ARE/E CENTRALES AND FOVEJE 181 



(C) Are^ Centrales and Fove^ 



The Area Centralis — An important feature characteristic of the best- 

 adapted diurnal eyes, and found in many twenty-four-hour eyes (as an 

 adjunct to their diurnal activity phase) is the area centralis. It is best 

 defined as a circumscribed retinal area within which the retina is so 

 constructed as to afford a marked local increase in resolving power. 



The name 'area centralis' is not too fortunate, for the area is not 

 necessarily near the center of the fundus — though it happens to be in 

 man, whose morphology has greatly influenced all anatomical termin- 

 ology. In the human and other primates, the macula lutea (= yellow 

 spot) of the retina is synonymous with 'area centralis', but the term 

 'macula' is most improperly applied to the areae of other vertebrates. 

 Similarly, the word 'fovea' is often badly misused, and it will be well 

 to get these three terms firmly and accurately in mind : 



An area centralis is only exceptionally pigmented, making of it a 

 yellow spot on the otherwise colorless retina. Only then can it properly 

 be called a macula lutea. This latter term should consequently be re- 

 served — if, indeed, there is any need for it at all — for the areae centrales 

 of the higher primates, and possibly the chameleons. No others are 

 known to have the diffuse yellow pigmentation of the inner layers of the 

 retina in the area centralis. 



Again, only certain areae centrales have a depression or pit in the 

 center; and it is just this pit, not the whole area, which should be called 

 a fovea. An area centralis can occur without a fovea — it may actually 

 be thickened, not thinned — but a fovea can exist only within an area 

 centralis. 



The various features of a full-fledged area centralis can best be set 

 forth if we enumerate them as steps in the evolution of such an area in 

 a hypothetical vertebrate. This animal must have taken on diurnality 

 and — unless of course he eventually dispenses with rods entirely — must 

 have a large-enough eye to be able to afford to devote a portion of the 

 retina to an area centralis without sacrificing the ability to see in dim 

 light with the greater part of the retina. 



The first obvious thing to do is to increase, locally, the number of 

 visual cells per unit area of the retina. This is brought about partly by 

 making them more slender, partly by packing them more closely to- 

 gether than they are outside the area. Since the rods are like so much 

 deadwood when it comes to affording highly-resolved images to the 



