Optometry Notes

OPTOMETRY · SEMESTER 2 Ocular Embryology Ocular Anatomy and Physiology START READING NOTES Contents of This Topic Ocular Embryology CONTENTS CHAPTER 6: CHAPTER 12: THE EYEGLOBE NEURO-OPHTHTALMIC_ ANATOMY CHAPTER 1. DEVELOPMENTAL DISORDERS. OPTIC PITS Ciliary Body PUPILLARY MEMBRANE. ANTERIOR CHAMBER. VITREOUS OPTIC NERVE DEVELOPMENT OF OCULAR ADNEXA. NASOLACRIMAL SYSTEM Hyaloid Arterial System BLOOD VESSEL PERMEABILITY AND BARRIERS Ocular Embryology THE EYE BASIC ANATOMY AND PHYSIOLOGY MOSHI PREFACE This book is written to provide the Assistant Medical Officer- Ophthalmology , the Optometrist and the Ophthalmic nurse, with basic knowledge of anatomy and physiology of the eye. The practical application of anatomic facts to ophthalmology, has been emphasised throughout the book. Clinical problems requiring anatomic knowledge for their solution, are presented at the end of each chapter. Weare greatly indebted to the members of the Advisory Board, Allied Health Sciences School KCMC, for their moral and financial support. Ophthalmologists Moshi CONTENTS CHAPTER 1: CHAPTER 2: CHAPTER 3: CHAPTER 4: CHAPTER 5: CHAPTER 6: CHAPTER 7: CHAPTER 8: CHAPTER 9: CHAPTER 10: CHAPTER 11: CHAPTER 12: OCULAR EMBRYOLOGY THE ORBITAL CAVITY THE EYELIDS THE LACRIMAL_ APPARATUS THE EXTRAOQCULAR MUSCLES THE EYEGLOBE THE CORNEA AND THE SCLERA THE ANTERIOR CHAMBER THE UVEAL TRACT THE CRYSTALLINE LENS THE RETINA AND THE VITREOUS NEURO-OPHTHTALMIC_ ANATOMY AND PHYSIOLOGY 14 22 27 37 40 46 52 61 67 83 CHAPTER 1. OCULAR EMBRYOLOGY 1. INTRODUCTION. Developmentally and functionally the eye is an extention of the central nervous system. The human eye development commences in the 4" week of gestational age and the tissues that will constitute the human eye, are derived from: e surface ectoderm; e neural ectoderm; e neural crest; mesodermal mesenchyme. Embryologic Derivation of Ocular Structures Surface ectoderm gives rise to: + Lens + Corneal epithelium + Conjunctival epithelium and lacrimal glands + Epithelium of eyelids and cilia, meibomian glands, and glands of Zeis and Moll + Epithelium lining nasolacrimal system Neural ectoderm gives rise to: + Retinal pigment epithelium + Neural retina + Optic nerve fibers + Neuroglia + Epithelium of ciliary body + Epithelium of iris, including iris sphincter and dilator muscles Neural crest gives rise to: + Corneal stroma (which gives rise to Bowman’s layer) + Corneal endothelium (which gives rise to Descemet’s membrane) + Most (or all) of sclera + Trabecular structures + Uveal pigment cells + Uveal connective tissue + Vascular pericytes During the embryonic development of the eye, the final differentiation and sangerent of clare conta by numerous inductive and suppressive These interactions are mediated by specific elements; recently, three elements have been identified , as making very important contributions to the series of sequential events: e growth factors; ¢ homeobox (homeotic) genes; One particular gene that appears to be important in the development of ocular structures is the Pax-6 gene. As it influence the development of lens, cornea and conjuctiva e neural crest cells. 2. CHRONOLOGY OF OCULAR DEVELOPMENT AND COMMON DEVELOPMENTAL DISORDERS. As stated above, the eye embryologically develops parallel with the central nervous system. During the third week of embryonic development, the three primary germ layers— ectoderm, mesoderm, and endoderm— have formed the embryonic plate.4 (Of these three, only ectoderm and mesoderm will take part in the developing ocular structures.) A thickening in the ectoderm, visible on the dorsal surface of the embryo, forms the neural plate, which will give rise to the central nervous system, including ocular structures. In humans, the central nervous system differentiates from ectoderm. First, the neural plate develops (neural ectoderm), then the neural groove and the neural folds . Later the walls of the groove fuse, forming the neural tube. The developing central nervous system is now recognised as the neural tube. The ectoderm now lining the tube is neural ectoderm and that surrounding the tube is surface ectoderm, which differ both in anatomic location and in differentiation potentials By day 22 (embryological age) and embryo size 2-3 mm, the optic sulci develop on the neural folds; later the sulci become pits, then optic vesicles. At the same tim: , which contribute extensively to the development of the eye. These neural crest cells come to lie between the neural tube and the surface ectoderm Mery of the neural crest cells migrate away from the neural tube,and form Congenital and developmental anomalies that involve cells derived from the neural crest have been mer ssaent under the term Formed by the deepening of the optic pits, the optic vesicles are continuous with the neural tube, by optic stalks ( future optic nerve).The cavity of the hollow optic vesicle communicates with that of the forebrain.Later the optic vesicle invaginates to form the optic cup with its two layers. Optic cup Optic stalk Surface ectoderm Lens vesicle Choroid fissure Mesectoderm (Neural crest) Hyaloid artery Fig. 1.1. Diagrammatic representation of the : optic cup, lens vesicle, optic stalk and embryonic ( choroids) fissure. At about sper of seit n one te ize 4. it Gradually the lens placode invaginates and the lens vesicle is formed ( by day 29). im th 33, the lens vesicle Soar from T_T OPTIC PITS The optic pits form as indentations on both sides of the neural tube in the forebrain region even before the tube is completely closed. On approximately day 25, after the neural tube has closed, the optic pits form lateral sac- shaped extensions, the optic vesicles. Neural crest cells and mesoderm collectively make up the mesenchyme, from which the connective tissue of the globe and orbit develop. Mesodermal cells and neural crest cells appear similar cytologically. If the origin is uncertain, mesenchyme is cited as the germ layer. As the optic vesicle evaginates, the tissue joining the vesicle to the neural tube constricts, forming the optic GD ait with wes offic stalk. The cavity of the optic stalk, as well as that of the optic vesicle, is continuous with the space that will become the third ventricle. While the wall of the optic vesicle is in contact with surface ectoderm, it thickens and flattens to form the

OPTOMETRY · SEMESTER 2 The Orbital Cavity Ocular Anatomy and Physiology START READING NOTES Contents of This Topic The Orbital Cavity THE WALLS OF THE ORBIT Frontal Superior Palatine Zygomatic Inferior Maxillary Lacrimal The Orbital Cavity CHAPTER 2. THE ORBITAL CAVITY THIS CHAPTER SHOULD BE READ WITH A SKULL MODEL IN FRONT BY THE END OF THIS CHAPTER YOU WILL KNOW: DEFINITION,SHAPE, MEASUREMENTS, RELATIONSHIPS THE WALLS OF THE ORBIT ORBITAL CONTENTS THE FUNCTIONS OF THE ORBIT OPENINGS OF THE ORBIT CLINICAL CONSIDERATIONS QaROnNn> 1. DEFINITION, SHAPE , MEASUREMENTS, RELATIONSHIPS. 1.1. The shape of the orbit. It is like a hollow pyramid or a pear with an open base facing forwards ja and the a of the pee or the stem of the pear facing The stalk of the pear represents the optic nerve and lies in an opening called the optic foramen. The medial walls are almost parallel. The apex of the orbit is Important nerves and vessels are crowded here, 1.2. Measurements of the orbit. a. Height: 35mm b. Width: 45mm c. Depth: 40-45mm from the entrance to apex d. Volume (adult): ~30cc 1.3. Relationships – the paranasal sinuses. Adjacent to the orbital cavities lie the paranasal sinuses. These are bony cavities filled with air, located as follows: the frontal sinus (FS ), superiorly; e the ethmoid sinus (ES ), medially; e the maxillary sinus ( MS ), inferiorly; e the sphenoid sinus posteriorly. Fig. 2.1. The relationship of the orbits to the paranasal sinuses. 8 2. THE WALLS OF THE ORBIT. The walls of the orbit are: The roof (superior wall) The floor (inferior wall) The medial wall The lateral wall The bones that make up the orbit are: Maxillary Zygomatic Ethmoidal Sphenoid (has a body and two wings – the greater and lesser wings) Frontal Palatine Lacrimal The frontal, sphenoid, and ethmoid are each a single bone and take part in the formation of both orbits. Frontal Superior Palatine bone orbital rim bone Optic eral Ethmoid bone Lesser sphenoid wing: Medial orbital Lateral —~ | ve orbital rim Greater sphenoid wing, Zygomatic Inferior Maxillary Lacrimal bone orbitalrim bone bone Fig. 2.2. The bones of the orbit. 2.1. The Roof of the orbit. The roof is triangular made up of two bones: e the frontal bone; e the lesser wing of the sphenoid bone. There is on the anterolateral part of the roof, called fossa . This is where the lacrimal gland lies. At the junction of the medial third and lateral two-thirds is found a small notch called the supraorbita/ notch or foramen. This notch can be palpated easily. In 25% of orbits the supraorbital notch is enclosed to form a foramen. At the superior medial corner is a less well- defined groove, the supratrochlear notch, through which pass the nerve and vessels of the same name. This allows the passage of the supraorbital nerves and vessels and is an important landmark while giving retro bulbar anesthesia. roves 4mm from the orbital the pul located on the frontal ae The frontal lobe of the brain and the frontal sinusis found lying above the orbital roof. 2.2. The Floor of the orbit. It is made up of three bones: e the maxillary bone; e the zygomatic bone, e the palatine bone. The maxillary bone makes up the largest part of the floor, and most of the remainder is provided by the zygomatic bone. The orbital process of the palatine bone is a small, flattened area at the top of the vertical arm and is located at the most posterior edge of the orbital plate of the maxilla. The floor does not reach all the way to the apex and is separated from the lateral wall posteriorly by the inferior orbital fissure. There is an oe at the posterior part of the floor called the inferior Below the floor is found another para nasal sinus called the maxillary sinus. This is an important structure clinically. 2.3. The Medial wall. The medial wall is rectangular. From front to back, it is formed by the frontal process of the maxilla, the lacrimal bone, the orbital plate of the ethmoid, and apart of the body of the sphenoid. A ridge on the frontal process of the maxilla that forms the anterior part of the medial orbital margin also forms the anterior lacrimal crest, which demarcates one border of the fossa for the lacrimal sac. The lacrimal bone, a small bone approximately the size of a thumbnail, It is made up of four bones: e the maxillary; e the /acrimal; e the ethmoid ¢ part of the sphenoid body. 10 The ethmoid bones form the largest part of the wall. These are very thin, paper-like bones that separate the orbital cavity from yet another paranasal sinus called the etfimotdal sinus. Those paperlike bones are called lamina papyracea thus the medial wall is the thinnest of the orbital wall. A groove that holds the lacrimal sac is found on the anterior part of the wall. This, the /acrimal groove, continues inferiorly as a canal that opens in the nose and is called the naso/acrimal canal. 2.4. The Lateral wall. This is the thickest and strongest wall, and is made up of two bones: The lateral wall is roughly triangular and is composed of the zygomatic bone in front and of the greater wing of the sphenoid bone behind. The zygomatic bone separates the orbit from the temporafossa whereas the two lateral walls, if extended posteriorly, would form approximately a 90- degree angle with each other e the zygomatic, e the greater wing of the sphenoid. The lateral wall and roof are fused together anterior| posteriorly by an opening called the Po The /ateral or marginal orbital tubercle (Whitnall’s tubercle) is a small, bony prominence located on the orbital surface of the zygomatic bone and is the attachment site for the aponeurosis of the superior palpebral levator muscle, the lateral palpebral ligament, and the lateral check ligament. The greater wing of the sphenoid separates

OPTOMETRY · SEMESTER 2 The Eyelids Ocular Anatomy and Physiology START READING NOTES Contents of This Topic The Eyelids ANATOMICAL SEGMENTS OF THE EYELID Palpebral Ligaments Tarsal Plate Palpebral Conjunctiva INNERVATION OF EYELIDS BLOOD SUPPLY OF EYELIDS HISTOLOGIC FEATURES TENON’S CAPSULE CONJUNCTIVAL INNERVATION The Eyelids CHAPTER 3. THE EYELIDS BY THE END OF THIS CHAPTER YOU WILL HAVE COVERED THE FOLLOWING ASPECTS OF THE TOPIC: EYELID -INTRODUCTION ANATOMICAL SEGMENTS OF THE EYELID INNERVATION AND VASCULARISATION OF THE EYELIDS EYELID MOVEMENTS SOME EYELID DISORDERS SOME CONJUNCTIVAL DISORDERS QaRWON> 1. INTRODUCTION There is a superior ( upper) and an inferior (lower) lid to each eye. The opened angles or canthi. The medial canthus, not so sharply angled like the lateral one but more , lies 6mm from the eyebal It is in which the caruncle and The medial canthal ligament anchors the tarsal plates to the anterior and posterior lacrimal crests via the anterior and posterior limbs respectively. Levator palpebrae superioris Orbicularis oculi muscle Fornix Skin Tarsus Meibomian gland : Palpebral Follicles 5 conjunctiva of cilia Cornea = Palpebral conjunctiva ols se Bulbar conjunctiva Opening of meibomian gland Fornix Fig. 3.1. Diagram of the upper eyelid in longitudinal section. 2. THE ANATOMICAL SEGMENTS OF THE EYELID. 2.1. The palpebral (lid) margin. It is 2mm wide, has a rounded anterior border bearing the cilia ( eyelashes ), and a sharp posterior border apposed to the globe. re are aoe 1100 lashes in 2 or 3 rows in the upper eyelid and 50 An eyelash survives for about 5 months; its replacement is fully grown in 10 Posterior to the cilia on the free eyelid margin is a narrow grey line , marking an avascular palpebral plane , the anterior boundary of the tarsal plate. The grey line corresponds to the muscle {NNN ( section of the pretarsal orbicularis) and it is not to be confused with the mucocutaneous junction. Next to the openings is the mucocutaneous junction, representing the anterior limit of the marginal strip of tear fluid. The lacrimal puncti are located at the Fi@dialendslontheuppenandlowenlids. Gray line Meibomian gland orifices Mucocutaneous junction Conjunctiva Lash follicle ‘onjut a Meibomian gland Orbicularis oculi muscle Fig.3.2. Diagrammatic representation of the palpebral margin. 2.2. The eyelid skin and subcutaneous tissue. The eyelid skin is the thinnest in the body; it is devoid of subcutaneous fat , but alas has a loose connective tissue base , hence the propensity of the eyelids to swell. . In oriental people, 2.3. The orbicularis oculi muscle. Is the muscle situated beneath the SKinNaNGIENISIFeSpOnSIDIEHOnEyenG The fibres have a circular orientation , hence the sphincter like function of the muscle. Innervated by the facial nerve (cranial nerve Vil), it is divided into three concentric zones: clogin “or and orbital. These divisions are both The palpebral ( pretarsal and preseptal ) parts are more involved in involuntary eye movements ( blink) , while the orbital portion is primarily involved in forced eyelid closure ( winking and blepharospasm). Part of the pretarsal orbicularis ( Horner's muscle ) , encircles both canaliculi &nd facilitates tear drainage. Around the eyelid margin , the pretarsal fibres form the muscle of Riolan. ————— eee | ane In the upper eyelid , the marginal arterial arcade lies 2mm superior to the margin, near the follicles of the cilia, and anterior to the tarsal plate. The peripheral arterial arcade lies aponeurosis and Muller's muscle. In the lower Pale there is often only SHelaberialarcadelocatedlanthe Within the tarsal plates, lie the . They are sebaceous gland The canals of the glands almost completely traverse the tarsal plates and open on the margin of the eyelid just in front of the mucocutaneous junction. They sorte tpl Into the eee – this lipid helps stabilise the tear film and 2.5. The orbital septum ( palpebral fascia ). It is a fibrous tissue The orbital arp serves as a barrier between the orbit and the eyelid to The orbital fat normally lies posterior to the orbital septum and anterior to the levator aponeurosis . It is an important landmark during eyelid surgery, Palpebral Ligaments The palpebral or tarsal ligaments are bands of dense connective tissue connecting the tarsal plates to the orbital rim and holding the tarsal plates in position against the globe during eye and lid movements. The medial palpebral ligament runs from the medial edge of each tarsal plate to the medial orbital rim, where it divides into two limbs. One limb attaches to the posteriorlacrimal crest and the other to the anterior lacrimal crest. Both limbs lie anterior to the orbital septum Tarsal Plate Each eyelid contains a tarsal plate (tarsus) that gives the lid rigidity and structure and shapes it to the curvature of the globe. The tarsal plate in the upper lid is approximately 11 mm high, and the inferior tarsal plate is approximately 5 mm high 2.6. The eyelid retractors. In the lower lid, the retractors are the ial ei orn a. The levator palpebrae superioris, a striated muscle innervated by the oculomotor nerve (cranial nerve II! ), is responsible primarily for eyelid opening. It originates on the lesser wing of the sphenoid bone above and in front of the optic foramen, and its sheath blends with the sheath of the superior rectus muscle. As the levator approaches the eyelid from its posterior origin at the orbital apex, a ligament, the superior transverse ligament (Whitnall’s ligament) may act as a fulcrum, changing the anteroposterior direction of the levator to Superoinferior Originates in the apex of the orbit, just above the annulus of Zinn, passes forward beneath the orbital roof towards the It becomes ELE in the lid, where the aponeurosis fans out on either It inserts into the skin ( causing the eyelid crease ) and the anterior surface of the tarsal plate. The muscular portion of the levator is approximately 40mm long , while the aponeurosis is b. Muller’s muscle originates at This srmpathreticaly innervated muscle, provides approximately 2mm of The is located between c. The capsulopalpebral

OPTOMETRY · SEMESTER 2 The Lacrimal Apparatus Ocular Anatomy and Physiology START READING NOTES Contents of This Topic The Lacrimal Apparatus THE PREOCULAR TEARFILM The Lacrimal Apparatus CHAPTER 4. THE LACRIMAL APPARATUS AND TEARS. BY THE END OF THIS CHAPTER YOU WILL HAVE COVERED THE FOLLOWING ASPECTS OF THE TOPIC: THE LACRIMAL SECRETORY APPARATUS THE PREOCULAR TEARFILM THE ANATOMY OF THE LACRIMAL DRAINAGE SYSTEM THE PHYSIOLOGY OF THE LACRIMAL DRAINAGE SOME DISORDERS OF THE LACRIMAL APPARATUS ARWONS The lacrimal apparatus function n of certain ocular tissues. Tears are thus essential in maintaining the functional integrity of the eye 1. THE LACRIMAL SECRETORY APPARATUS. The secretory system includes the main lacrimal gland, the accessory lacrimal glands, meibomian and Zeis glands, and the conjunctival goblet cells. 1.1. The Lacrimal gland located in the superior lateral quadrant of the orbit the lacrimal gland fossa, secretes tears ( aqueous secretion ) through a series of ducts into the superior fornix. If the upper lid is everted, the lacrimal gland can be seen above the edge of the upper tarsal plate It is divided by muscle, into . The superioris superior orbital portion is larger and almond shaped. The palpebral lobe is one-third to one-half the size of the orbital lobe and is subdivided into two or three sections. The lacrimal gland consists of lobules made up of numerous acini. Each acinus is an irregular arrangement of secretory cells around a central lumen surrounded by an incomplete layer of myoepithelial cells. A network of ducts connects the acini and drains into one of the main excretory ducts. There are approximately 12 of these ducts, which empty into the conjunctival sac in the superior fornix Histologically, the lacrimal gland is a branched tubuloacinar gland of the serous type,resembling the parotid gland in structure. The secretion of the oe is ral Be .Contains also} The blood supply of the gland is by the lacrimal artery , branch of the ophthalmic artery. The lacrimal vein drains into the superior ophthalmic vein and the lymphatic vessels into the preauricular nodes. 23 Innervation of the lacrimal gland: e the lacrimal nerve, part of the ophthalmic branch of the trigemen, is the afferent pathway in the reflex tear arc.Stimulation of receptors in the trigemen’s distribution, activates tear production from the lacrimal gland. e the efferent pathway is more complicated: parasympathetic fibers are supplied via the facial nerve Sympathetic pathways are still not completely understood. The gland receives vasomotor sympathetic innervation and secretomotor parasympathetic innervation. Reflex tearing occurs with stimulation of branches of the ophthalmic nerve or in response to external stimuli, such as intense light; the afferent pathway is through the trigeminal nerve, and the parasympathetic pathway is through the facial nerve 1.2. The accessory lacrimal glands of Krause and Wolfring, are located in the superior fornix and above the superior border of the tarsus, respectively Both contribute to the aqueous secretion of the tears. Aqueous lacrimal secretion is divided into basal and reflex secretion. The accessory glands are thought to be the main providers of basal tear secretion , in contrast to reflex secretion which is produced by the lacrimal gland , although the lacrimal gland may also have a role in basal secretion. 2. THE PREOCULAR TEAR FILM. The preocular tear film is a layer of tears which Covers the exposed Inteilaipebal portionvof tie Globeland Goitie’. The portion overlying the cornea is the precorneal tear film. The tear film, which covers the anterior surface of the globe, has several functions: (1) keeps the surface moist and serves as a lubricant between the globe and eyelids; (2) traps debris and helps remove sloughed epithelial cells and debris; (3) is the primary source of atmospheric oxygen for the cornea; (4) provides a smooth refractive surface necessary for optimum optical function;68 (5) contains antibacterial substances (lysozyme, beta-lysin, lactoferrin, IgA) to help protect against infection;69 and (6) helps to maintain corneal hydration by changes in tonicity that occur with evaporation .Recent measurements of the — tear film indicate a thickness of i to | The precorneal tear film consists of three layers ,each of which has separate functions. 2.1. The outer lipid layer. This is secreted by the meibomian glands and has three main functions: to retard the evaporation of the aqueous layer ; e toincrease surface tension so that tears do not overflow the lower lid margin; e to lubricate the eyelids as they pass over the surface of the globe. 24 2.2. The middle aqueous layer. This is secreted by the main lacrimal gland and the accessory lacrimal glands, and has four main functions: e it supplies oxygen to the corneal epithelium; e it has antibacterial substances, such as lysozyme and lactoferrin; e it provides a smooth optical surface; e it washes away debris. 2.3. The inner mucin layer. Secreted by the , its main function is to convert the corneal epithelium from a 3. THE ANATOMY OF THE LACRIMAL DRAINAGE SYSTEM. Some tear fluid is lost by evaporation and some by reabsorption through conjunctival tissue, but approximately 75% is passed through the nasolacrimal ioe oad ai The lacrimal Tools ald consists of the 3.1. The puncti are small openings located near the medial end of each eyelid. They are visible to the naked eye. A small aperture, the lacrimal punctum, is located in a slight tissue elevation, the lacrimal papilla, at the junction of the lacrimal and ciliary portions of the eyelid margin. Both upper and lower lids have a punctum. The puncta are turned toward the globe and normally can be seen only if the eyelid edge is everted slightly. Each punctum opens into a tube, the lacrimal canaliculus 3.2. The ampulae are about 2mm long, vertically orientated. 3.3. The canaliculi are about 8mm long. In 90% of cases, the upper and lower canaliculi form a common canaliculus which opens into the lateral wall of the lacrimal sac. The canaliculi are tubes in the upper and lower lids that join the puncta to the lacrimal sac. The first

OPTOMETRY · SEMESTER 2 The Extraocular Muscles Ocular Anatomy and Physiology START READING NOTES Contents of This Topic The Extraocular Muscles INDIVIDUAL EOM THAT MOVE THE EYEBALL STRUCTURE OF THE MUSCLE DUCTIONS VERGENCES AND VERSIONS MONOCULAR BINOCULAR PAIRED ANTAGONIST AGING CHANGES IN THE The Extraocular Muscles CHAPTER 5. THE EXTRAOCULAR MUSCLES (EOM BY THE END OF THE CHAPTER YOU WILL HAVE COVERED THE FOLLOWING ASPECTS OF THE TOPIC: DEFINITION AND TYPES OF EOM INDIVIDUAL EOM THAT MOVE THE EYEBALL EYEBALL MOVEMENTS PHYSIOLOGICAL LAWS SOME CLINICAL CONSIDERATIONS ARWONS> 1. DEFINITION AND TYPES OF EOM. The muscles of the globe can be divided into two groups: the involuntary intrinsic muscles and the voluntary extrinsic muscles. The intrinsic muscles— the ciliary muscle, the iris sphincter, and the iris dilator— are located within the eye; these muscles control the movement of internal ocular structures. The extrinsic muscles— the six extraocular muscles— attach to the sclera and control movement of the globe. Extra ocular muscles are a group of muscles that are found outside the eyeball and are attached either to the eyeball itself or to the surrounding structures. This is in contrast to the intraocular muscles, that are found within the There are 8 extra ocular muscles grouped into two categories: a. 6 muscles are attached to the eyebal/ and function as movers of the eye. b. 2 muscles are attached to the eyelid and the adjacent structures like the lacrimal sac. This chapter will be dealing with muscles that move the eyeball. The other muscles have been covered in the chapter dealing with the eyelids. STRUCTURE OF THE EXTRAOCULAR MUSCLES The extraocular muscles have a denser blood supply, and their connective tissue sheaths are more delicate and richer in elastic fibers than is skeletal muscle. Precise fine motor control and quick accurate movement of the extraocular muscles occur because of this dense innervation. Singly innervated fibers have the classic end plate (en plaque) seen in skeletal muscle. Muscle spindles and Golgi tendon organs have been identified in human extraocular muscle, although it is unclear whether these structures provide any useful proprioceptive information relative to the extraocular muscles.18 Afferent information regarding extraocular muscle proprioception is thought to be mediated by a receptor that is unique to extraocular muscle, the myotendinous cylinder (palisade ending) 2. INDIVIDUAL EOM THAT MOVE THE EYEBALL. There are 6 muscles that are attached to the outer surface of the eyeball. 4 of these rmuscles have a straight course from their origin to thei insertion on the sclera, hence they are called the RECTI muscles (singular — RECTUS which means STRAIGHT). The 4 recti muscles are: Superior rectus Inferior rectus Lateral rectus Medial rectus ao7D 28 The names of the muscles suggest the positions where they are attached or . Meaning that, the superior rectus muscle is attached on the superior aspect of the eye, the lateral rectus on the lateral aspect and so on. The other two muscles take a slanting of tilted course from their origin to insertion and hence called OBLIQUE muscles. They are: a. Superior oblique b. Inferior oblique Again, the names suggest the positions where they are inserted on the eye. The other two muscles that are NOT attached to the eyeball surface are: a. Orbicularis oculi b. Levator palpebrae superioris 2.1. Muscles origins, insertions and nerve supply. The 4 recti originate from a common tendinous ring ( of Zinn ). This is a thickened connective tissue ring that bridges the superior orbital fissure at the orbital apex. All four muscles insert anteriorly on the globe. A line connecting the rectus muscle insertions forms a spiral, as described by Tillaux. This spiral starts at the medial rectus, the insertion that is closest to the limbus, and proceeds to the inferior rectus, the lateral rectus, and finally the superior rectus, the insertion farthest from the limbus a. The Superior Rectus muscle. Arises above the optic foramen on the common tendinous ring. The superior rectus muscle parallels the roof of the orbit until it passes through a connective tissue pulley just posterior to the equator of the globe; at this point it follows the curve of the globe to its insertion .It then passes forward and inserts on to the sclera 7.7mm from the limbus and is curved slightly, with the convex side forward . A line drawn from the origin to the insertion along the muscle will form an angle of approximately 23 degrees with the sagittal axis — The muscle is innervated by the superior division of the oculomotor (cranial nerve I/I)which enters the muscle on its inferior face. Branches pass either through the muscle or around it to innervate the levator b. The Inferior Rectus muscle. Arises from the tendinous ring below the optic foramen. It passes forwards and inserts onto the sclera about 6.5mm from the limbus. The inferior rectus approximately parallels the superior rectus, making an angle of 23 degrees with the sagittal axis. The inferior rectus muscle parallels the orbital floor until it passes through a connective tissue pulley just posterior to the equator of the globe; at this point it follows the curve of the globe to its insertion, which is parallel to the insertions of the superior rectus The muscle is innervated by the inferior division of the oculomotor nerve which enters the muscle on its superior surface c. The Lateral Rectus muscle. The lateral rectus muscle has its origin on both limbs of the common tendinous ring and the spina recti lateralis, a prominence on the greater wing of the sphenoid bone. Arises from the lateral aspect of the tendinous ring. It passes forwards and laterally and inserts onto the sclera about 6.9mm from the limbus. The lateral rectus muscle parallels the lateral orbital wall until it passes through a connective tissue pulley just posterior to the equator of the globe;at this point it follows the curve of the globe to its insertion. The insertion parallels that of the medial rectus and is approximately 6.9 mm from the

OPTOMETRY · SEMESTER 2 The Eye Globe Ocular Anatomy and Physiology START READING NOTES Contents of This Topic CHAPTER 6. CHAPTER 6. THE EYEGLOBE THE FOLLOWING ASPECTS WILL BE COVERED BY THE END OF THIS CHAPTER: 1. EYEGLOBE – GENERALITIES, MEASUREMETS 2. EYEGLOBE -LAYERS AND CONTENTS 1. EYEGLOBE – GENERALITIES , MEASUREMENTS. The eye is a highly specialized organ of photoreception , the process by These changes result in nerve action potentials , which are subsequently relayed to the optic nerve and then to the brain , where the information is Processed and consciously appreciated as vision. All the other structures in the eye are secondary to this basic physiologic although they may be part of the system necessary for focusing and transmitting the light onto the retina, for example cornea, lens, iris, and ciliary body , or they may be necessary for nourishing and supporting the tissues of the eye, for example the choroid, aqueous outflow system, and lacrimal apparatus. Occupies only one-fifth of the orbital cavity. The eyeball The adult human eye averages 24mm in diameter. The normal anteroposterior diameter varies between 21 and 26mm. This measurement is characteristically smaller in hypermetropia and larger in myopia. 38 Rim of bone Extraocular muscle Iris Cornea Pupil Lens plic nerv Retina Extraocular muscle Fig. 6.1. Schematic representation of the Eyeglobe in the orbit. 2. EYEGLOBE -LAYERS AND CONTENTS. The eye is made up of three basic layers or coats and three different contents. The three basic layers are : e the outer fibrous, protective coat (cornea, sclera); e the middle vascular uveal tract (iris, ciliary body, choroid); e the inner neural layer (retina). The coats surround the three different contents : e the crystalline lens; e the aqueous humour; e the vitreous humour 39 Fig. 6.2. Diagram representing the layers and contents of the eyeglobe. 40 ← PREVIOUS TOPICNEXT TOPIC →VIEW MODULE NOTESVIEW SEMESTER NOTESALL OPTOMETRY NOTES Need These Notes as PDF? Request a formatted copy for offline study, printing or revision. GET PDF NOTES ON WHATSAPP

OPTOMETRY · SEMESTER 2 The Cornea and Sclera Ocular Anatomy and Physiology START READING NOTES Contents of This Topic The Cornea and Sclera INNERVATION ———7~ Basement INNERVATION. SCLERAL SPUR . SCLERAL OPACITY SCLERAL BLOOD SUPPLY SCLERAL INNERVATION The Cornea and Sclera CHAPTER 7. THE CORNEA AND THE SCLERA BY THE END OF THIS CHAPTER YOU WILL HAVE COVERED THE FOLLOWING ASPECTS OF THE TOPIC: 1. DEFINITION AND CORNEAL MEASUREMENTS 2. THE STRUCTURE OF THE CORNEA 3. CORNEAL PROPERTIES, FUNCTIONS, METABOLISM AND INNERVATION 4. THE ANATOMY AND FUNCTIONS OF THE SCLERA 5. SOME CLINICAL CONSIDERATIONS 1. DEFINITION AND CORNEAL MEASUREMENTS. The cornea isthe transparent avascular tissue with a smooth , convex surface , that It measures 11-12mm horizontally and 9-11mm vertically. It is 0.52mm thick centrally and 0.67mm thick at the periphery. The radius of curvature of the central cornea is 7.8mm ; the peripheral corneal curvature is less marked. 2. THE STRUCTURE OF THE CORNEA. The cornea consists of five layers : The epithelium; Bowman’s layer; The stroma; Descemet’s membrane; The endothelium. 41 ———7~ Basement membrane Epithelium——— Basement. — membrane Bowman's layer -_ —. Stroma-—-—— Pp ees moms | Descemet’s _ membrane -. Endothelium-~ Fig. 7.1. Diagram representing the 5 layers of the human cornea. 2.1. The epithelium. It accounts for 10% (0.05mm) of the total corneal thickness. five to seven cells thick. The epithelium thickens in the periphery and is continuous with the conjunctival epithelium at the limbus. It is a stratified (made up by 5-6 layers ), squamous nonkeratinised epithelium each of which contains a flattened nucleus and fewer cellular organelles than deeper cells There is a continual loss of epithelium Maintenance of the smooth corneal surface depends on replacement of the surface cells that constantly are being shed into the tear film. 42 Cell proliferation occurs in the basal layer, basal cells move up to become wing cells, and wing cells move up to become surface cells. A slow migration of basal cells occurs from the periphery toward the center of the cornea. This replacement and differenciation process takes about 7-14 days. Repair to corneal epithelial tissue proceeds quickly; minor abrasions heal within hours, and larger ones often heal overnight. New cells also derive from the mitotic activity of the limbal cells. Due to this continuous renewal potential, the corneal epithelium responds rapidly to repair disruptions. Because of its excellent ability to regenerate, the epithelium does not scar. Despite cells constantly being sloughed, the barrier function is maintained as the cell below moves into position to replace the one that has been shed. 2.2. Bowman's layer. Bowman’s layer is a dense, fibrous sheet of interwoven collagen fibrils randomly arranged in a mucoprotein ground substance. The second layer of the cornea is seit onic 8 to 14 um thick. It isa road acl Foion There is strong adherence between Bowman's and the basal lamina of the epithelium. It does not regenerate when damaged . Therefore, if injured, the layer usually is replaced by epithelial cells or stromal scar tissue. Bowman's layer sometimes is referred to as a “membrane,” but it is more correctly a transition layer to the stroma rather than a true membrane. It differs from the stroma in that it is acellular and contains collagen fibrils of a smaller diameter. 2.3. The stroma/ substantia propia The middle layer of the cornea is approximately 500 pm thick,It makes up 90% of the corneal thickness. When the stroma is 75% to 80% water, the negatively charged molecules located around each collagen fibril maintain this precise arrangement by their bonds with the water molecules, and corneal transparency is optimal. This regularity is partly responsible for cornea. It is composed Keratocytes (corneal fibroblasts) are flattened cells that lie between and occasionally within the lamellae Ground substance fills the areas between fibrils, lamellae, and cells. When stroma is damaged, keratocytesiincreaselininumberandisynthesize the connective tissue components. 2.4. Descemet’s membrane. Descemet’s membrane is considered the basement membrane of the endothelium. It is produced constantly and therefore thickens throughout life, such that it has doubled by age 40 years. This is the basal lamina of the sotiteal endothelium afi consists of Gollagehifbtls. Descemet's membrane consists of two laminae If the basement membrane is intact, this healing takes only days, but if the basement membrane is damaged, several weeks will be needed for recovery.1 Descemet’s membrane is a strong, resistant membrane and, if damaged, can be regenerated by the endothelial cells that secrete it. 2.5.The endothelium. Consists of a single layer of hexagonal cells. It contains metabolic aa eae for the maintenance of corneal With age, the number of endothelial cells gradually decreases. The endothelium is incapable of regeneration; . the endothelium, lies adjacent to the anterior chamber and is composed of a single layer of flattened cells. The very regular arrangement of these cells is described as the endothelial mosaic. Endothelial cells are not replaced when damaged but migrate to cover the afealicalsingithelcellsinvolveditorenlargefandiflatteny)esulting in a decrease in cell density. 43 3. CORNEAL PROPERTIES, FUNCTIONS , METABOLISM AND INNERVATION. The complex structure of the cornea gives some indication of the diverse functional demands upon this tissue. The cornea must be: transparent ,avascular, refract light , contain the intraocular pressure and provide a protective interface with the environment( physical barrier to trauma and infection ). Each of these functions is provided by a highly specialised substructural organisation , and in an absence of vessels. 3.1. Corneal transparency , its most important property, is due to anumber of related factors: regularity and smoothness of the covering epithelium; avascularity; e regular, homogenous, arrangement of the stromal components; e state of relative dehydration ( deturgescence ). The cornea is thus an 3.2. The optical function. The cornea has two primary functions: to refract light and to transmit light. Factors that affect the amount of corneal refraction include (1) the curvature of the anterior corneal surface, (2) the change in refractive index from air to cornea (actually the tear film), (3) corneal thickness, (4) the curvature of the posterior corneal

OPTOMETRY · SEMESTER 2 The Anterior Chamber Ocular Anatomy and Physiology START READING NOTES Contents of This Topic The Anterior Chamber The Anterior Chamber CHAPTER 8. THE ANTERIOR CHAMBER BY THE END OF THIS CHAPTER YOU WILL HAVE COVERED THE FOLLOWING ASPECTS OF THE TOPIC 1. DEFINITON, BOUNDARIES, DIMENSIONS 2. THE ANTERIOR CHAMBER ANGLE 3. THE POSTERIOR CHAMBER 4. THE AQUEOUS HUMOUR COMPOSITION AND DYNAMICS 5. CLINICAL CONSIDERATIONS 1. DEFINITION , BOUNDARIES , DIMENSIONS. The anterior chamber (AC ), is the space between the cornea ,lens and iris. It is bounded anteriorly be the corneal endothelium , posteriorly by the anterior capsule of the lens and iris. The anterior and posterior boundaries meet laterally, to form the anterior chamber angle. A sieve-like, porous structure called trabecular meshwork is seen at the angle. The anterior chamber is filled with an optically clear, transparent fluid called the aqueous STS humour leaves the anterior chamber by passing through the The volume of the AC varies, but averages 0.2mls. The errr of the anterior chamber varies too, depending upon age, sex and It is 3mm deep at its center, becoming shallower towards the periphery (angle). Chamber depth decreases with age at an approximately 0.01mm per year . It is shallowefiinifémaléstandilinuhyperopiay deeper in males and in myopia. 2. THE ANTERIOR CHAMBER ANGLE. This is the angle The normal angle width is roughly 40°. The a width tends to be grectest in Caucasians, less in black Africans 47 The angle can be visualised clinically with the aid of a special mirror called the gonioscope and the procedure ts called gonioscopy. Four anatomical structures can be identified gonioscopically at the angle: a. Schwalbe’s line: represents the peripheral termination of the inner layer of the corneai.e. the point where the endothelial cell layer merges with the sclera. b. Trabecular meshwork : this is the sieve like structure mentioned above through which aqueous humour leaves the anterior chamber. c. Scleral spur: is a spike like ridge of scleral tissue that projects internally into the anterior chamber. d. Ciliary body: , especially in eyes that high axial length as in monn or in eam who had undergone cataract extractions Schwalbe's line Trabecular meshwork | I | i } 1 Scleral spur | s | Le Iris processes ! ep) rs ail Fig. 8.1. Diagram representing the anatomical structures on the anterior chamber angle. 48 The trabecular Some of the fibres of the ciliary body are attached to the scleral spur posteriorly too. @ Edit with WPS Office This arrangement makes the scleral spur to be a “ connector ” between the ciliary body and the trabecular meshwork. When the ciliary body muscles contract, specifically the transverse muscles, it exerts a pull on the scleral spur, which in turn pulls the trabecular meshwork thus opening its pores. This is thought to be one of the ways through which aqueous flows out of the anterior chamber. 3. THE POSTERIOR CHAMBER: fg The contents of the posterior chamber are: a. The crystalline lens b. Suspensory ligaments of the lens c. Major part of the ciliary body d. Vitreous base Into the posterior chamber is secreted the aqueous humour produced by the ciliary body. The anterior and peso a chambers i 4. THE AQUEOUS HUMOUR COMPOSITION AND DYNAMICS. The aqueous humour is the transparent fluid that fills the anterior and posterior chambers . Fe a ae ror bloo Plssmaiheniiseereted by Ahseiiaryepihiciim The fluid passes through the pupil into the anterior chamber; circulates in the anterior chamber, providing oxygen and nutrients for the lens and corneal endothelium then slowly drains mostly through the trabecular meshwork. 49 4.1. Aqueous humour formation and functions. Aqueous humour is produced from blood plasma ifithelciliaryiprocessesl) Enters the posterior chamber from the ciliary processes by means of : e diffusion; e ultrafiltration; e active secretion. The active secretion or transport The rate of aqueous formation in humans is about 2u!/min . Aqueous formation has diurnal fluctuations the highest rate being usually It decreases during the day to reach a 50% decrease during sleep. The constant flow of aqueous helps: e to maintain an adequate intraocular pressure (IOP), values between 10 -21mmHg being necessary for the structural integrity and normal functioning of the eye. to supply nutrients to the anterior segment tissues; to carry away metabolic wastes; to facilitate immune responses ; to serve as an antioxidant. 4.2. Aqueous humour composition. The protein level is low , markedly lower than the plasma levels. This is explained by the presence of blood- aqueous barrier , which blocks the passage of large molecules while still allowing a flux of ions and low molecular weight solutes. The essentially protein-free status of the aqueous explains its optical clarity. Other components include: glucose, oxygen, ascorbate, glutathione, enzymes, growth factotrs, ions, electrolytes, carbon dioxide, lactate. 50 4.3. Aqueous humour outflow occurs by two mechanisms : e trabecular (pressure dependent ) outflow; e uveoscleral ( pressure independent ) outflow. | a. The trabecular ( pressure dependent ) outflow. Most of the aqueous is drained by this mechanism. The trabecular meshwork It functions From the trabecular meshwork the fluid ent erSisehlémmsicanaland drains into the episcleral veins. Decreased trabecular outflow is a characteristic of glaucoma and elevated b. The uveoscleral ( pressure independent ) outflow occurs across the Drains about 10% of the aqueous. ro account for a greater than normal percentage of outflow in It is increased by eycloplegics and decreased by miotics. 51 Fig. 8.2. Diagram of the anterior chamber angle and the flow of aqueous. 5. CLINICAL CONSIDERATIONS. The balance between the aqueous production and outflow maintains the intraocular pressure (IOP) and it is extremely important to the proper functioning of the eye. Decreased outflow facilities and elevated intraocular pressure is characteristic of a disease called glaucoma. An increased rate of aqueous formation is rarely, if ever, the cause of raised IOP. Blood in the anterior chamber is called hyphaema. Pus in the AC is called hypopion. Abnormal vessels growing in the AC angle are known as