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7-1 CHAPTER 7 VISION Hotoreception Is a Particularly Important Sense for Most Primates, Including Man, but It Is Not Unique to P

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7-1 CHAPTER 7 VISION Hotoreception Is a Particularly Important Sense for Most Primates, Including Man, but It Is Not Unique to P CHAPTER 7 VISION hotoreception is a particularly from the two eyes. The end-products of the important sense for most primates, activity of the visual system are sensations Pincluding man, but it is not unique to that represent the object and its primates or even mammals. Even mollusks surroundings. These sensations can be used have photoreceptors, but one may question to guide our immediate behavior, or they can whether they possess vision in the same be stored for future reference. Visual sensa- sense as we have it. Most objects reflect tions contain a great deal of information, and light, and because light travels at high speed, understanding these complex phenomena is it is possible to nearly instantly assess their no simple matter. The best place to begin shape, size, position, speed, and direction of the study of vision is at the eye itself. movement. The light rays emanating from Figure 7-1 shows a cross section through an object are gathered and focused onto an the human eye. It consists of two fluid-filled chambers separated by a transparent structure, the lens. Nearly the entire eye is covered with a tough, fibrous coating called the sclera that is modified anteriorly to form the transparent cornea. The human cornea is about 12 mm in diameter, about 0.5 mm thick in the center and 0.75 to 1 mm thick on the edge, and it is made of the same collagenous connective tissue substance as is the sclera, but the fibers of the cornea are oriented in parallel arrays that let light pass through with minimal scatter, whereas fibers of the sclera are random and light rays are scattered when passing through. The result is that light passes easily through the cornea, but not through the sclera. Lining the inside Figure 7-1. A section through the human eye illustrating the major structures. (Walls GL: The of the posterior two-thirds of the sclera are Vertebrate Eye and its Adaptive Radiations. New two membranes: the choroid, a pigment York, Hafner, 1967) layer containing the vascular supply for the eyeball as well as mechanisms for array of photoreceptors. Activities maintaining the integrity of the generated in the different photoreceptors by photoreceptors, and the retina that contains the light interact to produce a two- the photoreceptors and other neural elements dimensional representation of the object essential to our visual process. The fine which is transmitted to the brain. The brain structure of the retina will be considered in then reconstructs a three-dimensional detail later. representation using information received The human lens is about 11 mm in 7-1 diameter and 3.5 mm thick at its thickest In any case, light behaves as if it has point, and it is suspended in place by zonule characteristic wavelengths that are fibers that attach to the ciliary process measurable. The total spectrum of anterior to the retina. A set of smooth wavelengths of electromagnetic radiation is muscle fibers, the ciliary muscle, lie between quite broad, ranging from 1 nanometer (10-9 the ciliary process and sclera. Just anterior to m, abbreviated nm) to over 104 m, yet we see the lens is a pigmented structure called the only a small part of that spectrum (400-700 iris, that is like the diaphragm on some nm; Fig. 7-2). Some other animals have a cameras in that it has a hole in the center of broader sensitivity than man, being able to variable aperture, the pupil. The pupil is see in the ultraviolet region (less than 400 surrounded by two sets of muscles, one that nm), but not even these animals see the encircles the aperture, the sphincter pupillae, whole light spectrum, 380-760 nm. and one that runs radially out from it, the Light travels in straight lines from its dilator pupillae. source unless it passes through a high The anterior chamber of the eye is filled intensity electro-magnetic field or from a with aqueous humor, a watery fluid of low medium of one refractive index into a protein content that is formed from plasma. medium of another. In the first case, the The vitreous cavity contains a gelatinous light is bent along a curved path. We will substance, the vitreous or vitreous humor. not consider this further because it plays no In many people the vitreous is not role in the majority of normal human vision. completely clear, but contains particulate matter that is not transparent. This material may be stationery or may float around, producing "spots before the eyes,” the floating variety being called “floaters.” Figure 7-2. The spectrum of light visible to the human eye. At this point, we should spend just a moment reviewing the properties of light, which is a form of electromagnetic radiation. When light passes from one medium into Light is radiant energy emitted by matter another of a different refractive index, the di- when it is at high temperature, when it is rection of travel of the light beam is changed electrically excited, and when it is at the interface. This is the basis for the undergoing certain chemical or physical phenomenon of refraction, that we will processes. Early on, physicists attempted to discuss in detail in the next section. account for the properties of light as a Psychologically, light has three aspects stream of particles emitted by a luminous we need to consider. It has hue, a quality object. Later, they attempted to account for correlated with wavelength and analogous them in terms of waves, but light has properties both of waves and of particles1. QED, The Strange Theory of Light and Matter, by Richard Feynman (Princeton 1 Actually, light is an exceedingly University Press, Princeton, NJ, 1985) will complicated and interesting phenomenon. convince the reader that Newton only Even casual reading of the popular book, scratched the surface. 7-2 with pitch in hearing. It has saturation, a quality related to homogeneity or purity of wavelength and comparable to timbre or freedom from noise in audition. The color of a light is a function both of its hue and saturation. Finally it has brightness, a quantitative measure of the intensity of the light and somewhat analogous to loudness in hearing. The intensity of light is dependent upon the amplitude of its waves or alternatively the number of photons (particles of light). Optical Functions of the Eye Figure 7-3. Convergence of light by a convex lens. The process of light reception and A. Light from an object far away is brought to transmission has a number of parts. First, we focus at the focal point of the lens. B. Light from a will consider the optical functions of the eye: nearer object is brought to focus at a point farther accommodation2, the process of forming a way than the focal point of the lens. C. The radius focused image, and control of the amount of of curvature of the lens must be decreased to luminous flux at the retina. bring the light to focus at the focal point of the lens in A. Accommodation. A convex lens like that in the eye has the property of bending penetrating rays of light toward its axis (Fig. between them, the rays are bent as shown in 7-3A). This property results from the fact Figure 7-3A, where parallel rays of light that light travels more slowly in some emanating from an object far away are bent materials, e.g., water and glass, than in air as they pass through the convex lens in such (and more slowly in air than in a vacuum). a way that they converge onto a point in The amount of slowing in a material is space. Where they converge depends upon specified by its refractive index, which is the strength of the lens which is specified by the ratio of speed of light in a vacuum (the its focal length. The focal length of a lens is speed of light in air is slightly less, but very the distance from the center of the lens to the close) to the speed of light in the material point of focus of light from a distant object. (refractive index = speed in vacuum/speed in Focal length is a function of the refractive material). If light passes from one medium index of the lens material and the radius of to another at an angle to the interface curvature of the lens. The larger the radius of curvature, i.e., the less spherical the lens, the longer the focal length is. 2 It is unfortunate that the same term is A convenient way to specify the focal used for the process of forming a focused length or strength of a lens is in terms of the image and for changes in critical firing level diopter, the reciprocal of the focal length of that occur with small prolonged the lens expressed in meters. As you know, hypopolarizations. The only way to there are two kinds of lenses, convex lenses distinguish these usages is by the context in (converging lenses), like that shown in which they occur. 7-3 Figure 7-3, and concave lenses (diverging focusing objects at a distance. Under these lenses). With a convex lens the image of an conditions, light from near objects is brought object is formed on the side of the lens to focus behind the retina but is unfocused opposite from the object. With a concave on the retina, and the images of them are lens no actual image is formed, but physicists blurred. Because of the way in which the identify a virtual image that lies on the same ciliary muscle is attached (see Fig.
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