852 



THE FUNCTION OF THE RETINA IN VISION. 



The sharpest vision is obtained at the fovea centralis, where cones 

 alone are found, placed closely together. In the peripheral portions of 

 the retina the cones are placed less closely together, and here vision is 

 less acute. From this it may be concluded that the cones are more 

 important for vision than the rods. In order to see an object as dis- 

 tinctly as possible, the eyes are therefore turned involuntarily so that 

 the retinal images fall on the fovea centralis. This adjustment is known 



FIG. 298. Horizontal Section of the Right Eye: a, Cornea; 6, conjunctiva- c, sclera' d 

 taming the aqueous humor; e, iris; /, /', pupil; g, posterior chamber; I, canal 'of I 

 k, corneo-scleral junction; , canal of Schlemm; m, choroid; n, retina; o, vitreous t 



anterior chamber, con- 

 Petit; /, ciliary muscle; 



g, nerve-sheaths; p, nerve-fibers; Ic, lamina cribrosa'. " The Hne'^ *O islhe'op'dc axis, S r the visual axis, r the 

 position of the fovea. 



as -fixation; the visual ray drawn from the fovea to a point in the object 

 is called the visual axis (Fig. 298, 5 r). This forms with the optic axis of 

 the eye (0 A), which unites the centers of the spherical surfaces of the 

 refracting media, an angle of only from 3.5 to 7. The point of inter- 

 section lies, of course, in the nodal point (k n) of the lens (Figs. 279, 298). 

 Vision with the optic axes directed upon the object is known as direct 

 vision. 



