598 



RADIATION BIOLOGY 



illumination of the eye, the photoreceptor unit is found to be relatively 

 incapable of responding to a test flash of light (see Fig. 13-9). The rela- 

 tive refractoriness of the sense cell may persist for half a second or more 

 under some conditions. Such prolonged refractoriness may be charac- 

 teristic of the mechanism by which the end organ initiates repetitive 

 impulses in the attached nerve fiber. 



071- and Off-responses. The sea scallop, Peden irradians, was found 

 by Hartline (1938b) to have a retina with tv. o layers of sense cells. Nerve 



Fig. 13-9. Changes in latency of response as a function of recovery from the dis- 

 charge of an impulse in the Limulus photoreceptor unit. A steady light, denoted by 

 the horizontal black line in each record, has been on for some time and is responsible 

 for the discharge of impulses at infrequent intervals. An added flash of light (de- 

 noted by the black dot) elicits a response whose latency depends upon the state of 

 recovery of the sense cell. {Riggs, 1940.) 



fibers from the proximal layer respond to the onset of illumination, but 

 those from the distal one respond when the light is turned off. Hartline 

 points out, however, that the neural off-response may originate as such 

 in retinal receptor cells or may instead be developed in synaptic connec- 

 tions with secondary neurones. Unfortunately the histological evidence 

 is not clear as to the presence or absence of such synaptic connections. 



RESPONSES OF VERTEBRATE EYES 



The above discussion of invertebrate eyes was concerned with the 

 responses of primary or secondary neurones. In the typical vertebrate. 



