262 



THE SPECIAL SENSES 



and other cells with their branches, a layer of special- 

 ized structures called rods and cones, which are thought 

 by many to be the real terminals of the nerves, and, 

 deeper still, a layer of dark coloring matter which pre- 

 vents the light from entering the eyeball anywhere 

 except through the pupil and also shuts off reflections 

 within the eyeball itself. 



Optic nerve. The nerve fibres from all points of the 

 retina pass over its inner surface and converge at one 

 point where they unite to form the optic nerve. The 

 optic nerve then passes out through the wall of the 



eyeball to the brain. 

 At the point of exit 

 of the' nerve there is 

 no retina, and rays 

 of light falling up- 

 on it excite no ner- 

 vous impulses. It is 

 therefore called the 

 Hind spot. Since the 

 blind spot in the 

 right eye cuts out a 

 portion of the field 

 of vision to the right 

 of the centre, and in 

 the left eye to the 

 left of the centre, we 

 can see with one eye 

 what we cannot see 

 with the other. Vision 

 with two eyes is not, therefore, impaired. 



Sensation of sight. The excitation of the retina by 

 the exceedingly rapid vibrations which constitute light, 

 excites the rods and cones and the nerve structures 



FIG. 143. The right retina as it would be seen 

 if the front part of the eyeball with the lens 

 and vitreous humor were removed. The 

 white disk to the right marks the entry of the 

 optic nerve (blind spot); the lines radiating 

 from this are the retinal arteries and veins. 

 The small central dark patch is the yellow 

 spot, the region of most acute vision. 



