THE VISUAL PIGMENTS 



of the choroid is mainly nutritive but it is similar to erectile tissue 

 and plays a part in the regulation of the intraocular pressure. 



By means of the cihary muscle the form, and hence the power, of 

 the crystalline lens can be varied so that, whatever the distance of the 

 object regarded, a sharp image is cast on the retina. The total power 



^ ^RETINA 

 "" CHOROfO 

 ^SCLER/\ 



OPT/C 

 NEBVEL 



Fig. 1.1. Horizontal section of the human eye. 

 {Wolff, 1948) 



of the resting eye (that is, when relaxed for distant vision) is about 

 60 dioptres. In a young child, whose lens is still soft and phable, a 

 further 14 dioptres of power can be added by the maximum effort of 

 accommodation. Throughout Hfe the lens hardens, however, and in 

 elderly persons all accommodation has been lost and aids to reading 

 are necessary. 



2 



