. Biology; the story of living things. 360 THE MAINTENANCE OF THE INDIVIDUAL l)osterior part of the retina lies a region known as tlie yellow spot, or macula lutea. The central pitlike portion of the macula lutea, where cones predominate, almost to the exclusion of rods, is designated as the fovea centralis, since it is here that the keenest vision occurs. The retina is thinner at this point and the black pigment of the outer layer shows through from behind, making it dark purple in color, due to a layer of cells next to the choroid coat. The retina acts as the sensitized plate in a camera, fo
. Biology; the story of living things. 360 THE MAINTENANCE OF THE INDIVIDUAL l)osterior part of the retina lies a region known as tlie yellow spot, or macula lutea. The central pitlike portion of the macula lutea, where cones predominate, almost to the exclusion of rods, is designated as the fovea centralis, since it is here that the keenest vision occurs. The retina is thinner at this point and the black pigment of the outer layer shows through from behind, making it dark purple in color, due to a layer of cells next to the choroid coat. The retina acts as the sensitized plate in a camera, for outar Surface of retivict. *- pigment layer ....l-oct ...Cone- outar nuclear layer (outer , I crarjular *^layar inner- groiT7ixlaTr layer 1 ganglionic. jlell^layer fibens- of optic nerve on it are received the impressions of light and shade and color which are transformed and sent to the brain resulting in sensations of sight. Like the camera, the eye has a lens formed of transparent elastic material, a circumstance permitting a change of its form and, in consequence, a change of focus upon the retina. By means of this change in form, or accom- modation, both near and distant objects may be seen. In fishes, unlike mammals, accommodation is accomplished by shifting the position of the lens, as in a camera, rather than by changing its shape. In front of the lens is a small cavity, divided by the iris into inner surface of" retina. Detail of retina showing rods and cones. two chambers that communicate through the pupil, filled with a watery fluid, the aqueous humor, while behind it is the main cavity of the eye, filled with a transparent, almost jellylike, vitreous humor. The lens lies directly behind the iris and is attached to the choroid coat by means of delicate ligaments and by pressure of the two liquid media. In order to function properly, the surface of the eye must be kept moist, and various glands are located in the cavernous orbit of the eye and along the edges o
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