L1-MUST-SS-Eyeball-Fall 2024-2025 ST PDF

PHYSIOLOGY OF VISION https://books.lib.uoguelph.ca/human-physiology/chapter/vision/ Objectives of Current Presentation: At the end of this lecture, the student should memorize the following key concepts; 1. Define poles of eyeball 2. Describe eyelids and eyelashes 3. Discuss lacrimal sys...

PHYSIOLOGY OF VISION https://books.lib.uoguelph.ca/human-physiology/chapter/vision/ Objectives of Current Presentation: At the end of this lecture, the student should memorize the following key concepts; 1. Define poles of eyeball 2. Describe eyelids and eyelashes 3. Discuss lacrimal system of the eye 4. Comprehend different layers of the wall of eyeball 5. Illustrate Fundus oculi 6. Aqueous humor, intraocular pressure and glaucoma 7. Eye lens and its common disorders 8. Enumerate types of simultaneous movements of both eyeballs BY Associate Professor Doctor Abdou M.A.Ibrahim Structure of Eyeball  Poles of Bulbus Oculi  Human eyeball (bulbus oculi) is approximately globe- shaped, with a diameter of about 24 mm (nearly one inch).  It is slightly flattened from above downwards. Eyeball is made up of two segments, an anterior and a posterior segment. The anterior part is small and forms one sixth of the eyeball.  The posterior segment is larger and forms 5/6 of the eyeball. Posterior wall of this part is lined by the light-sensitive structure called retina.  Center of anterior curvature of the eyeball is called anterior pole and the center of posterior curvature is called posterior pole.  The line joining anterior & posterior poles is called optic axis. Which side of the eye in this diagram?  The line joining a point in the cornea, little inside to the anterior pole and fovea centralis (situated lateral to posterior pole) is known as visual axis. Light rays pass through the 🤔 visual axis of eyeball are not refracted. Optic nerve leaves the eye, little medial to posterior pole.  Orbit & Eyeball  Except anterior one sixth, the eyeball is situated in a bony cavity known as orbital cavity or eye socket.  A thick layer of fatty areolar connective tissue is interposed between bone and eyeball. It serves as a cushion to protect the eyeball from external force.  The eyeball are attached to orbital cavity by the extra ocular muscles.  Eyelids and Eyelashes  Eyelids closure protect the eyeball from foreign particles encountering its surface and cutoff the light during sleep.  Eyelids are opened and closed voluntarily, as well as by reflex action.  On average, most people blink up to 20 times per minute. This helps your eyes stay healthy by keeping them oxygenated and moist, and clearing out debris. While there are some conditions that can cause you to blink more or less frequently, a change in your rate of blinking is rarely a sign of a disease.  Margins of eyelids have sensitive hair called the cilia (eyelashes). Each cilium arises from a follicle, which is surrounded by a sensory nerve plexus. When dust particle touches the cilia, these sensory nerves are activated, resulting in rapid blinking of eyelids. It prevents the dust particles from reaching the cornea. Muscles of eyelids - Eyelid Muscles: Retractors. The eyelid  There are about 100 to 150 cilia in the upper eyelid and about 50 to 75 muscle retractors serve to open the eye. cilia in the lower eyelid. - The retractors of the upper eyelid are; The levator palpebrae superioris (CrIII)  Meibomian glands and some sebaceous glands are also found in the Müllers muscles (Sympathetic supply) eyelids. These glands open into the follicles of cilia. Infection of these Frontalis (Cr VII) glands leads to a painful suppurative lesion (minute acute abscess) - The lower lid retractors are; Capsulopalpebral muscle (Cr V) called sty. Inferior tarsal/palpebral muscle (Cr V)  Elliptical opening between the two eyelids is called palpebral fissure. The NB; Orbicularis oculi is a skeletal facial muscle average of palpebral fissure width is 31 mm in males and 29 mm in present in both upper and lower eye lids and responsible for closure of palpebral fissure and females. The average of palpebral fissure height at its longest measure supplied by facial nerve (Cr VII). is 11.5  Conjunctiva  Conjunctiva is a thin mucus membrane, which covers the whitish exposed part (anterior 1/6 of sclera) of the eye, then reflects to line the inner surface of upper and lower eyelids.  The part of conjunctiva covering the exposed part of sclera of eyeball is called bulbar portion.  The part of conjunctiva that lines the inner aspect of the eyelid is called palpebral portion.  Surface of conjunctiva is lubricated by thin film of tear secreted by lacrimal gland.  During closure or opening of palpebral fissure, the opposed portions of conjunctiva slide over each other with ease due to the film of tears.  Lacrimal Gland & Tears  Lacrimal gland is situated in the shelter of bone that is bounded by upper and outer border of wall of the eye socket (orbit).  Basal tear secretion occurs at a rate of about 1.2 µl/minute, although reflexive tear secretion can increase this up to 100-fold.  Tears enter the puncta at a rate of 0.6 µl/min; about 90 percent are reabsorbed through the nasolacrimal duct mucosa and 10 percent is drained into the floor of the nasal cavity.  From lacrimal gland, tear flows over the surface of conjunctiva then it is drained in multiple ductuli into lacrimal canaliculi then into lacrimal sac then into nasolacrimal duct.  Tear is a hypertonic fluid. Due to its continuous washing and lubrication, the conjunctiva is kept moist and is protected from infection. Tear also contains lysozyme that kills bacteria.  Secretion of tears is controlled by the parasympathetic fibers of facial (Cr VII) nerve (tears is deficient in central lesion of Cr VII).  Around 2 weeks after birth, your baby's lacrimal glands will begin to increase tear production, although you may still not notice much change. Usually, about 2 months after birth is when babies really start to produce more tears when they cry, creating tears that you can see. Layers (tunicae) of Eyeball  The outer layer-Tunica Fibrosa Outer layer preserves the shape of the eyeball. Posterior five sixth of this coat is opaque and uncovered by conjunctiva. Anterior one sixth has a transparent disc-shaped central structure called cornea. 1. Sclera Sclera is the tough opaque, white and fibrous outer layer of eyeball, that covers nearly the posterior five sixth of the eye. Anteriorly it is continuous with cornea. Sclera is formed by white fibrous tissues and elastic fibers. Posterior part of sclera, where it is pierced by the optic nerve is thin with perforations. It is named as lamina cribrosa. The transitional part of outer layer between sclera and cornea is called limbus. It is about 1 mm width. The diameter of limbus is the same as that of cornea (it separates the cornea from sclera). 2. Cornea Cornea is the transparent convex most central anterior portion of the outer layer of eyeball, which covers the iris and pupil. It is naked of conjunctiva normally. In some diseases the conjunctiva crosses the limbus and encroaches on cornea and this finding is known as pterygium. Diameter of cornea is about 12 mm horizontally and 11 mm vertically.  The middle layer-Tunica Vasculosa Middle layer surrounds the sclera completely and stretches to form the posterior wall of anterior chamber (iris). It is formed from; The choroid most posterior The middle ciliary body The iris that includes the light aperture (pupil) most anteriorly. 1. Choroid  Choroid is the thin vascular layer of eyeball situated between sclera and retina.  It lines most of tunica fibrosa and covers all tunica nervosa. Choroid is extended anteriorly up to the insertion of ciliary muscle (the level of ora serrata).  Choroid is separated from sclera by peri-choroidal space. Anteriorly, this space is limited by the insertion of ciliary muscle into sclera. Posteriorly, this space ends at a short distance around the optic nerve.  Inner surface of choroid faces the pigment epithelium (innermost layer) of retina.  Choroid is rich in capillary plexus, numerous small arteries and veins.  Middle layer-Tunica Vasculosa 2. Ciliary Body  Ciliary body is the thickened part of middle layer of eye, situated between choroid and iris. It is situated in front of ora serrata (the serrated junction between the retina and the ciliary body).  The ciliary body is a ring. Its outer surface is separated from sclera by the obliterated anterior end of peri-choroidal space and inner surface faces the most anterior part of vitreous body and lens.  The suspensory ligaments (zonules); The zonule, often referred to as the ciliary zonule, is the circumferential suspensory ligament that connects the eye lens capsule to the ciliary body. From the anterior surface of ciliary body, the iris arises.  The Ciliary body has three parts: i. Orbiculus ciliaris: It is continuous with choroid, and it forms the posterior two third of ciliary body. It is about 4 mm broad. ii. Ciliary body proper: It is made up of 2 sets of ciliary muscle fibers, outer longitudinal and inner circular. Ciliary muscles are innervated by the parasympathetic fibers of oculomotor nerve. iii. Ciliary processes (corona ciliaris): Ciliary processes are the finger-like projections from inner surface of ciliary body. There are about 70 ciliary processes, projecting centrally and responsible for aqueous humor secretion.  → Middle layer-Tunica Vasculosa 3. Iris  Iris is a thin colored curtain-like structure of eyeball, located in front of the lens. It forms a thin circular diaphragm with a circular opening in the center called the pupil. Iris includes 2 muscles: i. Constrictor pupillae or iris sphincter muscle: It is formed by circular muscle fibers. Contraction of this muscle causes constriction of pupil (myosis). ii. Dilator pupillae or pupillary dilator muscle: It is formed by radial muscle fibers. Contraction of this muscle causes dilatation of pupil (mydriasis).  Activities of these muscles increase or decrease the diameter of pupil and regulate the amount of light entering the eye. Thus, iris acts like the diaphragm of a camera.  Iris separates the space between cornea and lens into two chambers, namely anterior and posterior chambers. Both the chambers communicate with each other through pupil. Lateral circumferential border of anterior chamber is angular in shape. It is called iris-cornea angle or angle of anterior chamber (iridocorneal angle).  Inner layer-Tunica Nervosa Retina is a delicate light-sensitive membrane that forms the innermost layer of eyeball. It extends from the margin of optic disk to just behind ciliary body. Here, it ends abruptly as a dentated border known as ora serrata. Structurally, retina is made up of 10 layers from outside in: