104 VERTEBRATE PHOTORECEPTORS 



plays in the visual function of these forms is a matter which 

 apparently has not been determined with finality.^ 



The majority of mammals apparently show some degree 

 of fundus specialization, although in some there is no evi- 

 dence of a central area (e.g. rat, mouse, sheep). When 

 present this ^area centralis' may be band-shaped as in the 

 horse, but usually it is round. In the horse the band-shaped 

 'area' is said to be temporal and eccentric. In the fox 

 and hare it occupies an inferior position. Apparently among 

 mammals no true macula with a fovea is encountered until 

 we reach the monkeys. Most monkeys, the great apes, and 

 man possess a fovea of the concaviclivate type (Figures 64 

 to 66) in contrast to the convexiclivate type of the saurop- 

 sida (Figure 63). 



We see from the above that in most vertebrate animals 

 some degree of retinal specialization occurs in the fundus 

 for increased visual acuity. This may be very slight in some 

 forms and involve merely a greater crowding of the elements 

 per unit area in this region. In others, the crowding is suffi- 

 ciently extensive to cause a bulge or local thickening of the 

 retina — particularly in the external nuclear layer. When 

 this situation is obtained we speak of it as the area centralis 

 retinae. This may be essentially circular (e.g. turtle, Fig- 

 ure 62) or in the form of a stripe or ribbon-shaped area (some 

 birds). Further specialization for increased visual acuity 

 constitutes the foveas which are found in some saltwater 

 fishes, diurnal lizards, birds, monkeys, the great apes, and 

 man. 



Visual acuity or detailed vision must be affected also from 

 a structural standpoint by the relative numbers and connec- 

 tions of the bipolar cells, association cells and ganglion cells 

 in the retina. The numerical relation of these neurones to 

 the photoreceptors varies greatly in different forms. It has 

 been generally considered that individual cones make 

 connections with single bipolar cells, whereas many rods 



1 For a discussion of the various theories regarding the significance of the pecten, 

 V. Walls, G. L., 1942a, "The Vertebrate Eye," pp. 648-659. 



