Generalized mathematical modeling of aqueous humour flow in the anterior chamber and through a mesh channel in the human eye

Project Abstract

<p> In this work, we propose mathematical models for the processes that take place in the<br>human eye and how they contribute to the development of pathological states. We<br>considered and studied two related dynamics processes that take place in the eye.<br>Firstly, a generalized mathematical model of aqueous humour flow driven by<br>temperature gradient in the anterior chamber is presented. This predicts the flow<br>behavior when the ambient temperature is higher than the core body temperature. The<br>purpose of these models is to predict flow behavior in the presence of high ambient<br>temperatures. Secondly, we consider the aqueous humour flow through a trabecular<br>mesh channel in the presence of multiple constrictions or stenoses. A two dimensional<br>model for the fluid in the mesh channel with couple stress fluid in the core region and<br>Newtonian fluid in the peripheral region is developed. The purpose of these models is to<br>examine the flow behavior and investigate how this influences primary open angle<br>glaucoma (POAG). The models are solved analytically. The result obtained showed that<br>buoyant convective flow would always arise from the temperature gradient that is<br>present across the anterior chamber of the eye. Also, as the cornea height and<br>temperature increases, the fluid velocity decreases. It is observed that resistance to flow<br>and wall shear stress increased with the height of the stenoses. The result equally<br>indicated that intraocular pressure (IOP) increased with the wall shear stress as a result<br>of the multiple stenoses that narrows the trabecular mesh channel. The channel becomes<br>progressively less porous, this might lead to primary open angle glaucoma (POAG). <br></p>

Project Overview

<p> INTRODUCTION<br>This research work is based on mathematical models on aqueous humour flow in the<br>interior chamber of the human eye and its exit through the outflow pathways. We shall<br>consider this under two subheadings. A generalized mathematical model of aqueous<br>humour flow driven by temperature gradient and a model on fluid flow through a mesh<br>channel in the human eye. We shall also discuss any other information that may be very<br>necessary for proper understanding of our mathematical models, results and subsequent<br>analysis.<br>1.0.1 A Generalized Mathematical Model for the Aqueous Humour Flow Driven<br>by Temperature Gradient.<br>Vision is one of the most important human senses (BO 2009, Umit 2003,<br>Valdivia 2009, Zuhaila 2008). The human eye is one of the most complex organ and<br>complex structure in the biology of man. As a sense organ, the eye is the (optic)<br>window through which man Visualizes his environment and what happens in and<br>around him. The power of vision and conception all lie in the power of sight enabled by<br>the eye. If the eye is a major vital organ in man, its study is of prominent concern. This<br>is to enable (eye) health practitionersunderstand the mechanism of sight more and more;<br>and then find ways to improve the condition of human eye.<br>15<br>The eye as an organ does a variety of functions other than sense of sight. It also<br>tells to some extent some disease conditions. Such diseases produce some changes that<br>are observable in the eye (Smith, 2008). Human eye function is more sophisticated than<br>any man-made optical device (Valdivia, 2009). The eyes are often called the windows<br>to the soul; we communicate and express emotion with our eyes in ways that defy<br>words. When we are shocked or surprised, our eyes open wide. If we are confused, our<br>eyes squint; angry, they appear to narrow; excited, they brighten (Valdivia, 2009).<br>The eyes are responsible for four-fifth of all the information sent and processed<br>in the Brain. Also eighty percent of learning occurs through the visual pathways (Smith,<br>2008).<br>Vision is considered to be the most desirable of all human senses. Without it, a<br>person’s relationship to the surrounding world and the ability to interact with the<br>environment is considered seriously diminished. The visual system also helps to<br>maintain balance and posture in human beings (Wikipedia; free encyclopedia).<br>In humans, sight mechanisms are also complex, its complexity, in addition to that<br>of the eye, makes its study complex as well. This study has been a major challenge to<br>researches in this field for some time now. Though a great deal has been achieved, a lot<br>still need to be done. This includes understanding the relationship between the sight<br>mechanisms and the fluid in the eye.<br>The human eye is made up of different fluids. These include the aqueous humour,<br>the vitreous humour and the tear film. The aqueous humour lies in the anterior and<br>16<br>posterior chambers of the globe whereas the vitreous humour occupies the posterior<br>segment of the globe (fig.1). The anterior chamber lies between the iris and the cornea<br>and the posterior chamber is the region behind the Iris and anterior to the hyaloids<br>membrane. Thus, understanding the complex mechanisms that regulate aqueous humour<br>circulation is essential for management/treatment of some eye diseases (Adam et al,<br>2012). Of course, the secretion of aqueous humour and regulation of its outflow are<br>physiological important processes for the normal function of the eye (Jeffrey et al<br>2002). We seek to know whether significant thermally driven natural convection exists<br>within the anterior chamber when the ambient temperature is higher than the core body<br>temperature.<br>Generally, the flow in the anterior chamber is thought of to be driven by<br>temperature gradient. Hence, the general idea of anterior chamber convection appears to<br>have been adapted although attempts are still being made to actually understand the<br>driving force for the fluid flow in the eye. Other researchers believed that flow in the<br>anterior chamber appears to take place in a single convection cell, rising (that is,<br>opposing gravity) near to the back of the chamber and falling towards the front. It is<br>believed that there is little or no lateral motion of the fluid (Canning et al 2002).<br>Even though the thickness of the cornea is assumed to insulate the content of the<br>anterior chamber (fluid) from fluctuations, in areas where the ambient (room)<br>temperature is in excess of the body temperature 370C, wefind that this insulation action<br>may not be true and thus the ambient (room) temperature may not be constant. This is<br>17<br>mostly the case as found in the desert and equatorial regions of the world (for example<br>in Africa, Nigeria and Niger in Particular) where most of the parts has room or ambient<br>temperature greater than 400C(for example, recorded temperature extremes of 56.70C in<br>Death valley Califonia, USA (1913).55.00C in Kebili Tunisia (1931) and 46.40C in Yola<br>Nigeria (2010)) (Wikipedia, the free encyclopedia). yet there is aqueous flow. Thus, the<br>modeled equations by Canninget al (2002). Gabriel &amp; Alisteir (2002). Brain &amp;Fitt<br>(2003). Jeffrey and Barocas (2002) and Gonzalez and Fitt (2006) Adam et al<br>(2012).Zuhaila (2013). Crowder &amp; Ervin (2013). may not appropriately take care of this<br>peculiar situation. This is what inspired this work. As a result, we propose a<br>modification of the temperature gradient for the thermally-driving flow in the anterior<br>chamber of the eye<br>The major change in the existing model is in the equation and this is to take care<br>of the situation where the ambient (room) temperature is more than the body<br>temperature which is 370C.<br>In human eye, heat gain occurs through conduction, perfusion, metabolism,<br>blinking, tear flow, evaporation, and convection, but heat loss occurs only through<br>conduction, evaporation, convection and radiation. More factors are involved in heating<br>eye components than cooling. Hence, the human eye is more vulnerable when it is<br>exposed to high temperatures (high ambient temperatures, hyperthermia treatment, laser<br>surgery etc) than low temperatures (low ambient temperatures, cryosurgery treatment<br>etc). At the cornea, heat loss from the eye occurs through convection, radiation, and tear<br>18<br>evaporation. Hence temperature increases from outer surface of cornea towards eye<br>core when ambient temperature is less than 370Cand vice versa. Due to convective heat<br>transport of the blood vessels, the blood picks up energy from hot areas and deposits<br>this at cooler areas or vice versa. The temperature inside the human body depends on<br>the degree of temperature, duration of exposure and the environment conditions which<br>cause heat gain/loss from tissues (Gokul et al 2013).<br>Thus we develop a model that takes care of the case where normal room or<br>ambient temperature is more than the body temperature which is 370C which is a more<br>general case.<br>19<br>1.0.2 Fluid Flow through aMesh Channel in the Human Eye<br>The trabecular meshwork is a tissue located in the anterior chamber angle (the angle<br>structure include: the outermost part of the iris, the front of the ciliary body, the<br>trabecular meshwork and the canal of Schlemm) of the eye,(Artur et al, 2003; Satish,<br>2003; Fitt, 2010 and Adamet al,2012). The trabecular meshwork is a wedge shaped<br>lattice, spongy tissue composed of 12 – 30 trabecular layers posteriorly and 3 – 5 layers<br>anteriorly at its apex near the cornea (Patrick, 2006 and Mark, 2006). It is one of the<br>Fig. 1.1: The anterior chamber of the eye<br>Adapted from canning et al (2002)<br>20<br>outflow pathways for the evacuation of aqueous humour from the eye. (Artur (2003).<br>Michael et al, (2006). Paul( 2008)).<br>Fig.1.2Schematic diagram of outflow system of human eye<br>(adapted from Satish, 2003).<br>21<br>Fig 1.3: Scematic diagram of trabecular meshwork<br>(adaptedZahaila (2013)).<br>The aqueous humour is a colourless intraocular fluid that is secreted by the ciliary<br>epithelium. It flows in the posterior chamber bathing the lens, through the iris, in the<br>anterior chamber providing a transparent medium, nutrients, means for metabolic waste<br>removal to the avascular tissues, and pressuring the eye and then drains into the<br>episcleral venous system through the trabecular meshwork and the canal of Schlemm.<br>(Artur et al,2003; Patrick , 2006; Satish, 2003; Adamet al, 2012 and Ram et al, 2014).<br>As such, disrupting the delicate balance between aqueous humour inflow and outflow<br>may lead to elevation of intraocular pressure (IOP). a known risk factor for primary<br>22<br>open angle glaucoma (POAG) (Michael, 1999; Chimdi &amp;Umeh, 2000 and Patrick,<br>2006).<br>We find that particulates substances of different sizes, shapes and traits circulate inside<br>the anterior chamber. These particulates such as the red blood cells, white blood cells<br>and other particulates detachment from the eye eventually flow out of the anterior<br>chamber by squeezing themselves through the trabecular meshwork (channel). We shall<br>note that this mesh channel can be reduced in diameter or otherwise depending on the<br>size of these particulates substances. Thus, if we consider this as a non uniform channel<br>whose whole diameter depends on the nature of the ciliary muscles and its own<br>contractile and volume – regulatory properties, we can see that the particulates can<br>obstruct the channel so that we can consider this as flow through a cylindrical channel<br>which is easily described by the Naiver – stokes equations. Again, because the size of<br>the particles are relatively large compared to the diameter of the trabecular meshwork,<br>we can consider the flow of this fluid and the particulates as a bi – layer flow described<br>also by the Naiver – Stokes equations. Hence the action of the ciliary muscles (force) on<br>the trabecular meshwork can be likened to fat deposit in the flow channel regarded as<br>the stenosis. Here we consider multiple stenoses with the effects of slip condition on the<br>flow of aqueous humour in the mesh channel. When this ciliary muscles contract, it is<br>likened to clearance of the stenosis because this forces the ciliary muscles to<br>mechanically stretch the trabeculur meshwork thereby increasing the thorough flow of<br>23<br>aqueous humour (Canninget al, 2002). If it relaxes, it do contract the meshwork by<br>reducing the diameter and so making it difficult for thorough flow of the particles. The<br>presence of multiple stenoses or constrictions in the mesh channel can lead to increased<br>resistance to outflow with undesirable consequences. This can create an imbalance in<br>the production and drainage of aqueous humour. The intraocular pressure within the eye<br>builds up which might lead to primary open angle glaucoma.<br>Primary open angle glaucoma (POAG) is the second leading cause of blindness<br>worldwide after cataracts (Fitt, 2010 and Zuhaila, 2013). It is also known as chronic<br>glaucoma or “the silent thief of sight” because of the lack of early symptoms. Most<br>patients with POAG are not aware that they have the disease until significant vision<br>loss occurred.<br>Interestingly, the human eye appear to be particularly vulnerable to POAG when<br>compared to eyes of non – human species. The reasons for this high susceptibility<br>remains unknown (Patrick, 2006). Also the definite locus for the primary resisitance<br>moiety within the normal human eye as well as the added resistance in eyes with<br>POAG is not yet known (Patrick, 2006; Adamet al,2012 and Ezell Research<br>Symposium, 2013). Unfortunately, this lack of fundamental knowledge has prevented<br>the development of an effective anti – glaucoma therapy that could be used to<br>selectively target and weaken the primary resistive moiety to allow for decreased<br>outflow resistance in the trabecular meshwork.<br>24<br>The aim of this study is to investigate the mechanism in the trabecular meshwork<br>responsible for the generation of aqueous humour resistance in the human eye with the<br>hope that specific outflow resistance profile might be identified as this will help in<br>understanding the mechanisms involved in regulating aqueous humour outflow<br>resistance in glaucomatous human eyes.<br>A mathematical model is presented for the flow of aqueous humour through the<br>trabecular meshwork with multiple stenoses in order to predict changes in intraocular<br>pressure (IOP). The governing equations have been adapted from Gurju and<br>Radhakrishnamacharya (2013) and Gurju et al (2014).<br>Table 1: Standard Parameter Values for an Adult Human Eye.<br>Physical Quantity Typical Values<br>Radius of anterior chamber ı (m) 5.5×10-3<br>Total width of anterior chamberı(m) 11×10-3<br>Coefficient of linear expansion of aqueous humour ∝ (k) 3.0×10-4<br>Gravitational acceleration ı (m/s2) 2.75×10-3<br>Height of anterior chamber ıı (m) 1.0×10-3<br>Dynamic viscosity m of aqueous humour (Pa s) 1.0×103<br>Density Po of aqueous humour (Kg/m3) 1.0×103<br>Adapted from Fitt &amp; Gonzalez (2006)<br>25<br>1.1 MOTIVATION FOR THESE MODELS<br>1.1.1 A Generalized Mathematical Model For The Aqueous Humour Flow Driven<br>By Temperature Gradient.<br>We saw that the models already built on aqueous flow in the eye (Canninget al<br>2002; Jeffery &amp; Borocas, 2002;Satish, 2003;Gabriela &amp; Alistair, 2002; Braun &amp; Fitt,<br>2003; Jeffrey &amp; Gonzalez, 2004; Gonzalez &amp; Fitt, 2006; Zuhaila &amp; Fitt, 2008; Adam et<br>al 2012; Zuhaila, 2013 andCrowder &amp; Ervin, 2013) were based on temperaturegradient<br>where the inner body temperature was assumed to be higher than the ambient<br>temperature. It is this temperature gradient that caused the outflow of the aqueous<br>through the iris to the outer cornea.<br>However, through research, we discovered that the normal human body<br>temperature is about 370C and that the consideration of the authors were based on<br>external temperature being less than this 370C, in particular in Europe where<br>temperatures are far less than this body temperature most of the time. In the light of this<br>we see that this model may not have promptly taken care of situations where the<br>external temperature is greater than 370C or even close to 370C. Our question then was,<br>whether there is still aqueous flow in people’s eyes in such regions or places where such<br>temperatures does not subsist. A close observation shows that there is still aqueous flow<br>in people of such regions like in Africa, Malaysia and other Asian or temperate<br>26<br>countries of the world. This then means that the existing aqueous flow models may not<br>have promptly represented this very case. Hence, our desire to remodel aqueous flow in<br>human eyes taking into account the various temperature differences in different regions<br>of the world where people live.<br>1.1.2 Fluid Flow through A Mesh Channel in the Human Eye<br>Available statistics from the Federal Ministry of Health on the 2014 World Sight Day<br>(9/10/2014) as published in the editorial of the Sun Newspaper of 8th November, 2014<br>show that Nigeria is one of the countries with the highest blind people. Over 1 million<br>Nigerians are blind with over 3 million being visually impaired. Also 42 out of every<br>100 adults above the age of 40 are visually impaired. 2 out of every 3 blind Nigerians<br>lost their sight to preventable causes. In addition, Nigerians account for 1 in every 5<br>blind Africans. Globally, over 45 million people are blind while 135 million have<br>severe visual impairment.<br>Glaucoma is the second (the first is cataracts) leading cause of blindness globally<br>as well as in most regions including Nigeria. It generally results from an outflow<br>resistance of aqueous humour. When the drainage channel becomes clogged, aqueous<br>fluid cannot leave the eye as fast as it is produced, causing the fluid to accumulate.<br>This accumulated fluid leads to an increase in intraocular pressure (IOP). As a<br>consequence, the retina ganglion cells progressively suffer irreversible damages that<br>lead to visual field reduction and eventually to blindness, (Artur 2012). This condition<br>27<br>is more worrisome as glaucoma can only be stemmed; it cannot be cured (Chimdi and<br>Umeh 2002). Glaucoma presents an even greater public health challenge than cataracts<br>because the blindness it causes is irreversible (Nosiri et al 2009). Infact, vision loss<br>from glaucoma is silent, slow, progressive, irreversible but treatable (Robert, 2008).<br>However, a conclusive determination of where in the outflow pathways this<br>elevated outflow resistance is generated has been elusive. Also, the locus of aqueous<br>outflow resistance in the normal eye has not been equivocally determined (2013 Ezell<br>Research Symposium). Again, the fluid dynamics of the aqueous humour and the role<br>of the outflow channels is not fully understood (Adamet al 2012). This fact is also<br>evidenced by the great number of drugs used for the treatment of primary open angle<br>glaucoma. The drugs most commonly used either decrease the production of aqueous<br>humour in the ciliary body or increase the uveoscleral (unconventional) outflow.Drugs<br>acting directly on the trabecular meshwork have not yet been developed (Artur et al<br>2003, Patrick 2006,Zuhaila 2013). However, due to the quantitative significance of the<br>trabecular meshwork in the drainage of aqueous humour, there is need for a tissue<br>specific anti – glaucoma therapy.<br>Consequently, we model the fluid flow in the trabecular meshwork by<br>considering the slip condition and multiple constrictions or stenoses in the graded flow<br>channel and its influence on primary open angle glaucoma (P O A G).<br>28<br>1.2 OBJECTIVES OF THE STUDY<br>This study is undertaken on generalized mathematical modeling of aqueous humour<br>flow in the anterior chamber and through a mesh channel in the human eye. The<br>objectives of the study are to:<br>1. formulate a mathematical model that describes the fluid flow in the human eye<br>when ambient temperature is higher than core body temperature,<br>2. investigate the dynamics of the model and compare with that of existing models,<br>3. describe the pressure and flow velocity in a healthy/glaucomatous eye,<br>4. describe the velocity streamlines and pressure contours in healthy/glaucomatous<br>eye and<br>5. analyze the effect of resistance of the drainage system on the flow distribution<br>and intraocular pressure (IOP).<br>1.3 Anatomy and Physiology of the Eye<br>1.3.1 The Human Eye<br>The eye is a special ball- like structure situated at the face of human beings. As a<br>sense organ, the eye is the opticwindow through which man visualizes his environment<br>and what happens around him. Human memory and mental process rely heavily on<br>sight (Encyclopedia of Nursing and Allied Health). There are more neurons in the<br>29<br>nervous system dedicated to vision than any other of the five senses indicating how<br>important vision is.The human eye is not only the organ with the most intricate<br>anatomy, but also the most delicate. It has complicated structures and sophisticated<br>functions (Brubakar,1982).<br>The efficiency and completeness of our eyes and brain is unparallel in<br>comparison with any piece of apparatus or instrumentation ever invented. The eye can<br>automatically focus objects as far away as infinity and as near as 10cm. It has a wide<br>field of view of about 160° in the horizontal and about 120° in the vertical. It can<br>smoothly track fast moving objects. It can perceive colors in visual wavelengths. It can<br>efficiently process and analyze images of high resolution. These functions are<br>performed by a normal healthy human eye. The degrees of functionality may differ<br>among individuals. (Jayoung, 2007).<br>The human eye can also be considered as a biological system. Tear films, cornea,<br>iris, crystalline lens, anterior and vitreous humor and retina are all incorporated into the<br>eye ball. Each is well structured with living cells and is well coordinated to make<br>objects visible. The eye grows with age and loses or diminishes in functionality for<br>various health reasons. All of the characteristics differ among individuals. The recent<br>developments in cornea surgery and the use of intraocular lens add more variation to the<br>already existing biological divergence.<br>30<br>The human eye can be considered a neurosensory system which begins with the<br>transmitting of light energy into changes of membrane potential of the photoreceptors<br>on the retina. The neural images made by the architecture of the photoreceptors are<br>delivered from the eye to the brain through the optic nerve. Since the photoreceptors<br>outnumber the fibers inside the eye, there is a significant degree of image compression<br>between them. Various combinations of the fibers inside the optic nerve with the<br>photoreceptors explain visual perceptions such as color and motion and the visually<br>controlled behaviors such as accommodation and eye movements.<br>The eyes are responsible for 5<br>4 (four-fifth) of information sent and processed in the<br>Brain. Also, 80% of learning occurs through the visual pathways (Herbert (2008)).<br>The eye as an organ does Variety of functions other than sense of sight. It also tells<br>to some extent some disease conditions that produce some observable changes in the<br>eye (Umit, 2003).<br>The power of vision and conception all lie in the power of sight enabled by the eye.<br>If the eye is a major vital organ in man, its study is of prominent concern. This is to<br>enable (eye) health practitioners understand the Mechanism of sight more and more;<br>and then find ways to improve the condition of human eye.<br>In human, sight mechanisms are also complex. Its complexity, in addition to that of<br>the eye makes its study complex as well. This study has been a major challenge to<br>31<br>researchers in this field for some time now. Though a great deal has been achieved, a lot<br>still has to be done.<br>1.3.1.1 Structure of the Eye<br>The eye is the sense organ for seeing. The human eye is composed of the eyeball<br>and some accessory structures that serve to protect, moisten, lubricate, and move the<br>eyeball.<br>The eyeball or bulb fits into and is protected by the bones of the orbit and by<br>a thick layer of fascia and fat in which it is embedded. The anterior surface, not<br>surrounded by bone, is protected by the eyelids which are capable of instantaneous<br>closure to exclude foreign objects or too much light or heat.<br>The upper and lower eyelids are composed of loose connective tissue covered<br>by a thin skin and supported posteriorly by the tarsal plates of dense connective<br>tissue. These plates are provided with complex sebaceous glands called tarsal<br>glands. The skin turns inward at the edges of the eyelids, lining them with a mucous<br>membrane – the conjunctiva. This conjunctiva, at the base of the lids, is reflected<br>back over the anterior surface of the eyeball as a transparent layer, consisting only<br>of stratified epithelium.<br>Along the edges of the eyelids are the ciliary glands. Their secretions moisten<br>the eyelids and may keep them from adhering to each other. The lacrimal apparatus<br>32<br>consists of lacrimal glands, ducts, sacs and nasolacrimal ducts. The lacrimal gland<br>lies hidden from view, in the upper lateral side of the orbit. It produces secretions<br>that move over the anterior surface of the eyeball and drain into a tiny hole or<br>punctum at the medial end of each eyelid. Each punctum leads into a lacrimal duct,<br>which joins it to form the lacrimal sac at the medial side of the orbit. The lacrimal<br>Sac, in turn empties through the nasolacrimal duct into the nasal cavity.<br>The eyeball is a sphere about one inch in diameter (Crouch, 1982). Its walls<br>are composed of three layers, the outer-most of which is leathery and relatively<br>thick, the Sclerawhich forms anteriorly a transparent rounded bulge, the cornea. The<br>middle layer, the pigmented Choroid Coat,contains blood vessels for and reduces<br>reflection of the light within the eyeball. Anteriorly at the edge of the cornea, the<br>Choroid Coat thickens to form a ciliary body,which contains smooth muscle, fibers.<br>Around the anterior edge of the ciliary body is a thin muscular diaphragm, the iris<br>with a hole in the center called the pupil. The middle layer is also made up of the<br>transparent, crystalline lens, held directly behind the pupil by a suspensory ligament<br>that extends inward from the ciliary body. The remaining and inner most coat of the<br>eyeball is the retina,which contains the receptors for light and colour, the rods and<br>cones. The retina is continuous posterioly with the optic nerve. It diminishes in<br>thickness and in complexity. Posterior part of the retina is a depression in which the<br>retina is exceedingly thin and where the light and colour receptor alone, called cone<br>33<br>cells are present in great numbers about 146000 per mm2 (Wilson, 1979). This area<br>is known as the fovea centralis and is the point of greatest visual acuity. Just to the<br>nasal side of the fovea is the place where the optic nerve leaves the eye, the optic<br>disc. Since there are no light receptors on the optic disc, it is often called the blind<br>spot<br>1.3.1.2 Cornea<br>The cornea is the transparent, dome-shaped window covering the front of the eye. It is<br>a powerful refracting surface, providing 2/3 of the eye’s focusing power. Like the<br>crystal on a watch, it gives us a clear window to look through. The cornea is<br>responsible for focusing light rays to the back of the eye. Cornea is 78% water.<br>(Umit, 2003)<br>Because there are no blood vessels in the cornea, it is normally clear and has a shiny<br>surface. The cornea is extremely sensitive – there are more nerve endings in the cornea<br>than anywhere else in the body. The reactions of the cornea are quite important in disease<br>processes. It is vascular and therefore reacts differently from those tissues that have a<br>blood supply. Bowman’s layer has little resistance to any pathologic process because of<br>that it is easily destroyed and never generates. Descemet’s membrane, on the other hand,<br>is highly resistant and elastic and may remain in the form of a bulging balloon-like<br>structure, called a “descemetocele,” after all the other layers of the cornea are destroyed<br>(Umit, 2003)<br>34<br>1.3.1.2.1 The Layers of the Cornea<br>The adult cornea is only about 0.5 mm thick and is comprised of 5 layers: epithelium,<br>Bowman’s membrane, stroma, Descemet’s membrane and the endothelium.<br>· The epithelium is layer of cells that cover the surface of the cornea. The epithelium<br>is about 10% of the total thickness of the cornea. It is only about 5-6 cell layers<br>thick., about 50 μ.m (Davson, 1990) and quickly regenerates when the cornea is<br>injured. If the injury penetrates more deeply into the cornea, it may leave a scar.<br>Scars leave opaque areas, causing the corneal to lose its clarity and luster.<br>· Bowman’s membrane lies just beneath the epithelium. Because this layer is very<br>tough and difficult to penetrate, it protects the cornea from injury. Bowman’s layer<br>is a sheet of transparent collagen 12 μm thick.<br>· The corneal stroma represents certainly one of the most typical examples of highly<br>specialized connective tissue. Its functional efficiency is transparency. The stroma<br>is the thickness layer and lies just beneath Bowmans, it represents some 90 percent<br>of the corneal thickness. The stroma consists normally of about 7 percent of water<br>(values of up to 85 percent are given in the literature). It is composed of densely<br>packed collagen fibrils that run parallel to each other. This special organization of<br>the collagen fibrils gives the cornea its clarity.<br>· Descemet’s membrane lies between the stroma and the endothelium. The<br>endothelium is just underneath descemet’s and is only one cell layer thick. This<br>35<br>layer pumps water from the cornea, keeping it clear. If damaged or disease, theses<br>cells will not regenerate. Descemet’s membrane is about 10μm.<br>· The corneal endothelium is composed of a single layer of cubiodal cells which<br>function to keep the cornea dehydrated.<br>· Tiny vessels at the outmost edge of the cornea provide nourishment, along with the<br>aqueous and tear film.<br>· Functionally, the most important elements of the cornea are the substantial propria<br>(stoma) and its two limiting cellular membranes, the epithelium and endothelium;<br>damage to the cells of the two membranes, whether mechanical or by interference<br>with metabolism, causes the stroma to lose its transparency as a result, apparently,<br>of the imbibitions of water.<br>1.3.1.3 The Sclera<br>The sclera is a thick, opaque white tissue that covers 95% of the surface area of the<br>eye. It is approximately 530 microns (μm) in thickness at the timbus, thining to about 390<br>μm near the equator of the globe and then thickening to near 1mm (0.04 in) at the optic<br>nerve. At the posterior aspect of the eye, sclera forms a netlike structure or “lamina<br>cribroga” through which the optic nerve passes. The sclera also serves as the anchor<br>tissue for the extraocular muscles.<br>36<br>The cornea and sclera together form the outer-most covering of the eye and withstand<br>both the internal and external force of the eye to maintain the shape of the eyeball and to<br>protect the contents from mechanical injury.<br>In children, the sclera is thinner and more translucent, allowing the underlying tissue<br>to show through and giving it a bluish cast. As we age, the sclera tends to become more<br>yellow. The sclera becomes transparent when dried. This is assumed to be the result of<br>the concentration of the ground substance so that its refractive index becomes close to<br>that of the collagen. As this happens when the tissue is nearly dry, it acquires a uniform<br>refractive index (Umit(2003)).<br>The differences between the compositions of the various types of connective tissues,<br>for instance cornea and sclera are more often quantitative than qualitative. The water<br>content of cornea is somewhat higher than that of sclera. The cornea and sclera together<br>form the tough tunic of the eye, which withstands the intra-ocular pressure from within<br>and protects the contents from mechanical injury from without.<br>1.3.1.4 The Iris<br>The iris is a protected internal organ of the eye, located behind the cornea and the<br>aqueous humor, but in front of the lens. A visible property of the iris and the fingerprint is<br>the random Morphogenesis of their Minutiae. The phenotypic expression even of two<br>37<br>irises with the same genetic genotype (as in identical twins, or the pair possessed by one<br>individual) have uncorrelated minutiae. (Ales et al, 2000).<br>This is the part of eye that gives the eye its color (i.e blue, green, brown)<br>(Umit,2003). The opening in the center of the iris is the pupil. The iris act like a camera<br>shutter and controls the amount of light that enters the eye. It behaves as a diaphragm,<br>modifying the amount of light entering the eye.<br>The tissue of the iris consists of two main layers, or laminae, separated by a much less<br>dense zone (the cleft of sucks). The posterior lamina contains the muscles of the iris, and<br>is covered posteriorly by two layers of densely pigmented cells, the innermost (nearest<br>the aqueous humour) being the posterior epithelium of the iris, which is continuous with<br>the inner layer of the ciliary epithelium (Umit,2003).<br>The most importantfunction of the iris is in controlling the size of the pupil.<br>Illumination, which enters the pupil and falls on the retina of the eye, is controlled by<br>muscles of the iris. They regulate the size of the pupil and this is what permits the iris to<br>control the amount of light entering the pupil. The change in the size results from<br>involuntary reflexes and is not under conscious control. The tissue of the iris is soft<br>and loosely woven and called stroma.<br>The layers of the iris have both ectodermal and mesodermal embryological<br>origin. The visible one is the anterior layer, which bears the gaily – coloured relief and<br>38<br>it is very lightly pigmented due to genetically determined density of Melanin pigment<br>granules. The invisible one is the posterior layer, which is very darkly pigmented<br>contrary to the anterior layer. The surface of this layer is finely radiantly and<br>concentrically furrowed with dark brown colour. Muscles and the vascularised stroma<br>are found between these layers from back to front. Pigment frill is the boundary<br>between the pupils and the human iris and is a visible section of the posterior layer and<br>looks like a curling edge of the pupil. The whole anterior layer consists of the papillary<br>area and the ciliary area and their boundary is called collavette. The ciliary area is<br>divided into the inner area which is relatively smooth and bears radial furrows, the<br>middle area, heavily furrowed in all directions and with pigment piles on the ridges,<br>and the outer marginal area bearing numerous periphery crypts.<br>Among the pigment related features are the crypts and the pigment spots. The<br>crypts are the areas when the iris is relatively thin. They have very dark colour due to<br>dark colour of the posterior layer. They appear near the collavette, or on the periphery<br>of the iris. They look like sharply demarcated excavations. The pigment spots are<br>random concentrations of pigment cells in the visible surface of the iris and generally<br>appear in the ciliary area. They are known as moles and freckles with nearly black<br>colour (Ales et al2000).<br>Features controlling the size of the pupil are radial and concentric furrows.<br>39<br>They are called contraction furrows and control the size of the pupil. Extending radially,<br>in relation to the center of the pupil are radial furrows that are increased in the anterior<br>layer of the iris from which loose tissue may bulge outward and this is what permits iris<br>to change the size of the pupil (Ales et al2000). The concentric furrows are generally<br>circular and concentric with the pupil. They typically appear in the ciliary area, near the<br>periphery of the iris and permit to bulge the loose tissue outward in different direction<br>than the radial furrows. The collarete is a sinuous line which forms an elevated ridge<br>running parallel with the margin of the pupil. The collarette is the thickest part of the<br>human iris.<br>1.3.1.5 The Pupil<br>The pupil is the circular aperture of the iris, a contractile diaphragm which<br>helps to regulate the amount of light entering the eye. It aids to increase the depth of<br>focus for near vision (Ravindran, 2001). When maximally dilated, the diameter of the<br>human pupil may be less than lmm; when maximally contracted, it may be more than<br>9mm. The fibres of the sphincter and dilator muscles of the iris are intimately<br>connected with the iris stroma and areresponsible for the constriction of the pupil even<br>after sphincterotomy or sector iridectomy. Normally, the pupil is placed slightly<br>nasally and inferiorly. The normal diameter of thepupil is about 2mm to 4mm. The<br>size of the pupil varies with age. The pupilary size and reactivityare a function of<br>parasympathetic and sympathetic tone (Ravindran,2001).<br>40<br>A number of physical and physiological factors also affect the size of the<br>normal pupil including light intensity, light adaptation, refractive status, emotional<br>factors and age. The pupil tend to be larger in the myopic eye and also in youth and<br>adolescence but then become steadily smaller until about age 60.<br>The pupil during sleep is contracted rather than dilated. Two mechanisms are<br>responsible for the miosis of the pupil during sleep: diminution of tonus of the<br>sympathetically innervated dilator muscle; and diminution of inhibitory impulses from<br>the contex to the constrictor centre. Loss of this cortical inhibition during sleep allows<br>the sub cortical oculomotor centre to act freely.<br>About one fifth of the normal population has a difference of 0.4 mm in papillary<br>diameter between the two eyes. While the subject is alert, the pupil dilates. But when<br>tired, the pupils gradually become smaller.<br>The Anatomical path ways controlling the papillary reaction is given below:<br>When light is shown in one eye, there is ipsilateral constriction of the pupil (direct<br>light response). At the same time, there is constriction of the contralateral<br>pupil(consensual light response). The neural pathway for this reflex from a three neuron<br>are: the afferent neurons from retinal ganglion cells to the pretectal area; an intercalated<br>neuron from the pretectal complex to the parasympathetic nucleus;parasympathetic<br>outflow with the oculomotor nerve to the ciliary ganglion and from there to pupillary<br>sphincter.<br>41<br>The afferent limb of the pupillary light reflex begins in the retina with axons from<br>retinal ganglion cells. The fibres destined for mid brain connections separate from the<br>optic tract and enter the midbrain via the brachium of the superior colliculus to reach<br>the pretectal region. The intercalated neurons from the pretectal nuclei hemidecussate<br>through the posterior commssure and synapse in the Edinger-Westphal nucleus. As a<br>result of this mid brain decussation the Edingerwestphal nucleus receives equal drive<br>from both optic nerves. The efferent fibres from the Edinger-westphal are carried in the<br>superficial layer of the oculomotor nerve and eventually ends in its inferior division. It<br>then passes through the superior orbital fissure and synapses in the ciliary ganglion.<br>Post ganglion fibres which enter the lobe near the optic nerve to supply the ciliary<br>muscles are composed of smooth muscle fibres and have acetylcholine receptors. There<br>is a disparity in the number of cells which innervate the irissphincter and those which<br>innervate the ciliary muscle for every axon which leaves the ciliary ganglion to supply<br>the light responses, thirty axons serve the near response (Ravindran,2001). The latent<br>period of light reaction of the pupil is 0.2 seconds in the bright light and up to 0.5<br>seconds in dim light.<br>Dilatation of pupil: Dilatation of the pupil is mediated mainly through the<br>sympathetic nervous system producing contraction of the dilator muscle fibres of the<br>iris. The efferent pathway is more complicated than that of light reflex. Two neural<br>mechanisms are involved one active and the other passive. The active component<br>42<br>results from contraction of the radially arranged fibres of the dilator muscle via the<br>cervical sympathetic pathway. The passive component results from relaxation of<br>sphincter muscles caused by inhibition of visceral occulomotor nuclei. In terms of the<br>sympathetic pathway, the dilator fibres pass from the sympathetic centres of the<br>hypothalamus downwards with partial decussation in the midbrain. It then passes<br>through the medulla oblongata into the lateral columns of the cord. The descending<br>fibres, considered to be the first order preganglionic neuron synapses in the<br>intermediolateral portion of the spiral cord known as the cilio-spinal centre of budge.<br>Next, second order pregangloinic fibres exit the cord primary with the first ventral<br>thoracic root. The fibres then enter the paravertebral sympathetic chain which is closely<br>related to the pleura of the apex of thelung. Then they ascend up without synapsing<br>through the inferior and middle cervical ganglion to terminate in the superior cervical<br>ganglion.<br>1.3.1.6 Lens<br>The crystalline lens is located just behind the iris. The purpose is to focus light<br>onto the retina (Umit 2003)). The lens in the human eye is avascular even at birth and has<br>no innervation. Molecular make up is unique and it has 2/3 water and 1/3 protein. The<br>percentage of water decreases with aging. It has high Refractive index (RI) because of<br>high protein content and the high RI helps to focus light. Lens does not shed cells and so<br>increases in weight throughout life (Ravindran, 2001).The lens is encased in a capsular43<br>like bag and suspended within the -eye by tiny “guy wires” called zonules from ciliary<br>body which are inserted into the equatorial zone and gives rise to epithelial cells that form<br>long fibres reaching anterior posterior poles of lens. With further cell division, the lens<br>fibres are pushed to centre and from necleus. Other cells in the outer region form the<br>cortex surrounded by acellular capsule.<br>In young people, the lens changes shape to adjust for close or distance vision. This<br>is called accommodation, but with age the lens gradually hardens, diminishing the ability<br>to accommodate.<br>Accommodation is a procedure that changes the focusing distance of the lens. The<br>lens thickens, increasing its ability to focus at near objects. A young person’s ability – to<br>accommodate allows him or her to see clearly far away and up close. At about the age of<br>40, the lens becomes less flexible and accommodation is gradually lost, making closerange<br>work increasingly difficult. This is known as presbyopia (Umit,2003).<br>As noted earlier, the lens continue to grow throughout life. The thickness of human<br>lens increases by 0.02 mm each year. Antero-posterior diameter of lens is about 3.5 to 5<br>mm and equatorial diameter ranges from 6.5- 9 mm. The anterior surface of the lens is<br>more curved than posterior surface. The radius of curvature anteriorly is 8 – 14mm and<br>radius of curvature posteriorly is 4.5 to 7.5 mm. the refractive power of lens depends<br>on the curvature of anterior and posterior surface and RI of lens material. The average<br>RI of lens is 1.420 (Ravindran 2001). <br></p>