PURE-ROD ANIMALS; SUMMATION 217 



bulky and exceed the cones in total volume as well as in actual numbers 

 (Fig. 64, p. 148) — just as in most teleosts the huge cones outweigh the 

 more numerous, tiny rods (Fig. 94, p. 237). The difference in acuity- 

 performance of bulky cones versus slender ones is obviously very great, 

 for the retinal limit of resolving power is set by the distance on centers 

 between the cones. It is not so easily apparent why nocturnal animals 

 should have slender rods and other animals not only fewer but plumper 

 ones. The slenderness of a rat's rods has not been produced for its own 

 sake. The distance between centers of adjacent rods has nothing to do 

 with the overall sensitivity of the rod population — but the number of rods 

 which can conveniently be hooked to a bipolar cell (this being pro- 

 moted by slenderness and close aggregation) has everything to do with 

 it. In amphibians and lungfishes not only the visual cells but most 

 somatic cell-types are notoriously huge. It makes an interesting specu- 

 lation: did the unknown factor which made their cells so large doom 

 the amphibians forever to low visual acuity because their cones are 

 usually bulky, and to a not particularly high sensitivity also, because 

 their rods are so big? 



The thick outer nuclear layer resulting from the slenderness of noctur- 

 nal rods (the tiger holds the record here!) is pretty well counterbalanced 

 by the thinning of all other retinal layers due to the great extent of sum- 

 mation of visual cells in bipolars, and of these in ganglion cells, for the 

 sake of sensitivity and at a tremendous sacrifice in resolving ability. Noc- 

 turnal animals, on the whole, have thinner retinae than diurnal groups, 

 and have much more slender optic nerves. It is not at all unusual for 

 several thousand rods to be summated in one optic nerve fiber. The reti- 

 nal adaptations for sensitivity, both within the visual cells themselves and 

 in their relationship to optic nerve fibers, render the receptive tissue of a 

 nocturnal animal so extraordinarily sensitive to light that it cries out for 

 protection from any light stronger than that of the moon. We go on now 

 to consider how this protection has been obtained. 



(C) The Slit Pupil 



The elementary discussion of pupil mobility in section C of Chapter 7 

 was based upon the commonest form of the aperture — the circle. There 

 are a number of departures from this primitive shape, the most wide- 

 spread one being the slit, which in land animals, at least, is most com- 

 monly vertically oriented, for which a reason is given later (see p. 428). 



