THE EYE AND THE SENSE OF SIGHT 109 



through the pupil. Hence the interior of the eye usually 

 looks black. 



145. The Yellow Spot and the Blind Spot (Fig. 63). The 

 retina is not equally sensitive to light over its whole 

 surface. Only upon a single spot, about one twenty- 

 fourth of an inch in diameter, are perfectly definite out- 

 lines of images formed. This is called, from its color, 

 the yellow spot. 



About one tenth of an inch from the inner side of the 

 yellow spot is the optic disk, or blind spot, an elevated 

 surface where the optic nerve fibers enter the eye. These 

 are conducting nerve fibers only, not stimulated by light, 

 and that spot is therefore blind. 



Delicate fibers from the optic nerve run straight to the 

 yellow spot. Here the layer of ganglion cells is much 

 thicker than elsewhere, and in the rod-and-cone layer of 

 the yellow spot no rods, but cones only, are found. 



In the very center of the yellow spot is a colorless 

 depression, or pit, from which the various layers of the 

 retina have nearly disappeared, leaving only the rod and 

 cone layer. This is the point of most acute vision, the 

 spot upon which the image falls when, wishing to see with 

 the utmost distinctness, we look " straight at " an object. 



146. The Lenses (Fig. 63). The refracting media of the 

 eye are four in number. (1) The cornea has already been 

 defined. (2) The crystalline lens is a transparent, double- 

 convex body about one third of an inch in diameter and 

 one fourth of an inch thick, lying just back of the pupil 

 and kept in place by a sheet of transparent membrane 

 called the suspensory ligament attached to the circum- 

 ference of the lens and to the ciliary processes. (3) The 

 space between the iris and the cornea, called the anterior 

 chamber, is filled with a thin fluid like water, called the 



