368 ADVANCED PHYSIOLOGY 



only cones, and which lacks nearly all the other layers of the 

 retina. It is plain from its situation that light coming directly 

 into the eye falls on this spot. This is the area of clearest 

 vision, or in scientific terms, the fovea centralis, Fig. 167. 



The other layers of the retina are represented in Figure 180, 

 but no special description of them will be given. From the 

 figure it will be noticed that from the inner ends of the rod and 

 cone cells other nerve cells arise, which extend toward the inner 

 portion of the retina, and are there connected or closely as- 

 sociated with nerve fibres. Any impulse, then, which starts 

 from the rods and cones may pass up through these connec- 

 tions until it reaches the nerve fibres, and goes through them 

 to the brain. 



The nerve fibres thus arise on the innermost side of the 

 retina next to the vitreous humor. They all pass toward the 

 back of the eyeball and finally unite to form the large optic 

 nerve. This nerve passes through the various coats of the 

 eyeball and then goes to the brain. It does not leave the eye 

 directly at the back, in the line of entering light, but on the 

 side of the eye toward the nose. At the point where the 

 nerve leaves the eye there is a small area which, having no rods 

 or cones and no pigment (these structures alone are really 

 sensitive to light), is blind and is therefore called the blind 

 Spot. In the ordinary use of the eyes, however, we do not 

 notice the presence of any such area. 



EFFECT OF LIGHT IN THE EYE 



How does light act on the parts of the eye which are sen- 

 sitive to it the pigment and the rod and cone layers of the 

 retina? No one has any precise knowledge of this matter, 

 but there are two interesting theories as to its effect. 



The Chemical Theory. The chemical theory supposes thai 

 the chemical composition of the pigment layer is changed by 

 light, somewhat as is the sensitive plate in a camera when 

 i,he ahutter is opened. The theory rests upon these facts: 



