272 HUMAN PHYSIOLOGY CHAP. 



the delicate nerve endings. The impressions are carried 

 over the optic nerve to the occipital lobe of the brain, 

 where they are interpreted. The vibrations of ether 

 pass through the conjunctiva, the cornea, the aqueous 

 humor, the pupil, the lens, the vitreous humor, and 

 then are received on the retina. 



The point at which the optic nerve enters the eye- 

 ball is not sensitive to ether vibration, and, for that 

 reason, is known as the " blind spot" To find the blind 

 spot, close the left eye, focus the right steadily on the 

 triangle below, and then move the book slowly until it 



FIG. 147. To find the blind spot. 



is about a foot from the eye, when the large outline of 

 the blind spot on the right will disappear entirely. The 

 point of clearest vision, located at the centre of the retina 

 just outside of the blind spot, is called the " yelloiv spot" 

 188. Defective Vision. The lens, which is so impor- 

 tant in bringing the rays of light to a focus on the yel- 

 low spot of the retina, may be defective. Then the rays 

 cannot be properly focused. The lens may be too con- 

 vex or the eyeball too long, so that the light is brought 

 to a focus before reaching the retina ; this is called " near- 

 sightedness." The book must be held very close to the 

 eyes in order to read and the body bent over in writing. 



