Title: Principles of modern biology
Identifier: cu31924001154115 (find matches)
Year: 1964 (1960s)
Authors: Marsland, Douglas, 1899-
Subjects: Biology
Publisher: New York, Holt, Rinehart and Winston
Contributing Library: Cornell University Library
Digitizing Sponsor: MSN

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Responses of Higher Animals: The Receptors - 421 tremely sensitive to chemical stimulation. The taste buds of the oral cavity (Fig. 23-3) and the olfactory receptors of the nasal pas- sages (Fig. 23-4) are representative of chemo- receptors generally, and these structures were discussed previously (Chap. 16). EPITHELIUM
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TASTE BUD Fig. 23-3. A taste bud from the tongue. Receptors of Light, Especially the Eye. Light-receptive cells always contain one or more photosensitive substances, which un- dergo chemical reaction when energized by the absorption of the proper kind of light; and most photochemical substances are highly selective as to the wavelength of the light absorbed. The simplest photoreceptors are the red- pigmented "eyespots" of many flagellates and unicellular algae (p. 167). Eyespots are sensi- tive to changes in the quality, intensity, and direction of light, but are not capable of recording images of the form and pattern of surrounding objects. A true eye, in contrast, is a complex organ, which possesses a multi- cellular photosensory surface, together with a focusing mechanism, which projects a well- defined image upon the surface. Invertebrate animals have developed quite a variety of eyes, among which the simple and compound eyes of insects and other Arthropoda are most frequently encountered. Essentially the simple eye of an insect con- sists of a very small lens that focuses upon a group of light-sensitive nerve endings. The lens is anchored in a fixed position and can- not vary as to curvature. Consequently simple eyes possess no focusing capacity, and they do not form very clear images. The com- pound eye of the arthropod is more useful. The compound eye, being an aggregate of many simple eyes, constructs a mosaic of the light and dark regions of the surrounding locale, and is quite effective in detecting moving objects in the environment. A few invertebrates, such as the squid and octopus, possess eyes that can vary the focus for near and distant objects, and these eyes form fairly accurate images of the surroundings. The eye of man and other mammals is in many ways like a modern camera, equipped with color film. The human eye (Fig. 23-5) possesses a high-speed lens with an accurately variable focusing capacity; an efficient dia- phragm, called the iris, which controls the diameter of the pupillary opening; and a color-sensitive retina (Fig. 23-5). The eye is sturdily built, being protected at the sides and back by the tough opaque sclerotic coat, and in front by the strong transparent cor- nea. The black-pigmented choroid coat, which intervenes between the retina and the sclera, functions like the black paint on the inner walls of a camera. The choroid coat

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