INTRODUCTION TO OPHTHALMOLOGY.pdf
- 1. Anatomy and physiology the eye KIBET M.P BSc Ophthalmology
- 2. Introduction Vision is extremely important to human survival. More than half the sensory receptors in the human body are located in the eyes and a large part of the cerebral cortex is devoted to processing visual information. In this session we will be dealing with: 1)The accessory structures of the eye. 2)the eyeball itself 3)Image Formation
- 3. Accessory Structures Of the eye What are the accessory structures of the eye? The accessory structures of the eye include 1)The eyelids 2)Eyelashes 3)Eyebrows 4)The lacrimal apparatus 5)Extrinsic eye muscles
- 4. Contn..
- 5. Contn….. EYELIDS – The upper and the lower eyelids, or palpebrae ,shade the eyes during sleep, protect the eyes from excessive light and foreign objects and spread lubricating secretions over the eyeballs. EYELASHES ANS EYEBROWS – The eyelashes, which protect from the border of each eyelid, and the eyebrows, which arch transversely above the upper eyelids, help protect the eyeballs from foreign objects, perspiration, and the direct rays of the sun.
- 6. Contn… LACRIMAL APPARATUS – Each eye consists of a)1 lacrimal gland b) 2 lacrimal canaliculi c)1 lacrimal sac d)1 nasolacrimal duct The lacrimal glands are exocrine glands situated in the frontal bones on the lateral aspect of each eye just behind the supraorbital margin. Each gland is approximately the size and shape of an almond and is composed of secretory epithelial cells. The glands secrete tears composed of water mineral salts, antibodies and lysozymes, a bacterial enzyme.
- 7. Contn……. EXTRINSIC EYE MUSCLES – Six extrinsic eye muscles move each eye: 1)The superior rectus 1)superior oblique 2)The inferior rectus 2)Inferior oblique 3)Lateral rectus 4)medial rectus They are supplied by cranial nerves III,IV and VI. The extrinsic eye muscles move the eyeball laterally,medially,superiorly and inferiorly
- 9. Contn..
- 10. ANATOMY OF THE EYEBALL The adult eyeball measures about 2.5cm in diameter. Of its total surface area only 1/6th is exposed, the remainder is recessed and protected by the orbit, into which it fits. Anatomically, the wall of the eyeball consist of three layers: !) FIBROUS TUNIC- outermost protective layer -Sclera and the cornea 2) VASCULAR TUNIC-middle nutritive layer-the choroid, ciliary body and iris 3) NERVOUS TUNIC-innermost most layer-the retina
- 11. Contn..
- 12. Contn…
- 13. FIBROUS TUNIC The fibrous tunic is the superficial coat of the eyeball. It consists of the anterior cornea and the posterior sclera. CORNEA- It is a transparent coat the covers the colored iris. Because it is curved ,the cornea helps to focus light onto the retina 1)Consists of nonkeratinized stratified squamous epithelium in its outer surface. 2)The middle coat of the cornea consists of collagen fibres and fibroblasts. 3)The inner surface is simple squamous epithelium
- 14. Contn….. SCLERA- (scler=hard), the “white” of the eye is a layer of dense connective tissue made up mostly of collagen fibres and fibroblasts. The sclera covers the entire eyeball except the cornea . Function-It gives shape to the eyeball, makes it more rigid, and protects its inner parts. At the junction of the sclera and the cornea is an opening known as the scleral venous sinus (canal of Schlemm).A fluid called aqueous humor drains into this sinus.
- 15. VASCULAR TUNIC The vascular tunic or uvea is the middle layer of the eyeball. It is composed of three parts:-the choroid, ciliary body and iris. CHOROID-The highly vascularized choroid which is the posterior portion of the vascular tunic, lines most of the internal surface of the sclera. Its numerous blood vessels provide nutrients to the posterior surface of the retina. It contains melanocytes that produce the pigment melanin, which causes this layer appear dark brown in color. Melanin in the choroid absorbs stray light rays ,which prevents reflection and scattering of light within the eyeball. As a result the image cast on the retina by the cornea and the lens remains sharp and clear.
- 16. Contn… CILIARY BODY-In the anterior portion of the vascular tunic, the choroid becomes the ciliary body. The ciliary body appears dark brown in color because it contains melanin-producing melanocytes. The ciliary body consists of ciliary processes and ciliary muscles. The ciliary processes are protrusions or folds on the internal surface of the ciliary body. They contain blood capillaries that secrete aqueous humor. Extending from the ciliary process are zonular fibers (suspensory ligaments)that attach to the lens. The ciliary muscles is a circular band of smooth muscle. Contraction or relaxation of the ciliary muscle changes the tightness of the zonular fibers, which alters the shape of the lens, adapting it for far or near vision.
- 17. Contn…. IRIS-The colored portion of the eyeball is shaped like a flattened donut.It is suspended between the cornea and the lens and is attached at its outer margin to the ciliary processes. It consists of melanocytes and circular and radial smooth muscle fibers. The amount of melanin in the iris determines the eye color.(black ,blue, green) Principle function of the iris is to regulate the amount of light entering the eyeball through the pupil ,the hole in the centre of the iris.
- 18. Contn…. When bright light stimulates the eye, parasympathetic fibers of the oculomotor(III) nerve stimulate the circular muscles (sphincter pupillae) of the iris to contract , causing a decrease in the size of the pupil (constriction).In dim light ,sympathetic neurons stimulate the radial muscles (dilator pupillae) of the iris to contract. causing an increase in the pupils size(dilation).
- 19. RETINA The third and inner coat of the eyeball, the retina, lines the posterior ,three quarters of the eyeball and is the beginning of the visual pathway. The optic disc is the site where the optic(II) nerve exits the eyeball.Bundled together with the optic nerve are the central retinal artery ,a branch of the ophthalmic artery and the central retinal vein . The retina consists of a pigmented layer and a neural layer The pigmented layer is a sheet of melanin-containing epithelial cells located between the choroid and the neural part of the retina. Three distinct layers of retinal neurons- the photoreceptor layer, the bipolar cell layer and the ganglion cell layer.
- 20. Contn…
- 21. Contn… Photoreceptor cells are specialized cells that begin the process by which light rays are ultimately converted to nerve impulses. The two types of photoreceptors are :rods and cones. Each retina has about 6 million cones and 120 million rods. Rods allow us to see in dim light ,such as moonlight. Because rods do not provide color vision . Brighter light stimulate cones ,which produce color vision . Three types of cones are present in the retina: (1)Blue cones ,which are sensitive to blue light. (2)Green cones, which are sensitive to green light. (3)Red cones, which are sensitive to red light.
- 22. Rods and Cones
- 23. Contn.... OPTIC DISC-It is within the retina. It is the site where the optic nerve exits the eyeball and is also called the blind spot. It is called the blind spot because there are no rods and cones and due to the absence of these ,we cannot see images of the objects. MACULA LUTEA-It is also called the yellow spot. It is small flat spot in the exact center of the posterior portion of the retina. Fovea centralis-It is a small depression in the center of the macula lutea ,containing only cones. It is the area of highest visual acuity or resolution(sharpness of vision)
- 25. Contn…. LENS- Behind the pupil and iris, within the cavity of the eyeball,is the lens. Proteins called crystallines are arranged like the layers of an onion in the cells of the lens. The lens helps to focus images on the retina to facilitate clear vision.
- 26. Interior Of The Eyeball The lens divide the interior of the eyeball into two cavities, the anterior cavity and vitreous chamber. Anterior cavity –the space anterior to the lens,consists of two chambers. The anterior chamber lies between cornea and iris. The posterior chamber lies behind the iris and filled with aqueous humor. Vitreous chamber large posterior cavity of the eyeball It lies between the lens and the retina. Within the vitreous chamber is the vitreous body ,a transparent jelly like substance that hold the retina.
- 28. Intraocular Pressure The pressure in the eye is called the intraocular pressure. It is produced mainly by the aqueous humor and partly by the vitreous humor . Normally the pressure is about 16 mm of Hg(millimeters of mercury). The intraocular pressure maintains the shape of the eyeball and prevents it from collapsing.
- 30. IMAGE FORMATION There are three processes for an image formation on the retina. (a)The refraction (b)Accommodation (c) Constriction Refraction of the light rays- As the light rays enter the eye,they are reflected at anterior and posterior surfaces of the cornea. Both surfaces of the lens of the eye further refract the light rays so they come into exact focus of the retina.
- 31. Contn… Images formed on the retina are inverted . 75%of total refraction of light occurs at the cornea.The lens provide the remaining 25%of focusing power.
- 32. Accommodation and the near point of vision. The lens of the eye is convex on both its anterior and posterior surfaces, so that the lens will refract incoming light rays towards each other , so that they eventually intersect. Its focusing power increases as its curvature becomes greater. When the eye is focusing on a close object the lens become more curved , causing greater refraction of the light rays. This increase in curvature of the lens for near vision called accommodation. Near point of vision is the maximum distance from the eye that an object can be clearly focused with maximum accommodation. The distance is about 10cm in young adult.
- 33. Contn…
- 34. Constriction of the pupil Constriction of the pupil is the narrowing of the diameter of the hole through which light enters the eye due to the contraction of the circular muscles of the iris.
- 35. References… Gerard J Tortora andBryan Derrickson Principles of anatomy and physiology,13th edition ,page no 642-656.