Lecture :ANATOMY OF THE EYE By Professor Marianne Shahsuvaryan The tissues and structures surrounding the eye, protecting and supporting the globe: Orbit Extraocular Muscles Eyelids Lacrimal Apparatus The Orbit - the bony cavity in the scull - contains the globe, the extraocular muscles, the blood vessels, the nerves, fat Fig.1.1.) The Extraocular Muscles (EOM) - control the movement of the globe (Fig.1.2) - four rectus muscles: medial, lateral, superior, inferior - two oblique muscles: superior, inferior - medial rectus moves the eye towards the nose - lateral rectus moves the eye away from the nose - superior rectus moves the eye up - inferior rectus moves the eye down - superior oblique moves the eye down and away from the nose - inferior oblique moves the eye up and away from the nose Fig.1.1. Sagittal section of the eye in the orbit 1 – Frontal Bone, 2 – Superior Orbital Septum, 3 – Superior Tarsus, 4 – Cornea, 5 – Anterior Chamber, 6 – Pupil, 7 – Lens, 8 – Iris, 9 – Posterior Chamber, 2 10 – Zonules, 11 – Vitreous, 12 – Schlemm’s Canal, 13 - Ciliary Body, 14 – Inferior Tarsus, 15 – Inferior Orbital Septum, 16 – Maxilla, 17 – Levator Palpebrae Muscle, 18 – Superior Rectus Muscle, 19 – Orbital Fat, 20 – Optic Nerve, 21 – Retina, 22 – Choroid, 23 – Sclera, 24 – Inferior Rectus Muscle. 3 Fig 1.2. The extraocular muscles The Eyelids - the outer structures that protect the eyeball and lubricate the ocular surface (Fig 1.3) - join at the medial and lateral canthus - have the space between the two open lids – palpebral fissure - the margin or edge of the eyelid contains: hair follicles for the eyelashes (cilia) – anterior edge openings of oil – secreting meibomian glands – posterior edge - consists of the following layers from anterior to posterior: (Fig 1.1 ; 1.3) 1. skin 2. fibrous tissue and muscles – the tarsus containing meibomian glands; orbicularis oculi – a muscle that closes the eye; levator palpebral – a muscle that raises the upper eyelid 3. the conjunctiva The conjunctiva 4 - thin translucent mucous membrane that covers the inner surface of the eyelids (palpebral conjunctiva) and the outer front surface of the eyeball, except for the cornea (bulbar conjunctiva). The Lacrimal apparatus (Fig 1.3) consists of - lacrimal gland, located in the lateral part of the upper eyelid just under the upper orbital rim, responsible for tears production - the upper punctum and lower punctum located on the upper and lower eyelid margins near the nose - the upper canaliculus and the lower canaliculus join together and connect with the lacrimal sac - lacrimal sac - the nasolacrimal duct, which opens into the nasal cavity in the inferior nasal meatus. Fig.1.3. External eye and lacrimal system 1 – Lateral canthus, 2 – Lacrimal gland, 3 – Upper Punctum, 4 – Upper Canaliculus, 5 – Medical Canthus, 6 – Lacrimal Sac, 7 – Nasolacrimal Duct, 8 – Lower Canaliculus, 9 – Lower Punctum, 10 - Follicles of Cilia, 11 – Cilia, 12 – Openings of Meibomian Glands 5 The tear film The tear film is made of 3 layers, from anterior to posterior: 1. The lipid layer; secreted by the meibomian glands in the tarsus of the eyelid - lubricate the eyelid 2. The aqueous layer; secreted by the main lacrimal gland and the accessory lacrimal glands in the superior fornix of conjunctiva - provides oxygen and nutrition for the cornea - has antibacterial properties 3. The mucus layer; secreted by goblet cells in the bulbar and palpebral conjunctiva - makes the corneal surface hydrophyllic and wettable. When we remove the eye from the orbit, we can see that the eye is a slightly asymmetrical sphere with an approximate sagittal diameter or length of 24 to 25 mm. and a transverse diameter of 24 mm. It has a volume of about 6.5 cc. A cross-sectional view of the eye shows: Three different layers 1. The external layer, formed by the sclera and cornea. 2. The intermediate layer, divided into two parts: anterior (iris and ciliary body) and posterior (choroid). 3. The internal layer, or the sensory part of the eye, the retina. Three chambers of fluid: Anterior chamber (between cornea and iris), Posterior chamber (between iris, zonule fibers and lens) and the Vitreous chamber (between the lens and the retina). The first two chambers are filled with aqueous humor whereas the vitreous chamber is filled with a more viscous fluid, the vitreous humor. 6 The cornea: - transparent anterior 1/6th of the globe - vertical diameter ~ 11mm, horizontal ~ 11.7mm - central thickness 0.5mm, 1mm peripherally - axial refractive power of 43 diopters - avascular, oxygen supply mainly from the tear film, metabolic requirements from the aqueous humor and perilimbal vascular plexus - composed of 5 distinct histologic layers: 1. Epithelium 2. Bowman’s layer 3. Stroma 4. Descemet’s membrane 5. Endothelium Bowman’s layer: Does not regenerate after injury. Stroma: - 90% of thickness of cornea - composed of collagen fibrils of uniform diameter and regular spacing. Fibers in any one lamellae are parallel but perpendicular to fibers in adjacent lamellae. Descemet’s membrane: Does regenerate after injury Endothelium: - a single layer of flattened hexagonal cells facing the anterior chamber - highly active in maintaining corneal transparency by regulating the water content of the corneal stroma. Physical or metabolic damage to the endothelium leads to corneal swelling, overhydration and opacification. The limbus 7 - a transitional zone between the cornea and the sclera - abrupt transition occurs from avascular cornea to vascular limbus - this region contains the outflow apparatus of the aqueous humor (the trabecular meshwork and the canal of schlemn ). The sclera - White of the eye, posterior five sixths of the globe. - Irregular size and arrangement of collagen fibrils. - Thickness is 1 mm posteriorly near the optic nerve and 0.3 anteriorly where the extraocular muscles (EOM) attach. - The posterior scleral foramen is a canal which transmits the optic nerve, the central retinal artery and vein, and the sympathetic plexus to eye. This canal is 2 -3 mm in diameter and is bridged by a sieve-like structure called the lamina cribrosa. The Iris - a contractile diaphragm that controls the degree of retinal illumination. - has a central aperature, the pupil, located slightly nasally. - consists of the following layers from anterior to posterior: Fig 1.4. View of the human eye 1. stroma: contains the sphincter pupillae muscle (parasympathetic innervation). Heavily pigmented in persons with brown eyes, less pigmented in green and hazel irises, and least in blue. 2. pigment epithelium: contains the dilatator pupillae muscle (sympathetic innervation) The ciliary body (Fig 1.1.) 8 - part of the uvea = choroid + ciliary body + iris - a band-like structure made of muscle and secretory tissue that extends from the edge of the iris and encircles the inside of the sclera toward the front of the eyem responsible for uveascleral outflow - consists of a posterior portion (pars plana) and an anterior portion (pars plicata).The latter has 60-70 folds called the ciliary processes which secrete the aqueous humor into the posterior chamber. Contraction of the ciliary muscle relaxes the zonules allowing for increased curvature of the lens, thereby increasing its refractive power in the process of accommodation (near vision). The choroid - dark brown vascular sheet, lying between the sclera and the retina. - consists of large vessels and extensive network of fenestrated vessels, the choriocapillaris which is the major blood supply to the outer layers of the retina and to the whole macula. Bruch’s membrane - a relatively thin membrane lying between the choriocapillaris of the choroid and the retinal pigment epithelium (RPE) of the retina. - In AMD (age related macular degeneration),which is the most common cause of blindness above 65 years of age, there are yellow depositions in Bruch’s membrane called drusens, usually at the macula. The retina [Fig.1.5; 1.6] - Most internal layer of the eye, facing the vitreous - ends at the ora serrata anteriorly. - consists of 10 basic layers 9 Fig.1.5. Scheme of the human retina. 1. neural retina: inner layer, itself has 9 layers including the photoreceptor layer. 2. RPE (retinal pigment epithelium): outer layer that rests on Bruch’s membrane & choroid. Fig 1.6. The Retina - Photoreceptors are the outermost layer of the sensory retina, and are divided into: 1. cones: responsible for color vision and daytime high discrimination vision. They are highly concentrated at the fovea (7 millions). 2. rods: responsible for night vision (black and white) cruder perception and less resolution (120 millions). The Macula 10 The area of the retina at the posterior pole of the eye responsible for fine, central vision. Fig 1.7. Normal fundus The optic nerve (Fig.1.7). - formed by the axons of the 1.2 million ganglion cells while exiting the eye. - contains within its fibers the central retinal artery and the central retinal vein which emerge from a central depression called the physiologic cup - can be divided into 4 portions: 1. intraocular 2. intraorbital portion extending from the globe until the apex of the orbit 3. intracanalicular portion within the optic canal 4. intracranial portion that merges into the chiasm and then optic tract. Visual Pathways of the Brain In order for perception to occur, the physiological signal that starts in the retina must travel to the visual cortex. As we saw in the diagram of the retina, there are several layers of neurons which lead to the optic nerve. 11 Fig 1.8. Diagram of the Brain In the diagram of the brain (Fig 1.8) we see that the optic nerve travels from the retina to the lateral geniculate nucleus in the mid brain. The neurons then become the visual radiations which travel to the visual cortex at the back of the brain. The visual cortex is also called the striate cortex and the occipital cortex.