52 EVOLUTION OF THE HUMAN EYE 



sums up the correlation between the area centralis, the 

 position of the eyes in the head and the shape of the cornea 

 and lens as follows: "Sensitive areas of restricted dimensions, 

 omitting those cases in which the area is limited to a macula, 

 exist in the Carnivora, in which order the divergence is not 

 great. In the Ungulates, Rodents, Edentates and Mar- 

 supials, where we find great divergence of the axes, large 

 corneae and nearly spherical lenses, the sensitive areas are 

 larger, and probably the degree of difference in perception 

 over such areas compared with the more peripheral parts 

 is but little." 



Among Primates a fovea is met with in man, anthropoid 

 apes and monkeys, but not in lemurs. In Tarsius, which 

 it is generally agreed is more nearly allied to Primates than 

 it is to lemurs, Elliot Smith says, "There is no macula 

 lutea." It is found in some reptiles, and is present in 

 practically all birds. In writing of these latter, Casey 

 Wood 23 says: "The depth of the fovea may be regarded as 

 a measure of the sharpness of vision. Slonaker classifies 

 fovea? as deep, medium and shallow. The round fovea is 

 especially 'deep' in swift fliers and birds of prey; 'medium' 

 to 'weak' in most birds, except that it is 'shallow' in the 

 domestic pigeon and probably lacking in the hen." 



Anatomically the fovea consists in man of an area measur- 

 ing about 0.3 mm. in diameter, subtending an angle of about 

 70', and lying almost exactly in the optic axis of the re- 

 fractive media of the eye, at a point where the image pro- 

 jected by them is brought to the sharpest focus. It is 

 characterized by an unfolding of the inner layers of the ret inn , 

 which are spread apart so as to permit the direct unimpeded 

 access of rays of light to the percipient end-organs of the 

 retina. These end-organs consist exclusively of cones, ami 

 in the very center of the fovea (Dimmers' foveola) of cones 



