THE RETINA 29 



1. The macula liitea measuring from 1 to 3 mm. in diame- 



ter and subtending an angle of 4° to 12°. 



2. The rod-free area within the macula containing cones 



only. 



3. The fovea centralis (central pit) in the center. 



The human fovea lies approximately 3.5 mm. temporal to 

 the optic disc and about 0.8 mm. below the horizontal merid- 

 ian. The rod-free area, said to be circular, measures from 

 0.4 mm. to 0.8 mm. in diameter. It subtends an angle of 



^-^^^^^'^^^^^I^g^^ggS^'g^ 



Fig. 23. Photomicrograph of the fovea centrahs of the marmoset. X 100. 



approximately 3°. The human fovea is elliptical; the long 

 horizontal axis measures 0.3 mm. and the vertical axis 

 0.2 mm., making an area of 0.5 to 0.6 sq. mm. In the para- 

 foveal region the retina reaches its greatest thickness. Duke- 

 Elder states that on the temporal side it measures 275 /jl to 

 410 fjL and on the nasal side 220 ^u to 350 m- My own measure- 

 ments (meridian unknown) showed a thickness of 390 jjl 

 (Table 1). This is not as thick as the parafoveal region of the 

 chameleon and the hawk which have the thickest retinas 

 of all the forms measured in this study. 



Apparently some authorities use the term ''fovea" synony- 

 mously with the term '' macula," giving to the fovea a diame- 

 ter of 1 to 2 mm. and calling the center the foveola. 



There are so many points of interest centering about the 

 structure, distribution, and function of the fovea in verte- 

 brate eyes that a more detailed discussion will be resumed 

 in a later chapter. 



