40 VERTEBRATE PHOTORECEPTORS 



to Salzmann (1912), and 70 m according to Polyak (1941). 

 The foveal cones of the chameleon have been found to be 

 longer than human foveal cones (Table 1). They are also 

 longer in the marmoset. The diameter of the human foveal 

 cone is said to range from 1.5 ju to 3 /x. The usually accepted 

 diameter is approximately 2.5 ju. According to Polyak, in the 

 center of the outer fovea, the cone inner segment is 1.5 /x 

 thick; the outer segment 1 ju. In the slope of the outer fovea 

 the inner segments are 2.3 ju; the outer segments 1.3 ju- In 

 the edge of the outer fovea the inner segments are 2.5 ju in 

 diameter; the outer segments approximately 1.3 /x. He says 

 also that in the center of the fovea the dead interval between 

 two adjacent cones is 0.3 /x, whereas from 100 /x to 150 ij. 

 outside the center the dead interval is 0.5 ju (v. Polyak, 

 op. cit.. Figure 38). Some have estimated that the distance 

 between two adjacent cones is 2.5 ju- As regards the number 

 of cones in the rod-free region of the macula, various esti- 

 mates have been given. Duke-Elder states (p. 96) that there 

 are from 13,000 to 14,000 per 0.1 sq. mm. in the region of the 

 macula, which would mean 130,000 to 140,000 per sq. mm. 

 However, on page 91, he says there are 13,000 cones in the 

 region of the macula and 4000 at the region of the fovea. 

 These estimates seem not to agree. According to Osterberg 

 (cited from Wolff, 1940) there are 147,300 cones per sq. mm. 

 at the fovea. At a point where the rods begin (130 /x from 

 the center of the fovea) there are 74,800 cones per sq. mm.; 

 3 mm. farther there are 6000 per sq. mm., and 10 mm. from 

 the fovea they number about 4000. Polyak has estimated 

 that, in the center of the fovea, where all the inner nuclear 

 layers and blood vessels are practically absent (the terri- 

 tory coextensive with the foveola), there are 25,000 cones. 

 He says also that the human central territory, where rods 

 are absent, measures approximately 500 ijl across (1° 40', or 

 50' of arc on both sides of the ideal fixation-point). The 

 number of cones computed in this region is given as approxi- 

 mately 34,000. This would mean approximately 136,000 

 cones per sq. mm., and would be of similar order of magni- 



