10 COLOUR VISION 



of the foveal cones at 0'3 /",, there are about 300 in the long axis, and 

 60 in the short, 1300 1400 in O'l sq. mm. The diameter of the outer 

 limbs is 0'6 0'75 M. The cones are arranged in curved lines (Max 

 Schultze) or spirals (Fritsch), and are not quite regular. There are 

 small spaces between them, measuring from 0'05 to 0'27 of the trans- 

 verse section of the inner limb. Greeff says that the cones are very 

 closely packed in the fovea, and in a specimen of Heine's were 

 hexagonal in transverse section. 



Koster 1 examined three normal children's eyes and found that the 

 part completely free from rods occupied a circular area 0'44 0'55 mm. 

 in diameter, the part relatively free from rods 0'88 mm. In the eye of 

 a youth aged 20, the rod-free area was 0'901 mm. He concludes that 

 in the adult the rod-free area measures about 0'8 mm. in diameter, 

 subtending a visual angle of 3 3'. This is probably a maximum, and 

 there is physiological evidence to show that the rod-free area varies in 

 size in different individuals. 



Three areas must be carefully distinguished : 



Fovea Centralis, measuring 0'24 0'3 mm. in diameter, subtending 

 55' 70' ; 



Rod-free area, measuring 0'8 mm., subtending 3 3' ; 



Macula, measuring 1 3 mm., subtending 4--12. 



Dimmer 2 describes a fovea cen trails, 1*5 mm. in diameter (the macula 

 of Koster), containing in its centre a foveola (the fovea centralis of 

 Koster). 



Gullstrand 3 regards the yellow colouration of the macula lutea as 

 a post-mortem change, a view which is scarcely consistent with its 

 absorptive capacity for coloured lights during life. 



Fritsch 4 describes the site of clearest vision as the area centralis, 

 possessing a central depression, the fovea centralis, which may or may 

 not contain a foveola. 



Comparative Anatomy. The distribution of rods and cones in the 

 retinae of lower animals is of great theoretical importance. Many erro- 

 neous statements have gained currency and have been used as arguments 

 in favour of certain theories. The great variety in the forms of the 

 neuroepithelial cells prevents any generalised classification. 



Green 05 says that there are rods and cones in the retinae of most 



1 Arch.f. Ophth. XLI. 4, 1, 1895 ; Arch. d'Opht. xv 428, 1895 



2 Arch. f. Ophth. LXV. 486, 1907. 



3 Arch.f. Ophth. LXII. 1, 378, 1905; LXVI. 141, 1907. 



4 Ueber Ban u. Bcdeutung d. Area centralis des Menschen, Berlin, 1908. 



5 Graefe-Saemisch Handb. d ges. Augenheilkundc. . Theil i. Bd. I. Cap. v. 1900. 



