SENSORY ORGANS AND RECEPTION 



333 



eye responding to onset of illumination. It seems that only the eccentric 

 cell of each ommatidium is electrically active and gives rise to conducted 

 spikes in the optic nerve of Limulus. The function of the retinular cells and 

 their axons still awaits explanation (150, 164, 165). 



Other eyes have on off systems, the simplest of which is found in the 

 scallop (Pecteri). In the mantle eye of this animal there are two retinal 

 layers: one, internal, discharging to onset of light; the other, external, 

 to cessation of illumination or reduction of intensity (61). The highly 

 complex vertebrate eye contains on off systems, some fibres responding 

 with an on burst, continued discharge during illumination and an off 



3Qsec 



■j v w ~*^—m 



1 mm 



*ar~w^m—im 



2mm 



40 mm 



W""S^*P""W 



5 mm 



1 hour 



Fig. 8.22. Dark-adaptation of a Single Visual Receptor 

 in the Eye of Limulus 



The records show action potentials of a single optic nerve fibre in response to illumi- 

 nating the eye with a test flash of light. The test flashes were applied at various times in 

 the dark (indicated in the records) following a period of light-adaptation. Flash duration 

 (8 msec) shown as interruption of white band. Time scale, 0-2 sec. (From Hartline and 

 McDonald (62).) 



burst ; others respond with on off bursts only ; still others show only off 

 effects. 



The magnitude of the retinal action potential depends on the intensity 

 and, for short flashes, the duration of the light stimulus. Similarly, the 

 characteristics of nervous discharge (frequency, number of impulses) in 

 the optic nerve depend on the conditions of illumination. These relation- 

 ships between stimulus and response have been used extensively for measur- 

 ing certain visual functions, namely spectral sensitivity, dark-adaptation, 

 reciprocal relationship between intensity and duration of illumination, 

 etc. 



Over a wide range the magnitude of the retinal action potential is related 

 linearly to the logarithm of the light intensity, but the curves tend to 

 become sigmoid at low and high intensities. 



The lateral eye of Limulus has been used extensively in studies of photo- 



