ORGANS OF SPECIAL SENSE THE EYE 



355 



near the sides of the iris is a mere frayed edge of tissue. Yet 

 this thin layer is a very complicated structure, a microscopic 

 section showing eight well defined layers or strata. Next to 

 the choroid coat is a pigment layer. At first glance this 

 usually appears black, but when more carefully examined it 

 proves to be made up of granules of a purplish hue, a fact 

 which explains why this material is sometimes called the 

 " visual purple." This pigment is not laid down in an even 

 coating next the 

 choroid, but is ar- 

 ranged in a kind of 

 mosaic of six-sided 

 patches. 



Next the pigment 

 is a layer of cells of 

 very peculiar shape. 

 Each contains a nu- 

 cleus like any other 

 cell, but has two 

 outgrowths on op- 

 posite sides; Fig. 

 180. On one side 

 a fibre extends and 

 ends in numerous 

 small branches or 

 arborations; on the 

 other side opposite 

 the fibre, in some a 

 rod-like, in others a cone-like structure is found. These rods 

 and cones are all arranged parallel with one another, their ends 

 pointed toward the pigment layer, the rods in many cases 

 reaching into the pigment. At the back of the eye the cones are 

 much more abundant than on the sides of the eyeball, where the 

 rods predominate. In the very center of the back of the eye 

 directly behind the pupil, is a minute area in which there are 



FIG. 180. SECTION THROUGH THE RETINA 



Very highly magnified. The lower edge is the ont 

 toward the center of the eye. The rods and cones 

 are indirectly connected with the nerves. (Cajal) 



