Photoreception 



405 



are so connected that at least certain fish, turtles, anurans (frog), and high- 

 er placentals (cat) ha\e a ph\siological basis for color \ision. These data 

 support the behax'ior evidence for the occurrence of color vision in teleosts 

 and turtles and extend its possibilty to frogs and higher placental mammals. 

 Granit found that the optic nerve fibers of the snake have only one type of 

 cone sensiti\'ity curve. Although the sensitivity cur\es for the snake are bi- 

 modal, the fact that they are all the same precludes the existence oF a phA-sio- 

 logical basis for color \ ision. This is in agreement w iih the data on beha\'ior. 



PHYSIOLOGY OF PHOTORECEPTION 



Sensory end-organs which respond to radiation displa\' great morpholog- 

 ical variation. Despite this morphological diversity there exists a common 

 pattern (Fig. 115,A) of a light-focusing de\'ice, a photosensitixe membrane 

 (retina), postretinal neurones which connect the receptor with the central 

 nervous system, and the visual center or centers within the central nervous 

 system. In keeping with this common structural pattern certain similarities in 

 the basic mechanism of the responsive system are revealed in investigations 

 of the function of light-sensiti\'e end-organs, or photoreceptors. 



RETINA 



(photosensitive, 

 membrane:) 



LIGHT 



O^ 



VISUAL CENTFR IN 



i CENTRAL NERVOUS 



_^ SYSTEM 



LIGHT FOCUSING DEVICE 



GANGLIA AND SYNAPSES IN OPTIC 

 PATHWAY 



B 



LIGHT 



i* 



RESTING POTENTIAL 

 _i 



RETINAL ACTION 

 POTENTIALS 



ELECTRICAL POLARfTY 

 OF VERTEBRATE E-YEL 



TIME. 



Fig. 115. Diagrams illustrating QA) the elements in a visual pathway, and (B) the 

 electrical polarity and retinal action potentials of the vertebrate eye. 



The physiological problems of prime interest center about two parts of 

 the optic pathway (Fig. 115, .4): (1) the photosensitive membrane and im- 

 mediate connecting neurones, i.e., the peripheral structures; and (2) the 

 central nerxous system structures which receix'e and integrate the activity 

 originating in the retina. 



The Peripheral Visual Mechanism: The Photochemical Nahire of Photo- 

 reception. Today it is an accepted fact that the initial e\'ent in the process of 

 vision is a photochemical reaction. Surprisinglv, this fact was only recently 

 established. It w^as the evidences presented by Hecht,'^^- ''•^' ''^- '^''- ''^ stemming 

 from his ingenious researches on the marine clam, Mya arenaria, which 

 firmly established the photochemical nature of vision on a factual basis. 



