xvin THE SPECIAL SENSES AND SENSATIONS 271 



is suspended in a ligament which holds it in place and 

 changes its thickness so as to adjust (accommodate) the 

 eye for objects near by and far away. 



1. Cut through the cornea of the sheep's eye and you will find a 

 liquid called the aqueous humor. 



2. Cut away the iris and find the lens. Notice its size, shape, 

 curvatures, and transparency. Place the lens on print and notice 

 the result. 



3. Cut back on one side so as to remove the vitreous humor that 

 fills the back part of the eyeball. 



4. Find the blind spot, or the place where the optic nerve enters 

 the eyeball. 



5. Near the blind spot is a very slight depression at the center 

 of the eyeball, known as the yellow spot. Find it. 



187. The Course of Light. The rays of light coming 

 from an object radiate in all directions. If the eye is 



FlG. 146. The formation of an image on the retina. (From Huxley.) 



so turned as to receive some of the vibrations, those 

 which fall upon the pupil enter the eye and pass 

 through the aqueous humor, the crystalline lens, and 

 the vitreous humor. The muscles of the capsule of the 

 lens so change its curvature as to bring the rays to a 

 focus on the retina. There is now an image of the 

 object produced on the retina, and this image excites 



