THE PERCEPTION OF LIGHT 611 



So far as the rods themselves are concerned, sensitivity is further 

 increased by several expedients all of which tend to lower their threshold 

 by increasing the amoinit of rhodopsin available in a given area. Close 

 packing of the individual elements with this end in view is seen carried 

 to its greatest lengths in certain deep-sea Selachians {Efmopferus, etc.) 

 and Chimaeras, while the concentration of visual cells in a pure -rod or 

 rod-rich area centralis probably serves a similar purpose.^ To increase 

 the amount of available visual purple still further, the outer segments 

 of the rods may be lengthened to a remarkable extent as is again seen 

 in certain bathyjDelagic Teleosts {Lampanyctus, Argyropelecus, Verrier, 

 1935; Contino, 1939); the rods may be arranged in layers one above 

 the other (3 in the peripheral retina, 6 in the fovea in Bathylogus) ; 

 while these elements may become massive and thickened as occurs in 

 Amphibians and some nocturnal geckos (Fig. 433). 



The degree of summation in the retina is the principal expedient 

 employed to increase sensitivity whereby large numbers of visual 

 cells converge upon a single bipolar cell, and several bipolar cells 

 themselves converge upon a single ganglion cell and optic nerve fibre 

 so that a meagre stimulus applied to each of a large number of visual 

 receptors can be summated to produce one nerve impulse. In the 

 organization of the retina summation is a characteristic of the rods. 

 In Osterberg"s (1935) counting, there are 110,000,000 to 125,000,000 

 rods and 6,300,000 to 6,800,000 cones in the human retina associated 

 with about 1,000,000 optic nerve fibres, that is, an overall summation 

 of the order of 125 : 1. In the retinae of nocturnal types it is not 

 uncommon for many more receptors to be summated to a single 

 ganglion cell. 



An additional summation may occur more proximally in the visual pathway : 

 thus in the higher Primates there is a one-to-one relationship between the 

 terminals of the optic nerve fibres and the cells in the geniculate body, but in 

 the cat, which has a high degree of sensitivity to light, it would appear that 30 

 to 40 optic nerve terminals are related to each geniculate cell (Glees, 1941-42). 



The characteristics of a diurnal eye attuned to a high visual acuity 

 are, as we shall see,^ almost precisely the opposite of those of a nocturnal 

 eye (Fig. 747) : a forward location of the optic centre so that a large 

 retinal image is formed to allow the resolution of detail, an adequate 

 pupillary stop to eliminate aberrations, and a cone-rich retina with a 

 low summation to the ganglion cells so that the retinal image can be 

 accurately analysed. 



In the most exclusively diurnal types the retina inay contain cones only : 

 such a PURE -CONE BETiNA is found among Dipnoi in Neoceratodus ; among the 

 Polypterini in Calamoichthys ; in niost diurnal lizards ; in diurnal colubrid Calamoichthys 



1 p. 657. « p. 637. 



