70 THE ORGANS OF SPECIAL SENSE. 



the power of perception is supposed to lie, branches of 

 the optic nerve being distributed over it in all directions. 

 In fact, the retina is formed by a membranous expan- 

 sion of the optic or second cranial nerve, the special 

 nerve of sight, which passes into the orbit through the 

 optic foramen at the back and enters the eye-ball close 

 to the macula lutea or yellow spot. The exact spot where 

 the optic nerve enters the retina is not sensitive and is 

 known as the blind spot. In the center of the macula 

 lutea, however, which is in the middle of the retina pos- 

 teriorly, is a tiny pit, the fovea centralis, in which all 

 the layers of the retina except the rods and cones are 

 absent, and at this point vision is most perfect. It is, 

 therefore, always turned toward the object looked at, 

 and when one wishes to see an object distinctly, he must 

 keep moving his eyes over it that the rays from each 

 part may fall in turn upon the fovea centralis. 



Directly behind the pupil is the crystalline lens, 

 a rather firm gelatinous body enclosed in a capsule, 

 which is transparent in life but opaque in death. The 

 lens is doubly convex and is held in place by the sus- 

 pensory ligaments, which arise from the ciliary pro- 

 cesses. In front of it is the anterior chamber of the 

 eye, filled with a thin watery fluid called the aqueous 

 humor, while the larger space back of it, occupying 

 about four-fifths of the entire globe, is filled with a 

 jelly-like substance known as the vitreous humor. 



The chief artery of the eye is the ophthalmic. 



Light Rays. The eye is practically a camera and its 

 principal function is to reflect images. Although there 

 are several refracting surfaces and media, for practical 

 purposes the cornea alone need be considered. Ex- 

 cept for those rays which enter the eye perpendicu- 

 larly to the cornea, whose line of entrance is called the 

 optic axis, all rays are refracted when they enter the eye 

 and the point at which they meet and cross each other 

 behind the cornea is called the principal focus of 

 the eye. To focus properly, all the rays from any one 



