Photoreception -iqj 



the lower limit of visibility. At higher light intensities the photosensitive 

 substance (rhodopsin) of the rods is almost completclv bleached, and the 

 rods are thereby non-functional; in fact they reach maximal sensitixity only 

 after a half hour or more in the dark. The cones contain a photosensitive 

 material (iodopsin) which is not so readily affected by light, and therefore 

 they are able to function at a higher intensity range. The existence of these 

 two types of sense cells gives a combination of sensitivity ranges which ac- 

 counts for the extremely efficient functioning of the human eye over an in- 

 tensity range of at least eight logarithmic units. Other vertebrates and in- 

 ^'ertebrates detect flickering light over most of this range, despite the lack of 

 such dualit) of structure, but their ability in terms of other visual func- 

 tions is probabh' not so good. 



Apart from the fact that the rods and cones in duplex retinas function in 

 different intensity ranges, they differ in another very fundamental manner. 

 The cones are of several types which are differentially sensitive to light of 

 various wave lengths, and this differential sensitivity is the basis for color 

 discrimination (see page 434). All of the rods of any given animal seem to 

 have the same spectral sensitivity and probably serve only to detect differ- 

 ences in intensity. 



Visual Adaptations of Louder Vertebrates. The structure and function of 

 the eyes of vertebrates show an astounding range of adaptation to the condi- 

 tions under which the animal lives. Consequently, rather closely related 

 species which li\'e under different conditions may have quite different vision. 

 This range of adaptation has been described in considerable detail by 

 Walls,^'''-^ and the outline of the subject presented here is largely adapted 

 from his book. 



The general types of adaptations of the vertebrate eye may be listed as 

 follows: 



ADAPTATIONS TO ACTIVITY UNDER CONDITIONS OF LOW AND/oR HIGH LIGHT 



intensity: Adaptation to Arrhythmic Activity, during Both Day and Night. 

 Animals which are equally active by day and night may not sleep at all or 

 may merely take short naps. Such animals may be said to be arrhythmic in 

 behavior and must be capable of good vision o\'er a great range of light in- 

 tensity. Some of the mechanisms which permit such vision are: (1) a con- 

 siderable development of both rods and cones; (2) pigment migration from 

 the choroid to the layer of sense cells in order to screen intense light from 

 some of the rods and cones, especially the rods (e.g., Phoxinus, a fish); (3) 

 migration of rods and cones into the pigment layer in intense light (Aweiw- 

 rus, a catfish); (4) high mobility of the pupil, especially in higher vertebrates, 

 where the positional changes of pigment and xisual cells (2 and 3 above) are 

 minimal. 



Adaptations to Day-Time Activity. Animals which are diurnal, i.e., active 

 chiefly by day, have a high degree of visual acuity, brought about in several 

 ways: (1) A larger image, which may be obtained by increasing the size of 

 the eye and the relative proportions of the lens and eyeball, especially by 

 increasing the lens-retinal distance. Man, birds, chameleons, and all diurnal 

 animals have a much greater lens-retinal distance than arrhythmic animals 

 such as cougars, dogs, or dromedaries, and \cry much greater lens-retinal dis- 

 tances than nocturnal animals such as opossums, house mice, lynx, or frogs. 



