XXIX. THE SPECIAL SENSES {continued). 

 Visual SsirsATioNS. 



The eyes are the external organs by means of which 

 visual impulses are produced. So complex are these organs 

 that we must first study their anatomy and some of the 

 laws of light before we can appreciate the manner in 

 which they produce visual impulses. 



Form and protection of the eyeball. — The eyes con- 

 sist of two nearly 



III 



spherical bodies called 

 the eyeballs, which, 

 together with certain 

 protective structures, 

 completely fill the two 

 sockets in the facial 

 bones which we call the 

 orbits or eye sockets. 

 Both eyeballs and 

 their accessory parts 

 are identical in struc- 

 ture, hence a description 

 of one will serve for 

 both.^ In front the 

 eyeball is protected by 

 movable folds of the 

 lids are lined with a 



Fig. 212 — Section of eyeball in orbit; J, 

 frontal bone; 2, cheek bone; 3, eyebrow; 4, 

 eyelids; 5, folds of the conjuctiva lining eye- 

 lids and covering front of eye; 6, 7, 5, muscles 

 moving eyeball; c, cornea; a, aqueous humor; 

 i, iris; /, lens; ?', vitreous humor; cA, choroid 

 layer; r, retina; a, sclerotic layer; o, optic 

 nerve. 



skin called the eyelids. These 



> A sheep's skull with the bony parts and eyeballs uninjured serves 

 well for a demonstration o£ the structure of the eyeballs. (See Ex. LXX.) 



452 



