398 Comparative Animal Physiology 



(2) A marked increase in the relative number of cones and a decrease in 

 the absolute number of cones which contribute to any one tertiary neurone, 

 i.e., by increasing the concentration of individual nerve channels from the 

 retina to the brain. This results in an actual increase in the resolving power 

 of the retina. (3) Special areas with an especially high concentration of 

 cones and few, if any, rods, the so-called central areas or foveas. In the flat 

 central area of frogs or in the cup-shaped fovea of man and in the acutely 

 depressed fovea of birds, the concentration of individually connected cones 

 is increased at the expense of group-connected rods. This is merely a local 

 extension of the adaptation mentioned under (2), above. The structure of 

 the fovea is carried to its extreme in certain birds, where the shape of the 

 fovea, together with the difference in refractive index of vitreous body and 

 retina, spreads the image over a larger number of cones, thereby acting as a 

 localized negative lens. (4) Development of intraocular color filters which 

 absorb in the blue end of the spectrum and thereby reduce chromatic aberra- 

 tion at the retinal surface and consequently increase acuity. These filters 

 consist of (a) red or yellow oil droplets in the cones of turtles and most liz- 

 ards and birds; (1?) a yellowish lens in the lamprey, some lizards, snakes, in- 

 sectivores, in man, and in most Sciuridae— the lens is almost orange in 

 ground squirrels and prairie dogs; (c) a yellowish cornea in Amia, Esox, 

 Cyprinns, and a few other fishes; (d) a yellow fovea in man; and (e) retinal 

 capillaries in eels and most mammals (but not especially in diurnal animals). 



ADAPTATION TO NIGHT-TIME ACTIVITY. The noctumal animals have a num- 

 ber of adaptations which tend to increase the sensitivity of the eye. Some 

 of these are: (1) Greater sensitivity of the retina to dim light by an increase 

 in the number of rods and also in the number of rods connected to each ter- 

 tiary neurone, thereby permitting greater summation of sense-cell activity. 

 An increase in sensitivity by this method produces a simultaneous decrease 

 in visual acuity. This fact is largely responsible for the visual acuity differ- 

 ences between diurnal and nocturnal animals shown in Table 63. (2) Pres- 

 ence of a slit pupil instead of a round pupil, as in diurnal animals. The slit 

 pupil is able to close more tightly in bright light and thereby preserve the 

 dark adaptation of the retina. For this purpose the slit pupil can be much 

 more effective than the circular pupil of diurnal animals, and under condi- 

 tions of dim illumination it can be greatly expanded into a large circular 

 opening. This wide range of adaptability makes it an excellent nocturnal 

 adaptation despite the fact that it does not directly aid dim vision. (3) Pres- 

 ence of a tapetum lucidum, a reflective layer on the choroid. In all animals 

 the choroid consists of a highly absorbing layer which prevents internal re- 

 flection. However, in many nocturnal animals there is a highly reflective ma- 

 terial (crystalline guanine) in either the choroid or the retina, which is ca- 

 pable of reflecting light after it has passed through the sense cells so that it 

 passes through them again in the opposite direction. This doubles the sens- 

 itivity to dim light but also decreases acuity. Some of the reflected light pass- 

 es out through the pupil, and the animal is said to have "eyeshine," a phe- 

 nomenon commonly noted in cats. Eyeshine occurs in all classes of verte- 

 brates and is a special adaptation to nocturnal habits. 



ADAPTATIONS TO SPACE AND MOTION. Any objcct in visual space may have 

 a number of perceptible characteristics, such as size, shape, pattern, bright- 



