228 THE CONTENTS OF THE EYEBALL. 



the eye, and consists of the spread-out fibres of the optic 

 nerve, and the peculiar tips or " end organs" joined to 

 them. If you should take a cord, and fray out its threads 

 at one end, and spread them out on all sides, the cord 

 would answer to the optic nerve, and the spread-out 

 threads to its fibres in the retina, except that each 

 thread, in order to make the resemblance greater, ought 

 to have a very small rod or cone easily excited by 

 light, attached to its end. 



6. The Interior of the Eyeball is filled up by liquid or 

 jelly-like matters, surrounded by its coats, as the pulp 

 of an orange is surrounded by the rind. These sub- 

 stances are all transparent; they guide to the retina, 

 light which enters the eye through the cornea and pupil. 

 They are three in number, (i) The crystalline lens, 26, 

 27, 28, just behind the iris. It is soft and jelly-like. (2) 

 The aqueous (watery) humor, 30, a watery liquid be- 

 tween the crystalline lens and the inner side of the 

 cornea. (3) The vitreous (glassy) humor, 29, behind the 

 crystalline lens, a soft jelly filling up all the back part of 

 the cavity of the eyeball. 



7. The Use of Aqueous Humor, Lens, and Vitreous Humor 

 is to gather the rays or lines of light which enter the eye, 

 and so bend and direct them, that all those starting from 

 one point outside the eye meet again in one point on the 

 retina, and excite the same nerve-fibre. This enables us 

 to see things distinctly, because an exact image of the 

 thing looked at is made on the retina. In Fig. 56, O 

 answers to the lens of the eye; Z>, E, is the object looked 



6. How is the interior of the eyeball filled ? Use of these sub- 

 stances ? Their number ? Names ? Describe each. 



7. What is the use of aqueous humor, vitreous humor, and lens ? 

 How does their action enable us to see distinctly ? How is the image 



