Sight, and the Eye. 439 



blood vessels. This goes to prove that the optic nerve itself is not the 

 organ of sight, since its fibres are mingled freely with the blood vessels. 

 But this fact is still better proved by the blind spot. This spot is be- 

 tween the center of the retina and the nose, and is the place where the 

 optic nerve passes through the sclerotic and choroid coats of the eye 

 and the outer layers of the retina itself. After passing these it spreads 

 to left and right, ramifying all through the inner layers of the retina, as 

 stated. An image thrown upon this spot cannot be seen, as a simple 

 experiment will prove. Thus, fix the right eye on a distant object and 

 hold the end of the finger, or other small object, in front of the nose ten 

 inches away. Now move the object to the right three to four inches, 

 and it will become invisible, but after going further to the right it comes 

 into view again. This blind spot is the bundle of the optic nerve fibres, 

 and there are no rods or cones mingled with it in this place. 



Directly opposite the pupil on the retina is a round spot called the 

 macula lutea, or yellow spot, so called from its color. This spot is the 

 area of the greatest visual power, and whenever exact and clear sight is 

 required, the eye is automatically turned so that the image of the object 

 is thrown upon it. It is more densely packed with cones than any other 

 part of the retina, and they are longer than elsewhere, while the rods 

 are absent. The other layers over this spot are very thin, and around 

 it the nerve fibres are very abundant. Outside of the yellow spot the 

 rods appear among the cones, and increase in number till at the edge 

 of vision they are three times as numerous as the cones, and the latter 

 decrease in length as they diminish in number. To Max Shultze be- 

 longs the credit of having discovered that the function of the rods is 

 the sensation of light as distinguished from darkness merely, but that 

 of the cones is the sensation of color, or of one kind of light as distin- 

 guished from another. In the yellow spot, therefore, all the colors are 

 perceived. But as objects are passed to one side so as to be seen by 

 the edge of the retina, the power to distinguish color is lost before the 

 power of distinguishing forms. First, we become blind to red, later to 

 green, while yellow and blue are distinguished well out upon the edge 

 of the retina. On the other hand, it is shown that the region of the 

 yellow spot is not so acute in distinguishing faint light as the tract of 

 rods outside of it. This can be tested from within a dark room by 

 turning the eye toward a window that admits a very faint light. The 

 light will be seen better by the side of the retina than by its center. * 



Now, if it is true that the action of light is the cause of the habit by 

 which the modification and evolution of the eye has taken place, it 

 would follow that the tissues of the rods and cones must become ex- 



1 Henry T. Fink in Lit. Liv. Age, Vol. 144, page 25. Probably this differs with different 

 individuals. 



