Optometry Semester 2

Optometry Notes, Optometry Semester 2, Physical and Geometric Optics

Nature of light

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OPTOMETRY · SEMESTER 2 Nature of light Physical and Geometric Optics START READING NOTES Contents of This Topic Nature of light THE NATURE OF LIGHT LIGHT WAVES VISIBLE LIGHT Wavelength: Long Short Meets Microwave THE SUN’S RAYS THE RAY APPROACH A DIVERGING PENCIL Air 1.001 PHASE DIFFERENCE Nature of light NATURE OF LIGHT CHAPTER CONTENTS THE NATURE OF LIGHT……………………………………………………………………………………………………………………………….. 1 LIGHT WAVES……………………………………………………………………………………………………………………………………………… 3 VISIBLE LIGHT …………………………………………………………………………………………………………………………………………….. 5 THE RAY APPROACH ………………………………………………………………………………………………………………………………….. 7 REFRACTIVE INDEX ……………………………………………………………………………………………………………………………………. 9 PHASE DIFFERENCE…………………………………………………………………………………………………………………………………… 9 THE NATURE OF LIGHT Until the beginning of the 19th century, light was considered to be a stream of particles, emitted by a light source, which stimulated the sense of sight on entering the eye. The chief architect of the particle theory of light was Newton. With this theory, he provided simple explanations of some known experimental facts concerning the nature of light, such as the laws of reflection and refraction. According to Newton, the particles emitted by a source travelled in a straight line until the boundary of a new medium was encountered. He derived a relationship that predicted that the speed of light in a medium such as water would be greater than its speed in air. This is exactly the opposite of the result predicted by the wave theory. Foucault later found a value for the velocity of light in a medium that showed that Newton’s prediction was incorrect. This played a large part in discrediting the socalled corpuscular theory of light.Nature of Light Most scientists initially accepted Newton’s particle theory of light. However, during Newton’s lifetime, another theory was proposed. In 1678, a Dutch physicist and astronomer, Christian Huygens (1629-1695), showed that a wave theory of light could also explain the laws of reflection and refraction. In addition, his theory could account for the fact that light entering a medium from air at an angle would be bent. The wave theory did not receive immediate acceptance for several reasons. All the waves known at the time (e.g. sound and water waves) travelled through some sort of medium; but light from the Sun could travel to Earth through empty space. Furthermore, it was argued that if light were some form of wave, it would bend around obstacles; hence, we should be able to see around corners. It is now known that light does indeed bend around the edges of objects. This phenomenon, known as diffraction, is not easy to observe because light waves have such short wavelengths. Even though experimental evidence for the diffraction of light had been discovered by Francesco Grimaldi (1618- 1663) around 1660, for more than a century most scientists rejected the wave theory and adhered to Newton’s particle theory. This was partly due to Newton’s great reputation as a scientist. The first clear demonstration of the wave nature of light was provided by 1801 by Thomas Young (1773-1829), who showed that under appropriate conditions, light exhibits interference behaviour. That is, at certain points in the vicinity of two sources, light waves can combine. They can even cancel each other by destructive interference. Such behaviour could not be explained by a particle theory. A few years later, and building on Young’s experimental work, Augustin Fresnel (1788-1827) definitely signalled the end of 18th century physics and the birth of modern optics. He extended the wave theory of light to a large class of optical phenomena and developed the theoretical framework that became the foundation of modern optics. The most important development concerning the theory of light was the work of James Clark Maxwell, who in 1865 predicted that light was a form of high-frequency electromagnetic wave. His theory predicted that these waves should have a speed of 3 x 108 m/s. This value is in agreement with the experimentally measured speed. Light is transmitted in the form of transverse waves. In the diagram below we see that the electric and magnetic vectors associated with an electromagnetic wave are at right angles to each other and also to the direction of wave propagation. Figure 1.1: Schematic diagram of an electromagnetic wave propagating in the x direction. The electric field vector E vibrates in the xy plane, and the magnetic field vector B vibrates in the xz plane Although the classical theory of electricity and magnetism explained most known properties of light, some subsequent experiments could not be explained by the assumption that light was a wave. The most striking of these was the photoelectric effect, discovered by Hertz. Hertz found that clean metal surfaces emit charges when exposed to ultraviolet light. In 1905, Einstein published a paper that formulated the theory of light quanta and accounted for the photoelectric effect. He reached the conclusion that light is composed of corpuscles, or discontinuous quanta of energy. Furthermore, he asserted that light interacting with matter also consists of quanta, and he brilliantly worked out the implications of the photoelectric process. Nature of Light More specifically, Einstein showed that the energy of a photon is proportional to the frequency of the electromagnetic wave: E = h·f where h= 6.63 x 10-34 Js is Planck’s constant. This theory contains features of both the wave and particle theories of light. In view of these developments, light must be regarded as having a dual nature. That is, to best describe light’s behaviour, we need in some cases to consider light to be acting as a wave and in others to be acting as a particle. For example, classical electromagnetic wave theory provides adequate explanations of light propagation and of the effects of interference, whereas the photoelectric effect is best explained by assuming that light is a particle. The nature we consider light to have in a particular situation will depend on which is the most appropriate model for those circumstances.. . In this course, we consider Physical Optics which involves phenomena which can only be explained by reference to the nature of light, as well as phenomena which do not depend on the nature of light, but only on its path. This

Optometry Notes, Optometry Semester 2, Quality Control

Additional Study Notes: Spectacle Verification and Equipment Records

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OPTOMETRY · SEMESTER 2 Additional Study Notes: Spectacle Verification and Equipment Records Quality Control Additional Study Notes — newly authored explanations and examples. These sections supplement the supplied course material. START READING NOTES Contents of This Topic Learning objectives Verification from order to handover Choosing acceptance criteria Equipment control: an original record template Traceability and complaints Learning objectives Describe an ordered verification process and explain traceability, measurement checks and the response to nonconforming work. Verification from order to handover Start with an unambiguous order: patient identifier, right and left prescriptions, intended lens design and material, relevant fitting measurements and frame identification. Confirm that the finished pair belongs to the correct order before measuring it. Check the measured spectacle powers and any prescribed addition or prism against the order. Inspect the lens surfaces, edges, mounting and frame condition. Confirm the required lens position, fit and comfort on the wearer, then assess function and acuity as appropriate. Explain intended use and maintenance and arrange aftercare. This learning sequence draws on the College of Optometrists’ dispensing guidance; its UK legal provisions should not be treated as Tanzanian law. Choosing acceptance criteria A tolerance is a permitted difference from a specified target. It is not the same as measurement uncertainty, which describes the uncertainty associated with a measurement. Do not invent one universal tolerance for every power, lens type and parameter. The WHO spectacle-quality guide covers lenses, frames and ready-made spectacles. Use the applicable product standard, edition, manufacturer requirements and locally adopted rules when setting acceptance limits. Record which criterion was applied so a later reviewer can reproduce the decision. Equipment control: an original record template Equipment ID | location | procedure version | date | check performed | reference used | result | acceptance criterion | action | staff identifier. A routine performance check asks whether an instrument is behaving as expected. Calibration establishes its relationship to an appropriate reference under stated conditions. Cleaning or adjusting an eyepiece is not, by itself, proof of full calibration. Follow the instrument’s instructions for the actual checking and maintenance method. If a check fails, identify and segregate the affected equipment or product, document the finding, notify the responsible person and assess whether earlier work needs review. Release it only after the required corrective action and satisfactory recheck. Traceability and complaints Keep enough information to connect a finished order with its prescription, measurements, supplier, verification and handover. A complaint should prompt a structured reassessment of the patient’s intended task, symptoms, prescription, lens measurements and fitting. Do not assume that adaptation explains every difficulty. An illustrative nonconformance record contains the order identifier, observed defect, detection stage, immediate containment, investigation, correction, recheck and closure date. Reviewing these records can reveal repeated causes suitable for a quality-improvement project. Study References College of Optometrists: Sale and supply of spectacles WHO: Summary guide on quality standards for spectacles (2025) Source module: Lensometer and Benchmarking notes External references checked 13 September 2026. Worked numerical examples and teaching activities are original. ← PREVIOUS TOPICVIEW 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 Notes, Optometry Semester 2, Quality Control

Additional Study Notes: Quality Management in an Optical Service

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OPTOMETRY · SEMESTER 2 Additional Study Notes: Quality Management in an Optical Service Quality Control Additional Study Notes — newly authored explanations and examples. These sections supplement the supplied course material. START READING NOTES Contents of This Topic Learning objectives Quality across the patient pathway Structure, process and outcome: an original teaching example A practical improvement cycle Worked audit example Learning objectives Distinguish quality assurance, quality control and quality improvement; describe a measurable improvement cycle; and build an optical-service audit using explicit denominators. Quality across the patient pathway Quality assurance is the planned system used to prevent avoidable defects: agreed procedures, staff competence, suitable supplies and reliable records. Quality control is the checking activity that identifies whether a particular product or process meets the chosen requirements. Quality improvement uses measured changes to make the system perform better over time. These functions overlap but are not interchangeable. WHO describes good services through effectiveness, safety, attention to people’s needs, timeliness, fairness, coordination and efficient resource use. In an optical service, this means looking beyond whether a lens power is correct: the appliance must also be usable by the patient, delivered in time and supported by appropriate follow-up. Structure, process and outcome: an original teaching example Structure: a functioning lensmeter, accessible testing space, trained staff and an approved work instruction. Process: recording the prescription and fitting measurements, verifying the finished spectacles and documenting handover. Outcome: spectacles that meet the patient’s agreed visual task, with the expected measured performance. A high number of completed orders alone does not establish good outcomes. An audit question should specify the population, criterion and time window. Example: among all completed spectacle orders in one month, what proportion have a documented final verification? Numerator: orders with verification recorded. Denominator: all completed orders reviewed. Missing documentation should be reported explicitly rather than assumed to be satisfactory. A practical improvement cycle 1. Define the problem precisely. For example, too many orders require remaking because measurements were omitted. 2. Establish a baseline from a defined sample. Record the reason for every remake using consistent categories. 3. Identify possible causes with the staff who perform the work. A cause-and-effect diagram can group issues involving people, measurement methods, equipment, materials and work environment. 4. Test one manageable change, such as a mandatory measurement check before an order is released. Name a responsible person and a review date. 5. Compare the same measures after the change. Look for unintended effects, including longer waits or staff workarounds. 6. Keep, adapt or abandon the change according to the evidence. Update the procedure and repeat the measurement. Worked audit example These figures are invented for learning. In 80 completed orders, 12 require a remake: remake proportion = 12/80 × 100 = 15%. After a change, 6 of 100 orders require remaking: 6%. The absolute reduction is 9 percentage points; the relative reduction is (15−6)/15 × 100 = 60%. These are different quantities. This before-and-after comparison does not alone prove causation. Check changes in prescription complexity, staff, suppliers, sample size and classification. Include a balancing measure, such as median delivery time, so that fewer remakes do not hide unacceptable delays. Study References WHO: Quality health services Source module: Benchmarking in Managing an Optical Workshop External references checked 13 September 2026. Worked numerical examples and teaching activities are original. ← 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 Notes, Optometry Semester 2, Quality Control

Benchmarking in an Optical Workshop

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OPTOMETRY · SEMESTER 2 Benchmarking in an Optical Workshop Quality Control START READING NOTES Contents of This Topic RELATED TASK BENCHMARKING Benchmarking in an Optical Workshop IMPORTANCE OF BENCHMARKING TYPES OF BENCHMARKING 2. Competitive (External) Benchmarking 3. Functional (Process) Benchmarking 4. Generic Benchmarking 5. Performance (Metric) Benchmarking DESCRIBE TOOLS FOR BENCHMARKING 2. Technical and IT Performance Tools 3. Business Intelligence and Data Analysis Tools 4. AI Model and RAG Benchmarking Tools 5. Specialized and Industry-Specific Tools 6. Survey and Qualitative Tools STEPS OF BENCH MARKING 2. Documentation and Data Collection 3. Analysis and Comparison 4. Implementation and Integration 5. Review and Recalibrate DESCRIBE THE PROCESS OF BENCHMARKING IN MANAGING OPTICAL WORKSHOP RELATED TASK Define benchmarking Explain importance of benchmarking Describe types of benchmarking Describe tools for benchmarking Describe steps of bench marking BENCHMARKING Benchmarking is the structured process of measuring a company's products, services, or processes against industry leaders or internal standards to identify performance gaps and improvement opportunities. It involves comparing metrics like quality, time, and cost to adopt best practices, enhance competitiveness, and drive continuous improvement Benchmarking in an Optical Workshop Benchmarking in an optical workshop involves measuring, comparing, and improving the performance of optical systems, components, or manufacturing processes against established standards or best-in-class performance. It focuses on enhancing technical, operational, or financial metrics to optimize precision and efficiency IMPORTANCE OF BENCHMARKING Identifies Performance Gaps: It pinpoints exactly where an organization is lagging behind competitors in areas like efficiency, quality, or cost. Drives Continuous Improvement: Benchmarking fosters a culture of excellence by establishing high performance targets based on what is actually achievable in the market. Enhances Operational Efficiency: By analyzing the workflows of top performers, companies can streamline their own processes, leading to significant cost savings and waste reduction. TYPES OF BENCHMARKING 1. Internal Benchmarking Definition: Compares performance and practices between different departments, production lines, or shifts within the same company. Optical Workshop Use: Comparing the polishing speed and scrap rate of Line A (using newer machines) against Line B (using older machinery) to identify best practices. Goal: To standardize high performance across the entire organization 2. Competitive (External) Benchmarking Definition: Compares your workshop's metrics and processes directly against competitors in the optical industry. Optical Workshop Use: Analyzing the surfacing time, coating turnaround time, and lens error rates of a competitor to identify competitive strengths and weaknesses. Goal: To understand your market position and identify gaps in performance or product quality. 3. Functional (Process) Benchmarking Definition: Compares specific processes (like coating or lens design) against organizations that are "best-in-class," even if they are in different industries. Optical Workshop Use: A spectacle lab comparing its inventory management or shipping logistics to a high-volume courier service rather than another optical shop. Goal: To adopt superior operational techniques not currently used in the optical industry. 4. Generic Benchmarking Definition: Compares processes that are similar across various industries, such as customer service or payroll. Optical Workshop Use: Comparing the front-office handling of patient lens orders against best practices in hospitality to improve customer satisfaction. Goal: To find and implement "best-in-class" practices regardless of industry 5. Performance (Metric) Benchmarking Definition: Focuses on collecting quantitative data to measure performance gaps, often focusing on Key Performance Indicators (KPIs). Optical Workshop Use: Tracking metrics such as: Surface Accuracy: RMS roughness (nm). Production Volume: Lenses per hour. Turnaround Time: Hours to complete a prescription. Scrap Rate: Percentage of lenses failed. DESCRIBE TOOLS FOR BENCHMARKING 1. Digital and Website Benchmarking Tools These tools analyze website traffic, engagement, marketing channels, and user behavior compared to competitors. Similarweb: Used to compare website traffic, marketing channels, and engagement metrics against competitors. Google Analytics: Useful for tracking internal performance metrics to compare against historical data or external benchmarks. Mixpanel / Hotjar: Used for deep dives into user behavior and conversion benchmarking. 2. Technical and IT Performance Tools These tools measure system speed, efficiency, and resource utilization, including hardware (CPU, GPU, memory) and software. [1] Geekbench: Measures CPU single-core and multi-core performance for comparison. UserBenchmark: A free program that tests computer hardware and compares it with other user results. 3DMark: A standard tool for testing gaming PC performance, particularly GPUs. PerfKit Benchmarker: A tool from Google Cloud that provides preconfigured tests to measure metrics like latency and IOPS 3. Business Intelligence and Data Analysis Tools These platforms are used to visualize, interpret, and analyze benchmarking data. Tableau / Power BI: BI platforms that provide data visualization and analytics to compare performance across the organization. R / Python: Used for advanced statistical analysis and modeling. 4. AI Model and RAG Benchmarking Tools These tools evaluate LLMs and RAG (Retrieval-Augmented Generation) systems for consistency and accuracy. Mosaic AI Evaluation Suite: A commercial tool for enterprise-level benchmarking. Weights & Biases (W&B): Tracks experiments and metrics for LLMs. BenchmarkQED: A Microsoft tool designed for RAG systems. AgentBench: Evaluates LLMs in agent-based environments 5. Specialized and Industry-Specific Tools WHO Global Benchmarking Tool (GBT): Used by the World Health Organization to evaluate regulatory systems. Nomitech CostOS: Used for cost and financial benchmarking in project management. SAP Value Lifecycle Manager (VLM): A self-service tool for benchmarking business initiatives 6. Survey and Qualitative Tools APQC Benchmarking Tools: Offers databases, frameworks, and tools for benchmarking processes. Appinio: Streamlines data collection through surveys and market research. STEPS OF BENCH MARKING 1. Planning and Selection Select the Process to Benchmark: Identify specific areas critical to the optical workshop's success, such as polishing quality, lens coating consistency, CNC machining speed, or reduction of surface defects. Define Key Metrics: Define the metrics you want to collect, such as surface roughnes (\(Ra\)), center thickness tolerance, or cost per unit. Identify Partners: Choose organizations or internal teams to compare against, such as a competitor, a leader in similar technology, or a different internal workshop 2. Documentation and Data Collection Document Current Processes: Map out current operational workflows (e.g., grinding, polishing, coating) to identify gaps, inefficiencies, or areas for improvement. Collect Data: Gather performance data using methods such as benchmarking questionnaires, site visits, or industry reports. Ensure

Ocular Anatomy and Physiology, Optometry Notes, Optometry Semester 2

Neuro-ophthalmic Anatomy and Physiology

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OPTOMETRY · SEMESTER 2 Neuro-ophthalmic Anatomy and Physiology Ocular Anatomy and Physiology START READING NOTES Contents of This Topic Neuro-ophthalmic Anatomy and Physiology THE PHYSIOLOGY OF VISION Pituitary Chiasma Edinger Westphal Neuro-ophthalmic Anatomy and Physiology CHAPTER 12. NEURO-OPHTHALMIC_ ANATOMY AND PHYSIOLOGY BY THE END OF THIS CHAPTER YOU WILL HAVE COVERED THE FOLLOWING ASPECTS OF THE TOPIC: THE ANATOMY OF VISUAL PATHWAYS THE PHYSIOLOGY OF VISION COMMON SYMPTOMS IN NEURO-OPHTHALMOLOGY OCULAR MOTOR NERVES ANATOMY AND PATHOLOGY ASPECTS PUPILLARY PATHWAYS AND REFLEXES ARWONS> 1. THE ANATOMY OF THE VISUAL PATHWAYS. The route taken by the light-generated impulses the eye is called the visual pathway. The visual pathway is effectively a tract within the central nervous system and it is made up of the: optic nerves; optic chiasm; optic tracts; lateral geniculate bodies; optic radiations; visual cortex. Disorders affecting different parts of the visual pathway produce characteristic changes in the field of vision. The nature of visual field disturbances , therefore, can help in determining which part of the visual pathway is affected. 84 Temporal retina Optic tract Lateral ‘eniculate ody Optic radiations Visual cortex Fig. 12.1. Diagramatic representation of the visual pathways. 1.1. The Optic Nerve ( Cranial_Nerve II ). The optic nerve is made up by the axons of the retinal ganglion cells . It may be divided anatomically, into four portions: intraocular (1mm); intraorbital (25mm); intracanalicular (9mm); intracranial (16mm). 85 a. The intraocular portion. This part includes the optic disc and the portion of the optic nerve that lies re The optic disc lies in the nasal retina, medial to the macula. Also called the optic nerve-head or papilla, it represents the confluence of approximately 1.2 million ganglion cell axons. It's pale pink in colour, slightly oval vertically . It has a central cup and a peripheral rim of neural tissue The central retinal vessels emerge at the centre of the optic disk, pass over the rim, and radiate out to supply the retina. The optic disk derives its blood supply via the short ciliary arteries from the ophthalmic artery, while the retina is fed by the central retinal artery. The nerve fibres of the intraocular portion are not myelinated. They traverse the sclera through the lamina cribrosa . Just posterior to the sclera, the fibres acquire a myelin coating. b. The intraorbital portion. The orbital portion of the nerve extends from the globe to the optic canal. It is 3-4mm thick, double the diameter of the intraocular portion as a result of the acquisition of myelin and meningeal sheath. Posteriorly , the meningeal sheath is continuous with the meningeal sheath around the brain. Thus, a rise in the intracranial pressure will be communicated to the subarachnoid space around the optic nerve, and may result in papilloedema. The length of the intraorbital portion of the optic nerve is longer than the anteroposterior dimensions of the orbit. The extra length of the intraorbital optic nerve allows unimpeded globe rotation as well as axial shifts within the orbit. c. The intracanalicular portion. 86 In addition to the optic nerve , the canal contains the ophthalmic artery and sympathetic fibres of the sympathetic carotid plexus. In the canal the nerve is firmly anchored (the dura and the periosteum are fused). As aresult, a small lesion in the optic canal can produce a compressive optic neuropathy even before it becomes easily visible upon neuroimagining. d. The intracranial portion. The optic nerve exits the posterior opening of the optic canal , enters the middle cranial fossa and continues posteriorly ascending to join the optic chiasma. Above the optic nerve lie the inferior surface of the frontal lobe, the olfactory tract , and the anterior cerebral and anterior communicating arteries. The lateral aspect of the optic nerve if often immediately adjacent to the internal carotid artery . Inferiorly and medially , the posterior ethmoid and sphenoid sinuses are adjacent to the nerve. The blood supply of the optic nerve is through the pial network of vessels. Arterial branches feeding the pial network originate from the ophthalmic artery . The venous drainage of the optic nerve is chiefly by the central retinal vein and to a lesser extent via the pial venous system. Both systems drain into the ophthalmic venous system ( superior and/or inferior ophthalmic veins ) in the orbit and less commonly directly into the cavernous sinus. 1.2. The Optic Chiasm. Fibres from the two optic nerves merge to form the optic chiasm. The chiasm is located anterior to the hypothalamus and above the sella turcica. 87 Fig. 12.2. Relation between the optic nerve and chiasm, the sellar structures and the third ventricle (III ). The chiasm overlies the sella turcica with the pituitary gland below, being The relatively large space between the chiasm and the pituitary explains the fact that only large tumours of the pituitary gland will compress on the chiasm and cause visual field defects. The chiasm forms the floor the third ventricle and carotid arteries bound it at either side. The circle of Willis surrounds the chiasm. 88 Anterior cerebral artery | Internal j carotid / artery I Posterior communicating Basilar artery artery Fig. 12.3. Relationship between the Optic Chiasm and the Circle of Willis. Fibres from each nasal retina cross in the optic chiasm , whereas fibres from each temporal retina do not cross. Slightly more than half of the fibres (65% ) decussate , because the nasal retina contains more ganglion cells than the temporal retina. Lesions of the chiasm characteristically involve crossing nasal fibres. As such , they cause bilateral temporal visual field defects, that respect the vertical line. 1.3. The Optic Tracts. The optic tracts begin at the posterior aspect of the optic chiasm , diverge laterally , and continue posteriorly to terminate primarily in the lateral geniculate bodies. A group of fibres leave the optic tract just before the lateral geniculate body for the pretectal area; the pretectal pathway controls light-mediated pupillary constriction. 89 Each optic tract contains crossed nasal fibres from the contralateral

Ocular Anatomy and Physiology, Optometry Notes, Optometry Semester 2

The Retina and Vitreous

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OPTOMETRY · SEMESTER 2 The Retina and Vitreous Ocular Anatomy and Physiology START READING NOTES Contents of This Topic The Retina and Vitreous ANATOMISTS CLINICIANS (HOW DOES THE EYE SEE?) The Retina and Vitreous CHAPTER 11. THE RETINA AND THE VITREOUS BY THE END OF THIS CHAPTER YOU WILL HAVE COVERED THE FOLLOWING TOPICS: 1. DEFINITION 2. GROSS ANATOMY 3. HISTOLOGY 4. THE RETINAL BLOOD SUPPLY 5. RETINAL BIOCHEMISTRY AND PHYSIOLOGY 6. COLOUR VISION 7. CLINICAL CONSIDERATIONS 8. THE VITREOUS. 1. DEFINITION. The BEEN is the inner neural layer or the nervous coat of the eye. It contains receptors that sense the light from the outer world and transmit it to the brain for higher processing. It is a thin transparent membrane with a purplish red colour in a living subject. Its thickness varies from 0.6mm near the optic disc to 0.1mm at its peripheral termination called the ora serrata. The outer surface is in contact with the choroid and the inner surface with the vitreous. The retina is firmly attached to the optic disc margin and at its peripheral a Externally, the peripheral termination of the retina corresponds with the site of insertion of the medial and lateral recti muscles. 2. GROSS ANATOMY. There are several prominent structures in the retina that can be identified with the help of an instrument called the ophthalmoscope. The procedure is called ophthalmoscopy or funduscopy , and the part of the eye visible on funduscpoy is called fundus. 68 2.1. The optic disc. This is most prominent structure of the retina and it is located nasally. It is a yellow-pink ,oval to round structure, also called papilla or optic nerve head . It represents the beginning of the optic nerve,( the second cranial nerve- the nerve that is responsible for vision ). The optic nerve head or the optic disc measures 1.75 mm vertically and 1.5mm horizontally in diameter. The centre of the disc has a circular depression that appears whiter than the rest of the disc. This is called the cup of the disc and is a location where the retinal arteries Fine vessels are seen to pass over the surface of the disc, which give it the pinkish appearance. The retina is firmly attached to the margins of the disc. There is no retinal tissue over the disc,thus it is insensitive to light and is referred to as the blind spot. 2.2. The area centralis. ‘Temporal to the disc is an area called the area centralis. This is the most posterior part of the globe and central part of the retina. Clinically, this area is called the posterior pole. It measures about 5-6 mm in diameter and is enclosed within the At the centre of the posterior pole is an area measuring about 1.5 mm called the macula lutea by the clinicians and fovea by the anatomists. It has a yellowish appearance when seen by an ophthalmoscope due a yellow pigment called the xanthophyll. The macula lutea or the fovea is located about 3mm temporal and 1mm inferior to the disc. The photoreceptor layer of the fovea contains only cones. The centre of the fovea is depressed and is called the fovea centralis by the clinicians and foveola by the anatomists. It measures about 0.35mm in diameter. 69 It is thus apparent that anatomists and clinicians differ in their terminologies as follows: ANATOMISTS CLINICIANS Area centralis Posterior pole Fovea Macula lutea (or just the macula) Foveola Fovea centralis ( or just the fovea) As clinicians, we will stick to the clinical terminologies henceforth to avoid confusion. The macula is responsible for central and colour vision. The sharpest central vision is achieved at the fovea 2.3. The peripheral retina. The remainder of the retina outside the posterior pole is termed peripheral retina, although further subdivision exists but that is beyond the scope of this work. The peripheral termination of the retina shows teeth like projections called the ora serrata. The retinal cells at the ora serrata continue over the ciliary body to form the non- pigmented epithelial layer of the pars plana. In the peripheral retina the predominant photoreceptors are the rods. The retina is divided into temporal and nasal halves by an imaginary line that runs vertically through the fovea. _ The centre of the optic disc is used to divide the retina into 4 quadrants: e Supero- nasal; ° supero-temporal; e infero-nasal; e infero-temporal. This division helps the clinician locate and document the position of lesions on the surface of the retina. With this division in mind one can visualise the retinal vessel's distribution on the surface of the retina. 70 The retinal artery emerges as a single vessel called the Central Retinal Artery , which then divides in to 2 branches the superior and inferior retinal arteries Each of these then further divides into 2 branches, one for the temporal and the other for the nasal quadrants of the retina. The same applies with the distribution of the Central Retinal Vein. Fig. 11.1. Anatomical landmarks of the left eye fundus. A- anatomical macula / clinical posterior pole; B- anatomical fovea / clinical macula; C- anatomical foveola / clinical fovea; D- optic disc; E- optic cup. 71 GD ait with wes offic 3. HISTOLOGY. The retina, the layer which develops from the inner and outer layers of the embryological optic cup, is divided into two major portions: e The Retinal Pigment Epithelial layer ( RPE); e The Neurosensory layer. 3.1. The Retinal Pigment Epithelium (RPE ). a. Anatomical features. The RPE is a GifigléllayerOnicellS deriving embryologically from the outer — layer of the optic cup. It is located between the choroid and the neurosensory retina, the apices of the cells pointing towards the vitreous while the base rests on its basement membrane towards the choroid. The RPE is a continuous monolayer of cuboidal / columnar cells which extend from the optic disc margin to the ora serrata. From ora serrata it continues over the ciliary body as the pigmented

Ocular Anatomy and Physiology, Optometry Notes, Optometry Semester 2

The Crystalline Lens

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OPTOMETRY · SEMESTER 2 The Crystalline Lens Ocular Anatomy and Physiology START READING NOTES Contents of This Topic The Crystalline Lens THE STRUCTURE OF THE CRYSTALLINE LENS Anterior Pole Bow Region The Crystalline Lens CHAPTER 10. THE CRYSTALLINE LENS BY THE END OF THIS CHAPTER YOU WILL HAVE COVERED THE FOLLOWING ASPECTS OF THE TOPIC: DEFINITION THE STRUCTURE OF THE CRYSTALLINE LENS BIOCHEMISTRY AND METABOLISM OF THE LENS THE FUNCTIONS OF THE LENS SOME DISORDERS OF THE CRYSTALLINE LENS ARWONS> 1. DEFINITION. The crystalline lens is a biconvex, transparent structure located directly behind the pupil. It lies posterior to the iris and anterior to the vitreous, Suspended in position by the The equatorial diameter of the adult lens is 9- 10mm. The anteroposterior length (approximately 4-5mm ), varies with accommodation. Fig. 10.1. Diagram of the anterior segment of the eye showing the lens held by the zonules to the ciliary body. 62 2. THE STRUCTURE OF THE CRYSTALLINE LENS. The lens is composed of : e capsule; e lens epithelium; ° cortex; e nucleus. 2.1. The capsule is a transparent elastic membrane . Anteriorly is considerably thicker than posteriorly. 2.2. The lens epithelium jis a single layer of metabolically active epithelial cells, situated under the anterior capsule. Continuously , new cell are formed The newly formed cells migrate posteriorly and differentiate into fibres. This change is associated with increase in protein content and loss of organelles (mitochondria, nucleus). The fibres are laid down in a concentric manner, their tips meeting to form sutures. The fibres crowd and compact the previously formed ones, with the oldest being the most central. 2.3. The cortex is made -up by the outermost fibres, most recently laid. The fibres have a high content of crystalline proteins which are responsible for the high refractive index of the crystalline lens. 2.4. The nucleus consists of all the fibres laid down before birth. There is no distinct morphologic differentiation between cortex and nucleus; rather, the transition between these regions is gradual. 63 Anterior Pole Surrounding Germinative Capsule ry, Zone Epithelial Cells . Equator a r Bow Region Cortical Fibers Nuclear Fibers Posterior Pole Fig. 10.2. Diagram showing the structure of the Crystalline Lens. 3. BIOCHEMISTRY AND METABOLISM OF THE CRYSTALLINE LENS. The human lens has a protein concentration of 33% , highest of any structure in the body; lens proteins can be divided in two groups based on their water solubility : a. Crystallins / water soluble (alpha, beta, gamma) & b. Water insoluble The crystallins are the major lens proteins. The crystalline lens lacks innervation and has no vascular system, it depends totally on the aqueous and vitreous for its nourishment. 4. THE FUNCTIONS OF THE CRYSTALLINE LENS. The functions of the lens are: e to maintain its own clarity (transparency); e torefract the light; e to provide accommodation. 64 4.1. The most important aspect of lens physiology is the maintenance of transparency. Lens transparency is highly dependent on cellular hydration; perturbation of cellular hydration can lead to lens opacification Responsible for the transparency is , also the homogenous structure of fibres , the small size of the extracellular space and the stable structure of the major lens proteins, the crystallins. As such, the lens is remarkable, unique, for its ability to preserve transparency throughout the human life span. 4.2. Refraction of light. The lens has a high refractive index (1.41 ) achieved by the high protein content of the fibres. The refractive power of the lens is 15 dioptres. 4.3 Accommodation , the mechanism by which the eye changes focus from distant to near images, is produced by a change in the lens shape, resulting from the action of the ciliary muscle on the zonules. When the ciliary muscle contracts , the zonular fibres relax allowing the lens to become more spherical . Thus, the axial thickness of the lens increases , its diameter decreases, and its dioptric power increases , producing accomodation. Accommodation is controlled by the parasympathetic system, via cranial nerve Ill (oculomotor ). 5. CLINICAL CONSIDERATIONS. Accomodation diminishes with age; clinically evident loss of accommodation is called presbyopia. It can be corrected with spectacles. The opacification (loss of transparency )of the lens is called cataract. 65 Opaque lens Fig. 10.3. A. Slit lamp photograph of an eye with cataract. B. Diagrams of cataract. When the opacity of the lens interferes with the patient’s daily activities , the cataract can be surgically removed. The absence of the crystalline lens is called aphakia. The most common method of replacing the lost optical power after cataract removal, is the implantation of an intraocular lens. Correction with an intraocular lens is called pseudophakia. 66 Cataract is the leading cause of blindness in the world. 17 million people are blind from cataract. The most common type of cataract is the senile cataract. Cataracts may be congenital ( eg. rubella syndrome) ; can also result from injury or disease: secondary cataracts. In all cases , treatment consists of surgical removal of the opacified lens. 67 ← 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

Epidemiology and Biostatistics, Optometry Notes, Optometry Semester 2

Introduction To Epidemiology-2

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OPTOMETRY · SEMESTER 2 Introduction To Epidemiology-2 Epidemiology and Biostatistics START READING NOTES Contents of This Topic Learning Objectives Epidemiology Definition od Epidemiology Definition of epidemiology Key words of the definition: Types of Epidemiology Distribution (DESCRIPTIVE EPIDEMIOLOGY) Distribution cont… Determinants (Analytic Epidemiology) Definition of Health Definition of host Host Definition of vector Definition of Reservoir Diseases with animal reservoirs (also known as zoonoses): Agent The determinants of health include: In a Specified population how does epidemiologist differ from clinicians? Applications and Achievements of epidemiology • Non-communicable Diseases(NCDs) Types of epidemiology Descriptive Epidemiology Examples of descriptive epidemiology Examples of descriptive epidemiologycont… Analytic Epidemiology Analytical Epidemiologycont… EVALUATION Key Points Key points cont… aims/Uses/Applications of epidemiology Uses/Applications of epidemiology cont… Read on reference Introduction to Epidemiology Learning Objectives By the end of this session, students are expected to be able to: Define epidemiology, health and disease Describe Types of epidemiology Describe the application of epidemiology and achievements of epidemiology Explain the determinants of health and disease -INTRODUCTION TO EPIDEMIOLOGY Epidemiology The word epidemiology comes from the Greek words epi = meaning on or upon demos = meaning people logos = meaning the study of or doctrine Thus the word “epidemiology” simply means the study of what is happening to people -INTRODUCTION TO EPIDEMIOLOGY Definition od Epidemiology As per perkins 1873 it is that branch of science which treats epidemics As per frost 1927 it is the science of mass phenomenon of infection disease. As per greenwood 1934 the study of any disease as a mass phenomena -INTRODUCTION TO EPIDEMIOLOGY Definition of epidemiology « The study of the distribution and determinants of health related states or events in specified populations and the application of this study to the control of health problems. » -INTRODUCTION TO EPIDEMIOLOGY Key words of the definition: Study  Basic science Distribution  time, place, person Determinants  Cause, risk factors Event  Health status Population  Community Application  Information for action Three closely-related components (distribution, determinants and frequency) encompass all epidemiological principles and methods -INTRODUCTION TO EPIDEMIOLOGY Types of Epidemiology DESCRIPTIVE EPIDEMIOLOGY ANALYTICAL EPIDEMIOLOGY -INTRODUCTION TO EPIDEMIOLOGY Distribution (DESCRIPTIVE EPIDEMIOLOGY) Distribution is concerned with the frequency and pattern of health events in a population Distribution: Descriptive Epidemiology What, who, when, and where Frequency: number, rates, and risk Quantify diseases to determine magnitude Patterns: time, place, and person -INTRODUCTION TO EPIDEMIOLOGY Distribution cont… Frequency: refers to the number of health events e.g. Number of cases of meningitis Number of cases of diabetes in a population Number of people with mental disorder in a population Number of children under one year of age vaccinated for measles -INTRODUCTION TO EPIDEMIOLOGY Distribution cont… Pattern of disease refers to the occurance of health related events or disease by time, place and person. Time pattern may be; seasonal, annual, monthly, weekly, daily, hourly, weekends or weekdays. Place pattern includes; residence (urban/rural), geographical variations, country variations Personal patterns includes; age, sex, gender, marital status, educational level -INTRODUCTION TO EPIDEMIOLOGY Determinants (Analytic Epidemiology) Causes and influences Compare between exposure groups to determine causal relationships Evidence for control and prevention Why and how -INTRODUCTION TO EPIDEMIOLOGY Definition of Health Health: A state of complete physical, mental, and social well-being and not merely the absence of disease or infirmity. (World Health Organization) Definition of Disease Disease: A disorder of structure or function in a human, especially one that produces specific symptoms or that affects a specific part. Definition of agent Is an microorganism/microbe that is cable of causing a disease -INTRODUCTION TO EPIDEMIOLOGY Definition of host • Host: An organism which harbors or nourishes another organism (parasite). • Intermediate host: An organism in which a parasite passes its larval or nonsexual existence. • Definitive host: An organism in which the parasite develops to an adult and sexually mature stage. -INTRODUCTION TO EPIDEMIOLOGY Host Host Is an organism capable of being infected by an agent. Examples… Host factors include: Age Sex Social class Personality Genetic factors Education Marital status Definition of vector An organism which is capable of spreading infection by conveying pathogens from one host to another. vectors of medical importance i. Mosquitoes ii. Tsetse flies iii. House flies iv. Ticks v. Mites vi. Bedbugs and Lice vii. Snails viii. Rodents ix. Fleas -INTRODUCTION TO EPIDEMIOLOGY Definition of Reservoir Reservoir: The habitat in which disease-causing organisms normally live and multiply without necessarily being affected Reservoirs can be human, animal, or environmental. o Diseases with human reservoirs: Smallpox (symptomatic) HIV (asymptomatic) Diseases with animal reservoirs (also known as zoonoses): Brucellosis (can be found in goats, sheep, cattle, pigs) Plague (can be found in rats and other wild rodents) Anthrax (can be found in cattle, sheep, goats, and other herbivores) o Environmental Histoplasmosis (caused by a fungus that is often found in areas with lots of bird/bat droppings such as caves) Legionnaires’ bacillus (caused by aquatic bacteria that grow in warm water) Note: a reservoir is different from a vector or disease carrier, which are agents of disease transmission. -INTRODUCTION TO EPIDEMIOLOGY Agent An etiological factor which are necessary for bringing about a particular disease in a susceptible host. Examples of agents are: Plasmodium Yesinia pestis Mycobacterium tuberculosis The determinants of health include: Income and social status Education Physical environment Social support networks Biology and genetic endowment Health services (availability and accessibility) Gender -INTRODUCTION TO EPIDEMIOLOGY In a Specified population how does epidemiologist differ from clinicians? Epidemiologist and clinicians differ on how they view “the patient”. Epidemiologist will be concerned with collective health of people in the community while clinician will focus on the individual case. e.g. patient with diarrheal disease- what is source?, other people infected/exposed?, interventions to prevent additional cases > epidemiologist will take account -INTRODUCTION TO EPIDEMIOLOGY -INTRODUCTION TO EPIDEMIOLOGY Applications and Achievements of epidemiology Epidemiology is used to: Describe the etiological factors in causation of disease. Study the natural history of disease, from good health to subclinical changes until occurrence of clinical disease, where the outcome can be recovery (with or without disability) or death.

Ocular Anatomy and Physiology, Optometry Notes, Optometry Semester 2

The Anterior Chamber

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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

Ocular Anatomy and Physiology, Optometry Notes, Optometry Semester 2

The Cornea and Sclera

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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

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