Eye Anatomy

9 Key excerpts on "Eye Anatomy"

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  eBook - ePub

  The Eye: The Physiology of Human Perception

  • Britannica Educational Publishing, Kara Rogers(Authors)
  • 2010(Publication Date)
  • Britannica Educational Publishing

    (Publisher)

  CHAPTER 1 ANATOMY OF THE EYE

  Close-up of a human eye. Shutterstock.com

  T he human eye is an amazingly complex structure that enables sight, one of the most important of the human senses. Sight underlies our ability to understand the world around us and to navigate within our environment. As we look at the world around us, our eyes are constantly taking in light, a component fundamental to the visual process. The front of the human eye contains a curved lens, through which light reflected from objects in the surrounding environment passes. The light travels deep into the eyeball, passing all the way to the back of the eye, where it converges to a point. A unique set of cells at the back of the eye receives the light, harnessing its energy by converting it into an electrical impulse, or signal, that then travels along neurons in the brain. The impulses are carried along a neuronal pathway that extends from the back of the eye all the way to the back of the brain, ultimately terminating in a region known as the visual cortex. There, the electrical signals from both eyes are processed and unified into a single image. The amount of time between the moment when light enters the eye and when a unified image is generated in the brain is near instantaneous, taking only fractions of a second.

  Scientists’ knowledge of the intricacy and the complex associations of the vital structures within the human eye expanded tremendously in the 20th and early 21st centuries. Although some of this knowledge was obtained from studies of the eyes of animals, a significant amount of information was also gained from studies of diseases of the human eye. With this knowledge came an understanding of the function of each of the eye structures. Each structure contributes in a specific way to the visual process, and collectively they underlie a broad range of visual functions, from the perception of an object’s shape, size, and colour to the perception of distance.

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  The Eye Pt IA

  • Hugh Davson(Author)
  • 2012(Publication Date)
  • Academic Press

    (Publisher)

  The combined optical and sensory apparatus is organized in the two symmetrically constructed and oriented eyeballs (Fig. 2). This arrangement allows a large section of visual space to be imaged on both retinas simultaneously, that is, binocularly. Between retinal points or areas of the two eyes, a relationship of equal subjective directional value (corresponding retinal points) exists, and images on corresponding retinal points are perceived as a single image. The coordination of stimuli received by corresponding retinal points is facilitated by juxtaposition of neurons which carry impulses from corresponding photoreceptor groups in the two eyes. This juxtaposition occurs in the optic chiasm by partial crossing over of neurons from one side to the other. The left halves of both retinas have their cortical representation in the left occipital iobe, and the right halves in the right lobe. Processing of a retinal stimulus received by the brain is a complex procedure and finally determines the extent and quality of our visual perception. For descriptive purposes, the anatomic structures that make up the human organ of sight may be grouped under four main headings: the eyeball, its protective apparatus, its motor and supporting apparatus, and the visual pathway. Within the scope of this chapter, our discussion is necessarily confined to the gross and microscopic anatomy of the human eye, orbit, and adnexa. Reference to ultrastructure is made only where it clarifies the minute anatomy. II. The Eyeball A. General Topography 1. SHAPE Although the eyeball is generally referred to as a globe, it is only approxi-mately spherical and consists of segments of two spheres placed one in front 4 RAMESH C. TRIPATHI AND BRENDA J. TRIPATHI of the other (Fig. 1). The anterior corneal portion is smaller and more curved than the posterior, has a radius of curvature of about 8 mm, and comprises one-sixth of the surface area of the eye.

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  Functional and Clinical Neuroanatomy

  A Guide for Health Care Professionals

  • Jahangir Moini, Pirouz Piran(Authors)
  • 2020(Publication Date)
  • Academic Press

    (Publisher)

  Chapter 14

  Visual system

  Abstract

  The sense of sight and our visual system is important for almost everything required for normal living. The eyes allow response to all types of stimuli, as they convert stored photochemical energy into nerve impulses. The visual brain cortex can then interpret these. Our visual system functions by allowing light to enter through the lenses of the eyes, with focusing and inversion of the image onto the retina. Finally, the visual cortex creates the images that we perceive. Recall that visual function is one of the higher cortical functions of the human brain (see Chapter 6 ). Other predominant structures of the eyes include the cornea, sclera, iris, pupil, choroid, and retina.

  Keywords

  Eyeball; Visual acuity; Convergence; Visual pigments; Blindness; Visual cortex

  The sense of sight and our visual system is important for almost everything required for normal living. The eyes allow response to all types of stimuli, as they convert stored photochemical energy into nerve impulses. The visual brain cortex can then interpret these. Our visual system functions by allowing light to enter through the lenses of the eyes, with focusing and inversion of the image onto the retina. Finally, the visual cortex creates the images that we perceive. Recall that visual function is one of the higher cortical functions of the human brain (see Chapter 6 ). Other predominant structures of the eyes include the cornea, sclera, iris, pupil, choroid, and retina.

  Anatomy of the eyeball

  Most of the eyeball lies within the eye socket, also known as the bony orbit . Just a small anterior surface is exposed. The eyeball is composed of three layers of tissues: the fibrous layer containing the sclera and cornea, the vascular layer containing the choroid, ciliary body, and iris, and the inner layer containing the retina, optic nerve, and retinal blood vessels.

  Fibrous layer

  The outermost layer of the eyeball is called the fibrous layer . It is made up of the white-colored sclera and the transparent cornea . The sclera is commonly referred to as the white of the eye

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  Veterinary Anatomy of Domestic Animals

  Textbook and Colour Atlas

  • Horst Erich König, Hans-Georg Liebich, Horst Erich König, Hans-Georg Liebich(Authors)
  • 2020(Publication Date)
  • Thieme

    (Publisher)

  17 Eye (organum visus) H.-G. Liebich, P. Sótonyi and H. E. König The eye, the organ of vision, consists of various parts, which are able to receive light stimuli from the environment, register the stimulus and convert it into an electrical signal, which is con-veyed to the brain. The receptor neurons contain photosensitive molecules that are chemically transformed by light impulses and react with neural activity of surrounding cells. The resulting sig-nal travels along neurone chains to reach cognitive centres in the brain, where the final image is formed. Vision is based on a very complex system, that involves all parts of the eye, including its accessory structures (adnexa), as well as various parts of the brain, see Chapter 15 “ Brain ” (p. 771): ● eyeball: fibrous, vascular and inner layers of the eyeball (sclera, cornea, choroid, ciliary body, iris, retina), ● adnexa: ocular muscles, eyelids, lacrimal apparatus, ● optic nerve and ● visual area of the cerebral cortex. 17.1 Eyeball (bulbus oculi) 17.1.1 Shape and size of the eyeball There is considerable variation between species in regard to the form and the size of the eyeball between species and individuals. It is roughly spherical in carnivores (20 – 24 mm in diameter), while in the horse, its width (50 mm) exceeds its height (42 mm) and length (45 mm). The eye of an ox is comparatively smaller than that of a comparably sized horse (40 – 43 cm). Corrected for body size, the cat has the largest eyeball, followed by the dog, the horse and the ox, with the pig having the smallest. The outline of the eyeball is not evenly rounded, but displays a larger curvature in its posterior part than in its anterior part, where the cornea bulges forward. The division of the two seg-ments is marked by a visible groove, the scleral groove (sulcus sclerae) ( ▶ Fig. 17.1).

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  Principles of Medical Physiology, 2/E

  • Sabyasachi Sircar(Author)
  • 2016(Publication Date)
  • Thieme

    (Publisher)

  Functional Anatomy of the Eye 113 Fig. 113.1 Anatomy of the eye. Ciliary body Cornea Sclera Retina Fovea Choroid Vitreous humor Aqueous humor Optic Nerve Optic Disk Extraocular muscle Iris Lens Canal of Schlemm Suspensory ligament Functional Anatomy of the Eye 745 The newly formed cells elongate into fibers. How- ever, the older cells cannot be cast off since they lie deep inside: They accumulate in the center as the nucleus of the lens and undergo sclerosis. The lens is surrounded by a hyaline fibrous membrane called the lens capsule, which is attached to the zonule. The zonule is split medi- ally into anterior and posterior laminae, which enfold the lens capsule and blend with it. Later- ally the zonule is attached to the inner surface of the ciliary body. The main source of energy of the lens is glucose, most of which is metabolized through Embden– Mayerhof pathway with the HMP shunt pathway and citric acid cycle making smaller contributions. In hyperglycemia, the sorbitol pathway, which is normally negligible, metabolizes large amounts of glucose. The sorbitol produced is trapped inside the lens because the lens capsule is impermeable to sorbitol. The sorbitol retains water osmotically, causing swelling of the lens and resulting in cataract formation. Uveal tract The uveal tract is made of highly vascular con- nective tissue and is innervated by sensory nerve fibers of the trigeminal as well as vasomotor fib- ers. Because of the presence of nociceptive fibers, the inflammation of the uveal tract (uveitis) is intensely painful. The uveal tract consists of three parts: the choroid, ciliary body, and the iris. The iris forms a circular diaphragm which, together with the lens, partitions the aquous chamber into the anterior and posterior chambers. It encloses a central aperture called the pupil and contains two muscles that control pupillary movements: (1) The sphincter pupillae is a circular bundle of muscle fibers running round the pupillary margin.

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  Principles of Ophthalmology

  • H. E. Hobbs(Author)
  • 2013(Publication Date)
  • Butterworth-Heinemann

    (Publisher)

  Chapter 1 THE OPTICAL MECHANISM OF THE EYE Vision is the most accurate of the senses and upon it we depend most for knowledge of our environment and for the co-ordination of our physical skills. Its importance to the higher animals and man is readily apparent when we contrast the primitive photo-sensitive cells of lowly forms of life to the perfection of the complex visual organs of primates. That the increasing independence and control over their environment which the latter enjoy owes much to the evolution of more refined visual mechanisms is clearly indicated by animal experiments which demonstrate the increasing dominance of visual over other sensations in behaviour as the evolutionary scale is ascended. This accuracy of sensation implies a similar precision in the mechanism by which it is mediated and this we find in the anatomical detail of the eye. It is apparent, too, in the physiological processes which subserve visual sensation and those which are responsible for maintaining normal ocular functions. Both of these present adaptations of general physiological processes to suit the needs of visual and ocular function: the former, notably in the rod and cone mechanism of the retina and the latter in the special provisions for maintenance of the ocular tension and the optical system of the eye. Consideration of some of the details of retinal sensation will be reserved for a later chapter; but a brief account of the optical mechanism of the eye is needed as a prelude to discussion of ocular disease. Its anomalies enter into the symptoma-tology and diagnosis of most ocular disorders and some knowledge of it is essential to their interpretation. The analogy with a camera is apt, for the eye possesses in the lens and cornea a refracting apparatus comparable with that of the camera lens.

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  External Senses

  • A.C. Damask(Author)
  • 2012(Publication Date)
  • Academic Press

    (Publisher)

  Above 680 nm is the infrared region. Thermal effects in the infrared can cause spurious random excitation of the chemical vision process, and such noise can reduce the sensitivity of the eye by lowering the signal-to-noise ratio. Between the photoreceptors, the rods and cones, and the optic nerve to the brain lie several stages of neurons which analyze the image. All of these neurons are within the retina. With other senses such analysis customarily is a function of the brain. It is therefore sometimes said that the retina of the eye evolved as an extended part of the brain. In this chapter we will omit the geometrical optics of the eye, which is classical physics and is covered in many texts. [See Bennett and Francis (1962).] We will instead trace the process, as it is now understood, from the receptor to the brain. Eye Anatomy The eye is an approximate spherical ball whose main features for vision are the cornea, iris, lens, and retina. The first three of these focus light on the retina which contains photoreceptors. These, in turn, convert light quanta into electrical impulses, which the optic nerve transmits to the brain, Fig. 6.1. The cornea is a protective cover for the lens but it also serves to focus light on the retina. The lens in vertebrates adjusts the focusing for near and far vision. By a combination of small muscles around the periphery, the lens is kept in tension, which is a shape with large radius of curvature. When these muscles relax, the lens approaches a more spherical shape, much as a balloon filled with water, Fig. 6.2. The lens, however, is built up in thin layers like an onion. The inner layers are the oldest and, with increasing age, become more separated from the blood supply. Thus, not only is optical accommodation, i.e., change in radius of curvature, de-creased with age, but also the inner parts tend to have an amber tint, called yellowing, with age. Because of this, one becomes less sensitive to blue light.

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  Biology in Physics

  Is Life Matter?

  • Konstantin Yu. Bogdanov(Author)
  • 1999(Publication Date)
  • Academic Press

    (Publisher)

  Optics of the Eye The human eye is the organ that gives us sight, which enables us to receive more information about the surrounding world than any of the other four senses. The human eye (Figure 8.1) is a wonderful instrument relying on refraction and lenses to form images. A human eye and a camera have many things in common.The retina, a membrane that lines the back of the eye, plays the role of film in a camera. It contains an array of photoreceptor cells called rods and cones that convert the light energy into electrical signals. These photoreceptor nerve cells react to the presence and intensity of light by sending nerve impulses to the brain via the optic nerve. A diaphragm, called an iris (the colored part of the eye), automatically adjusts to control the amount of light entering the eye. The hole in the iris (the pupil) is black because no light is reflected through it, and little light is reflected back from the interior of the eye. The lens focuses the light and creates a real, but inverted, image. The lens makes corresponding corrections for focusing at different distances. It is flexible and can change its shape and its focal length. This control is exercised by the ciliary muscles. The way the eye focuses light is interesting, because most of refraction that takes place is not done by the lens itself, but by the aqueous humor, a liquid on top of the lens. When light comes into the 169 170 Chapter 8. Optics of the Eye FIGURE 8.1. Structure of the eye. eye, it first is refracted by this liquid, then refracted a little more by the lens, and then a bit more by the vitreous humor, the jelly-like substance that fills the space between the lens and the retina. Johannes Kepler, a German astronomer, was the first one to guess that the image of the outer world forms on the retina. He came to this conclusion in 1604, before he discovered the main laws of the movement of heavenly bodies. His predecessors thought the eye lens was the organ sensitive to light.

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  Biochemistry of Vision

  • Hitoshi Shichi(Author)
  • 2012(Publication Date)
  • Academic Press

    (Publisher)

  Right and left sides of a vertical planar object perceived by the eye are recognized by the specific (opposite) side of the brain. For example, sup-pose you stand in front of a cardboard that has its right half colored in red and left half in blue. Visual information of the red color received by each eye (whether individually or together) is decoded by the left side of the brain and information of the blue color by the right side of the brain. A vertebrate eye is often compared to a photographic camera. As we see below, however, this analogy does not extend far. The cornea, the transpar-ent tissue in the anterior (front) part of the eye, is a permanently fixed lens cover (Fig. 2). The lens focuses an image on the retina, the film of the eye. In the primates the focusing is effected by regulating the thickness of the lens by the ciliary muscle. At rest the lens is less convex. To focus on a near object, the ciliary muscle contracts in such a way that the lens gets thicker. In the amphibians and fish, however, such a mechanism is absent and the thickness of lens remains constant. Unlike the photographic film, the retina is regenerable and reusable and contains a computer unit (neurons) that programs complex visual information for the brain. Metabolic requirements for the complex retinal function are catered to by the pigmented epithelium, the heavily pigmented unicellular layer located behind the retina. The space between the lens and the retina is filled with a viscous transparent substance called the vitreous humor or vitreous body. The vitreous body is important for the eye to maintain its shape. The outside surface of the cornea is protected by a thin film of tear. The lens and the inside surface of the cornea Light • Optic nerve Optic chiasma Fig. 1. The optic chiasma. Visual information is divided at the optic chiasma. I. Transparent Tissues 3 Fig. 2. Cross section of the vertebrate eye.

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