Nerve Ophthalmic nerve V1: (sensory) ! Scalp, forehead ! Upper eyelid ! Conjunc;va ! Cornea ! Nose, frontal sinuses ! Part of meninges (Dura) Maxillary nerve V2: (sensory) ! Lower eyelid ! Cheek, nares and upper lip ! Upper teeth and gums ! Maxillary, ethmoid and sphenoid sinuses,nand part of meninges Abducens Nerve — Soma;c efferent for LRM — Runs alongside the internal caro;d in the cavernous sinus — Enters the orbit SOF — Long course → vulnerable to injure at many levels (i.e. petrous temporal bone Fx, aneurysms, mass lesions) — Other problems can damage: stroke (infac;ons), demyelina;on, infec;ons (meningi;s),neuropathies (DR) — Rare: Wernicke-‐Korsakoff syndrome (thiamine↓), Tolosa –Hunt syndrome Orbit Blood Supply — Ophthalmic artery: is the first major branch of the intracranial por;on of the internal caro;d artery. It passes beneath the op;c nerve and accompanies it through the op;c canal into the orbit — Central re4nal artery: is the first intraorbital branch, which enters the op;c nerve — Lacrimal artery, supplying the lacrimal gland and upper eyelid — Long and short posterior ciliary arteries Orbit Blood Supply — Muscular branches to the various muscles of the orbit — Medial palpebral arteries to both eyelids — Supraorbital and supratrochlear arteries Lids Blood Supply — Arterial arcades of the eyelids: Formed by the most anterior branches of the ophthalmic artery (lateral and medial palpebral branches), which make an anastomosis with the external caro;d circula;on via the facial artery Eye Blood Supply — Short posterior ciliary arteries: supply the choroid & parts of the op;c nerve — Long posterior ciliary arteries:supply the ciliary body and form the major arterial circle of the iris — Anterior ciliary arteries: derived from the muscular branches to the rectus muscles. They supply the anterior sclera, episclera, limbus, and conjunc;va and contribute to the major arterial circle of the iris Venous Drainage — Superior and inferior ophthalmic veins: drain the vortex veins, the anterior ciliary veins, and the central re;nal vein. Communicate with the cavernous sinus via the superior orbital fissure and the pterygoid venous plexus via the inferior orbital fissure Ocular Adnexa — Eyebrows: prevent sweat, water, and other debris from falling down into the eye socket, also important to human communica;on and facial expression — Eyelids: is a thin fold of skin that covers and protects the eye — Lacrimal Apparatus: is the physiologic system containing the orbital structures for tear produc;on and drainage Eyelids — — — — Protect against injury and excessive light Keep eyes moist by spreading tears Maintain ocular surface Glands – source of cysts, tumours – Meibomian glands – modified sebaceous glands located in the tarsal plate – Glands of Zeis – modified sebaceous glands associated with lash follicles – Glands of Moll – modified sweat glands — Lashes – Lack erector pilorum muscles – Posi;on determined by surrounding orbicularis oculi, muscle of Riolan and tarsal plate Eyelids — From superficial to deep, they are the skin layer, a layer of striated muscle (orbicularis oculi), areolar ;ssue, fibrous ;ssue (tarsal plates), and a layer of mucous membrane (palpebral conjunc;va) — Orbital septum: the fascia behind the orbicularis muscle that lies between the orbital rim and the tarsus and serves as a barrier between the lid and the orbit — The levator and inferior rectus muscles are supplied by the third cranial (oculomotor) nerve Lid Retractors — Responsible for opening the eyelids — In the upper lid: the levator palpebrae superioris & Muller's (superior tarsal) muscle — In the lower lid: the main retractor is the inferior rectus — The smooth muscle components of the lid retractors are innervated by sympathe;c nerves Lid Margins Anterior lamella: — Skin, Cilia, sebaceous glands (Zeis), sweat glands (Moll) — Orbicularis Posterior lamella: — Tarsus: small orifices of modified sebaceous glands (meibomian, or tarsal glands) — Conjunc;va Lympha;cs — Lympha;cs from the lateral segment of the lids run into the preauricular and paro;d nodes — Lympha;cs draining the medial side of the lids empty into the submandibular lymph nodes Sensory Nerve Supply First and second divisions of the trigeminal nerve (V) 1st Ophthalmic: the lacrimal, the frontal, and the nasociliary nerves — Frontal: supraorbital and supratrochlear — Nasociliary: the infratrochlear nerve, which is sensory to the medial canthus, conjunc;va, lacrimal sac, canaliculi, and caruncle 2nd Maxillary: Infraorbital, zygoma;c nerve (zygoma;cotemporal, zygoma;cofacial) Lacrimal Apparatus — Lacrimal gland, the accessory lacrimal glands, the canaliculi, the lacrimal sac — Provides aqueous secretory component of tear film — 24 hour secretory volume=10ml — Main lacrimal gland situated under superolateral orbital rim — The accessory lacrimal glands are located in the conjunc;val fornices and the superior tarsal border (fewer in the lower eyelid) — 20-‐40 Krauss glands in the superior fornix+several Wolfring glands Lacrimal Gland — The blood supply is derived from the lacrimal artery — The nerve supply is by: (1) the lacrimal nerve (sensory), a branch of the trigeminal first division (2) the great superficial petrosal nerve (secretory), which comes from the superior salivary nucleus (3) sympathe;c nerves accompanying the lacrimal artery and the lacrimal nerve Extraocular Muscles — Rectus Muscles: originate at a common ring tendon(Zinn annulus) — Oblique Muscles: control primarily torsional movement and, to a lesser extent, upward and downward movement of the globe — The superior oblique is the longest and thinnest of the ocular muscles — The inferior oblique muscle originates from the nasal side of the orbital wall just behind the inferior orbital rim and lateral to the nasolacrimal duct Nerve supply — The oculomotor nerve (III) innervates the medial, inferior, and superior rectus muscles and the inferior oblique muscle — The abducens nerve (VI) innervates the lateral rectus muscle — The trochlear nerve (IV) innervates the superior oblique muscle Spiral of Tilaux — The rectus muscles insert in sclera gradually farther from the limbus beginning with the medial rectus at 5mm (range 3.0 to 6.0mm), inferior rectus 6mm, lateral rectus 7mm and superior rectus 8mm — The line of inser;on is called the spiral of Tillaux which is also the line of inser;on of posterior Tenon's capsule The Conjunc;va — Thin, transparent mucous membrane that covers the posterior surface of the lids joining the eyeball to the lids — Palpebral conjunc4va start at lid margins and is firmly adherent to tarsal plates — Bulbar conjunc4va covers anterior surface of the sclera, loosely aqached to Tenon (except at the limbus) — Blood & nerve supply: anterior ciliary and palpebral arteries; the first (ophthalmic) division of the V nerve Conjunc;va — Epithelium – 2-‐5 cell layers thick — Stroma – substan;a propria – Richly vascularised connec;ve ;ssue – Separated from epithelium by basement membrane — Glands -‐ Mucin secretors and accessory lacrimal glands – Goblet cells • Located in epithelium – dense inferonasally – Crypts of Henle • Located along upper third of superior tarsal conjunc;va and inferior tarsal conjunc;va – Glands of Manz Tenon Capsule — Fibrous membrane that envelops the globe from the limbus to the op;c nerve — Adjacent to the limbus, the conjunc;va, Tenon's capsule, and episclera are fused together — More posteriorly, the inner surface of Tenon's capsule lies against the sclera, and its outer aspect is in contact with orbital fat and other structures within the extraocular muscle cone — Sends a tubular reflec;on around each of these EOM (check ligaments) — Lower segment: IR & IO form the suspensory ligament (Lockwood) The Eyeball — Dimensions: ! At birth: 16-‐17 mm ! 3 yrs: 22.5-‐23 mm ! 13 yrs: 24 mm (adult length, most growth has occurred) ! Weight: 7.5 grams ! Volume: 6.5 cc Tunics: ! Fibro;c (sclera, cornea) ! Vascular (Iris, ciliar body, Choroid) ! Nervous (Re;na) The sclera — Opaque, tough outer fibrous protec;ve coa;ng of the eye, maintains shape and provides aqachment for EOM — At the inser;on of the rectus muscles→ 0.3 mm thick; elsewhere it is about 0.6 mm thick — Irregular arrangement of collagen bundles with occasional fibroblast and melanocytes — Episclera outer surface of the anterior sclera, thin layer of fine elas;c ;ssue connected to Tenon, which contains blood vessels — Lamina fusca the brown pigment layer on the inner surface of the sclera that forms the outer layer of the suprachoroidal space The Cornea — From ectoderm→avascular (isolated from IS) — 65 to 75% of the eye’s capacity to focus light on the re;na (40-‐45D) — Adult cornea is 550 m thick in the center and 11.75 mm in diameter horizontally and 10.6 mm ver;cally — From anterior to posterior, it has five layers : 1. Epithelium 2. Bowman's layer 3. Stroma 4. Descemet's membrane 5. Endothelium — Nourished by tears, oxygen, and the aqueous humor of the AC The Cornea — Epithelium: thin mul;cellular epithelial ;ssue layer (non-‐kera;nized stra;fied squamous epithelium) of fast-‐growing and easily regenerated cells, replaced every 7-‐10 days (limbal epithelial stem cells) — Bowman's Layer: tough layer composed of collagen (mainly type I collagen fibrils), that protects the corneal stroma — Stroma: 90% thickness. Collagen type I (200 layers) interconnected keratocytes, which are the cells for general repair and maintenance The Cornea — Descemet’s layer: thin acellular layer that serves as the modified basement membrane of the endothelium. Collagen type IV (less rigid than I) 5-‐20 μm thick — Endothelium: only one layer of cells, responsible for maintaining the essen;al deturgescence of the corneal stroma. Is suscep;ble to injury as well as loss of cells with age. Failure of endothelial func;on leads to corneal edema The Cornea — The transparency: uniform structure, avascularity, and deturgescence — Sources of nutri;on: vessels of the limbus, the aqueous, and the tears. The superficial cornea also gets most of its oxygen from the atmosphere — The sensory nerves of the cornea are supplied by the first (ophthalmic) division of the fi_h (trigeminal) cranial nerve The Uveal Tract — Vascular tunic of the eye — Iris, the ciliary body, and the choroid — The primary func;on is to supply nutri;on to the eye, both in health and in disease — Clinically, mirrors vascular diseases. Inflamma;on in the uveal tract, or uvei;s reflects local and systemic noxious agents — Degenera;ons are due to vasculopathies such as arteriosclerosis, diabetes mellitus, and autoimmune diseases The Iris — Is a musculovascular diaphragm with a central opening, the pupil — The sphincter and dilator muscles of the iris regulate the pupillary size — 12 mm in diameter and 0.5 mm in thickness, with a 3-‐mm, slightly nasal, off-‐center pupillary aperture in the res;ng state — The iris is thickest near the collareqe and thinnest at the iris root — The blood & nerve supply : major circle of the iris and fibers of the ciliary nerves The Ciliary Body — Ring of ;ssue between the iris and anterior edge of re;na 3 main func;ons: — Accommoda;on — Aqueous humor produc;on — Produc;on of lens zonules and vitreous components — Pars plicata → Corrugated anterior zone — The ciliary processes arise from the pars plicata and their covering ciliary epithelium (responsible for the forma;on of aqueous) — Pars plana →flaqened posterior zone The Ciliary Body — The ciliary muscle is composed of a combina;on of longitudinal, circular, and radial fibers and is responsible for accommoda;on — The func;on of the circular fibers is to contract and relax the zonular fibers, which originate in the valleys between the ciliary processes — The blood vessels supplying the ciliary body are derived from the major circle of the iris The Choroid — The posterior segment of the uveal tract, between the re;na and the sclera — Is a thin (0.2-‐mm), spongy, pigmented, vascular lamina — The internal por;on of the choroid vessels is known as the choriocapillaris — The choroid is bounded internally by Bruch's membrane and externally by the sclera — Serves to nourish the outer por;on of the underlying re;na The Lens — Is a biconvex, avascular, colourless, and almost completely transparent structure, about 4 mm thick and 9 mm in diameter — It is suspended behind the iris by the zonule, which connects it with the ciliary body — Gradually becomes larger and less elas;c throughout life — 65% water, 35% protein — There are no blood vessels, or nerves, nourishment derived from the aqueous and vitreous humors The Re;na — Is a thin, semitransparent, highly organized mul;layered sheet of neural ;ssue that lines the inner aspect of the posterior two-‐thirds of the wall of the globe — It extends almost as far anteriorly as the ciliary body, ending at that point in a ragged edge, the ora serrata (white arrow) — 2 components-‐neural re;na and RPE — The outer surface of the sensory re;na is apposed to the re;nal pigment epithelium and thus related to Bruch's membrane, the choroid, and the sclera Re;nal Pigment Epithelium — Monolayer of columnar cells, external to neural re;na — Selec;vely permeable barrier between neural re;na and vascular choroid (blood-‐re;na barrier) -‐ ;ght junc;ons — Phagocytosis of photoreceptor ;ps — Absorp;on of light (reduces scaqer in eye) — Transport/storage of metabolites & vitamins — Low regenera;ve capacity The Re;na (1) (2) (3) (4) Internal limi;ng membrane Nerve fiber layer (GC axons) Ganglion cell layer Inner plexiform layer (GC, Amacrin, and Bipolar connec;ons) (5) Inner nuclear layer (Bipolar, Amacrin, and Horizontal cell bodies) (6) Outer plexiform layer (Bipolar, Horizontal, and Photoreceptors) (7) Outer nuclear layer (Photo. nuclei) (8) External limi;ng membrane (9) Photoreceptor layer (10) Re;nal pigment epithelium The Macula — Macula lutea – Oval yellowish area at the center of the posterior part of the re;na – 5mm in diameter – Lies 3mm to the lateral side of the op;c disc — Fovea centralis-‐ 1,5 mm-‐diameter, thinnest part of area of the re;na (0.25 mm), containing only cone photoreceptor — Histologically it is characterized by thinning of the outer nuclear layer and absence of the other parenchymal layer The Re;na — Blood supply : Choriocapillaris supplies the outer third of the re;na, and branches of the central re4nal artery, supply the inner two-‐thirds — The fovea is supplied en;rely by the choriocapillaris and is suscep;ble to irreparable damage if detached — The re;nal blood vessels have a nonfenestrated endothelium, which forms the inner blood-‐re;nal barrier — The endothelium of choroidal vessels is fenestrated. The outer blood-‐re;nal barrier lies at the level of the re;nal pigment epithelium The Vitreous — Clear, avascular, gela;nous body that comprises two-‐thirds of the volume and weight of the eye — Highly viscoelas;c-‐shock absorber role — The outer surface of the vitreous (the hyaloid membrane) is normally in contact with the following structures: the posterior lens capsule, the zonular fibers, the pars plana epithelium, the re;na, and the op;c nerve head The Vitreous — The base of the vitreous maintains a firm aqachment throughout life to the pars plana epithelium and the re;na immediately behind the ora serrata — The aqachment to the lens capsule and the op;c nerve head is firm in early life but soon disappears — Liquefies with age and shrinks-‐ detaches from re;na (floaters) — 99% water — The remaining 1% includes two components, collagen and hyaluronic acid — Can tear re;na and result in RD The Op;c Nerve — 1 million axons that arise from the ganglion cells of the re;na (nerve fiber layer) — The orbital segment of the nerve is 25-‐30 mm long — It travels within the op;c muscle cone, via the bony op;c canal, and thus gains access to the cranial cavity The Op;c Nerve — The intracanalicular por;on measures 4-‐9 mm — A_er a 10-‐mm intracranial course, the nerve joins the opposite op;c nerve to form the op;c chiasm — 80% of the op;c nerve consists of visual fibers that synapse in the lateral geniculate body on neurons whose axons terminate in the primary visual cortex of the occipital lobes — 20% of the fibers are pupillary and bypass the geniculate body en route to the pretectal area The Op;c Nerve — The surface layer of the op;c disk receives blood from branches of the re4nal arterioles — In the region of the lamina cribrosa, comprising the prelaminar, laminar, and retrolaminar segments of the op;c nerve, the arterial supply is from the short posterior ciliary arteries — The anterior intraorbital op;c nerve receives some blood from branches of the central re4nal artery The Op;c Nerve — The remainder of the intraorbital nerve, as well as the intracanalicular and intracranial por;ons, are supplied by a pial network of vessels derived from the various branches of the ophthalmic artery and other branches of the internal caro;ds The Op;c Chiasm — Located near the top of the diaphragm of the sella turcica — Junc;on of the two op;c nerves — Provides crossing of the nasal fibers to the opposite op;c tract and passage of temporal fibers to the ipsilateral op;c tract — The macular fibers are arranged similarly to the rest of the fibers except that their decussa;on is farther posteriorly and superiorly — Receives many small blood vessels from the neighbouring circle of Willis The Visual Pathway " " " " " " " " " " " " 1) Optic nerve 2) Optic chiasm 3) Optic tract 4) Lateral geniculate body 5) Optic radiation 6) Visual cortex 7) Superior colliculus of the midbrain 8) Putamen 9) Long association bundle – inferior occipitofrontal fasciculus 10) Pulvinar of the thalamus 11) Calcarine fissure 12) Posteroinferior horn of lateral ventricle The Visual Pathway — Light — Photoreceptors — Op;c Nerve — Op;c Chiasm (par;al crossing) — Op;c Tract — Lateral Geniculate Nucleus(LGN) — Op;c Radia;ons — Primary Visual Cortex Primary visual cortex — Visual informa;on reaches the cortex via V1 — Area 17 (striate cortex) — Most complex region of the cortex — Other areas →Extrastriate visual cortex: V2,V3, V4, and MT (middle temporal area) or V5 Primary visual cortex — Six iden;fiable layers — Layer 1 is close to the cor;cal surface — Layer 6 adjoins the white maqer below — Projec;ons from the LGN arrive in layer 4 Visual Cortex — The separa;on of a visual image into its component parts con;nues even past the lateral geniculate nucleus and primary visual cortex — Temporal cortex: form and colour (“what” pathway) — Parietal cortex: mo;on and deep percep;on (“where” pathway) The Visual Pathway Rules: — Each hemisphere gets input from both eyes — A given hemisphere gets informa;on from the opposite half of the visual word
