Photoreception 



431 



Pecten; the fibers from the proximal sense cells exhibited a pattern similar to 

 the Limulus pattern (Fig. 133). The fibers from the distal retina exhibited 

 "off" bursts only. The difference in optic nerve discharge must be attributed 

 to the cells of the distal retina. The cells of either area are capable of being 

 excited only by cessation of illumination or— and this is the more plausible 

 view— they serve in the capacity of ganglion cells and receive their excitation 

 from the distal sense cell layer. Hartline and Wilska/'' using microelectrodes, 

 found that the fibers in the anterior portion of the optic lobe of Liviulus, pre- 

 sumably coming directly from the retina, exhibited the familiar discharge 

 pattern (Fig. 133). The fibers in the posterior portion of the lobe exhibited 

 "off" bursts on cessation of illumination, similar to that shown in the verte- 

 brate picture. These neurones are never active during illumination, and the 

 frequency and duration of the "off" burst depend on the intensity and dura- 

 tion of the preceding illumination. The "off" burst discharge is inhibited by 

 re-illumination, just as it is in the vertebrate eye^^ and in Pecten. In the 

 vertebrate eye, each third order neurone, from which the frequency patterns 



J2lJ 



400 



500 

 WAVE-LENGTH 



600 



700 



Fig. 135. Visibility curve for a single visual sense cell QLimulus^. Visibility of each 

 spectral band is the reciprocal of the relative intensity necessary to produce a specified 

 number of nerve impulses. From Graham and Hartline.*" 



(Fig. 134) were recorded, is separated from the primary neurone, the sense 

 cell, by at least one interspersed neurone, offering considerable complexity of 

 interrelations (see Fig. 110). The complexities of the vertebrate frequency 

 patterns are attributable to this retinal ganglionic layer. 



Measurement of Visual Functions by Means of the Optic Nerve Dis- 

 charge: SPECTRAL SENSITIVITY. Employing different wave length bands of 

 stimulating light, Graham and Hartline'*" measured the intensity necessary to 

 produce a constant number of nerve spike potentials in an optic nerve fiber. 

 The reciprocal of the intensity may be taken as a measure of sensitivity. The 

 resultant plot of visibility against wave length gives an action spectrum 

 (Fig. 135) that resembles those obtained by other techniques and from other 

 animals and, indeed, is similar to the spectral absorption curve of rhodopsin. 



THE RECIPROCITY LAW. Rccords of the result of illumination of the Limulus 

 eye with brief flashes of light are shown in Figure 136.*"' The technique 

 used in obtaining the data of Figure 136 is identical with the technique 



