EVOLUTION AND SIGNIFICANCE OF FOVEA 109 



change can take place in the retina all the fibers coming 

 from the temporal side of the retina should remain uncrossed 

 for the macular area develops in the line where the temporal 

 and the nasal fields of the retina meet, and one-half of it 

 transmits fibers which remain uncrossed, and the other half 

 transmits fibers which cross to the other side of the brain. 

 Hence until the rearrangement of the fibers in the optic 

 chiasm has been completed the true macula cannot develop. 

 Intimately linked with this process of evolution is the devel- 

 opment of a wide range and a greater exactitude in the 

 conjugate movements of the eyes." 



One is left somewhat in doubt as to what is implied by the 

 ^ ' true macula. ' ' Speaking of mammalian retinas Duke-Elder 

 (1939, p. 101) says, " . . . . only in some monkeys and man 

 is there a true macula with a fovea subserving binocular and 

 stereoscopic vision." If the true macula implies binocular 

 vision, then it cannot be confined to anthropoids and man, 

 for in many birds the temporal macula is supposed to sub- 

 serve the function of binocular vision. In the owl the visual 

 axes are almost parallel (Figure 3). Wolff (1940) also makes 

 the statement: ^'Only man and some monkeys have a true 

 macula and fovea centralis subserving binocular and stere-. 

 oscopic vision." In discussing the bird retina he speaks of 

 an area with a fovea, although Casey Wood (1917) describes 

 this avian region as a macula and Duke-Elder also refers 

 to it as a macula. 



The macular region in anthropoids and man is the region 

 of highest visual acuity where the layers of the retina are 

 spread apart, so as to produce a depression known as the 

 fovea. The displacement of the retinal layers at the fovea 

 produces a thickening in the parafoveal region. The struc- 

 tural conformation is exactly the same in principle in anthro- 

 poids as it is in diurnal saurians regardless of the difference 

 in shape between the foveas of these two groups of animals. 



Duke-Elder defines the macular region in man as follows, 

 ''The region of the retina at the posterior pole of the eye is 

 specially differentiated to subserve the highest visual acuity. 



