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 



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 



OLFACTORY CELL 

 (SMELL RECEPTOR) 



AFFERENT 

 NERVE FIBER 



Fig. 23-4. Olfactory cells from mucosa of the nose. 



