306 



occupies about the central point of the retina and functions onh' in monocular 

 vision. The temporal fovea functions in binocular vision. In man it is 

 located about the center of the retina, but in the owl it is some distance 

 to the temporal side of the center. In the case of two fovese, the one 

 at the center of the retina (fovea nasalis) functions in monocular vision; the 

 other (fovea temporalis) corresponds in position to that of the owl, and functions, 

 likewise, in binocular vision. If a trough-like fovea is present it would function 

 in acts of sight anywhere between monocular and binocular vision. 



The area centralis also has a variety of forms, number and positions corre- 

 sponding to those of the fovea and similarly named. A simple fovea is always 

 surrounded by a round area which is frequently on or in a band-like area extending 

 horizontally across the retina. If a trough-like fovea should be present it would 

 lie along a band-like area. Frequently when two foveso are present the areas 

 surrounding each are connected by a band-like area. 



We may thus have various combinations of area and fovea. The most com- 

 mon is a simple fovea surrounded by a round area. Further, this round area 

 may be continuous with a band-like one. Or two fovefu may each be surrounded 

 by a round area, one of which may be continuous witli a band-like area, or each 

 may be so connected. 



When one considers the prevalence of a fovea in the diflerent vertebrates he 

 finds that, though each class has representatives which possess a fovea, by far the 

 greater number have only an area centralis. Many have been examined in which 

 not even an area has been observed. 



The following tabulation represents in a condensed form the results, so far as 

 I have been able to collect them, of all investigations up to the present time:. 



