178 ADAPTATIONS TO DIURNAL ACTIVITY 



Where the cones are slender, hence numerous per unit of retinal 

 area, their nuclei pile up in several layers. This is true in lizards and 

 particularly in birds; and in all cases, in the pure-cone spots in duplex 

 retinae referred to above and treated at length in the next section. The 

 snakes are quite conspicuous, among pure-cone forms, for having single 

 outer nuclear layers — the reason being that the cones are generally fatter 

 than their own nuclei (Fig. 68) , since only a few snakes (e.g., Dryoph'ts, 

 Malpolon, Sepedon) have taken advantage of their diurnality to obtain 

 high visual acuity by slenderizing their cones. 



Though the outer nuclear layer tends to be thin, the inner nuclear 

 and ganglion layers tend to be thick in diurnal animals. This is an 

 expression of the reduction of summation (see pp. 47, 67) , of the increase 

 in the number of neurons per number of visual cells, for the preservation 

 of the high resolving power which the multiplication and slenderization 

 of the cones tends to produce. A diurnal retina can thus often be dis- 

 tinguished at a glance from a nocturnal one, for in the former the inner 

 nuclear layer is usually thicker than the outer, this situation being re- 

 versed in the nocturnal retina (Fig. 72). A considerable portion of the 

 characteristic thickening of the inner nuclear layer of diurnal retinae is 

 due to the greatly increased numbers of horizontal and amacrine cell- 

 bodies ; for, as diurnaUty is adopted and perfected by a vertebrate group, 

 these integrative cells are multiplied even faster than the straightforward 

 conductive ones (bipolars and ganglion cells) and may, as in birds, 

 come to outnumber the latter. Though it would seem that ganglion: 

 bipolar: visual-cell ratios would take up and finish the job of fixing 

 visual acuity where the size and quality of the image and the concen- 

 tration of cones leave off, the 'switchboard' effects of the horizontally in- 

 tegrative neurons have a mysterious and very considerable concern with 

 the sharpening of the mental picture, probably by manipulating contrast 

 phenomena. This particular specialization makes the bird retina the 

 thickest of all— though it should not be thought that the variation of 

 retinal thickness from group to group of animals is a very great one, 

 for it is surprisingly slight. 



Minimization of the Physiological Scotoma — The 'blind spot' of 

 the retina may, in thoroughgoing diurnal eyes, be called upon to modify 

 itself in sympathy with the efforts toward improving detail- and form- 

 perception. The insensitive head of the optic nerve, called the 'disc' 

 from its usual appearance when seen with the ophthalmoscope, causes 



