HUMORS OF THE EYE. 157 



Emily. The cornea then, is in front of this mem- 

 brane, and of course a space must be left between them. 

 But what do you call it ? 



Dr. B. It is called the iris, and is the seat of the 

 colour of the eye, which varies you know, by infinite 

 shades, in different individuals. 



Emily. If I understand it right, the iris has a hole in 

 its centre, forming the pupil, as it is called ; so that the 

 shape of the iris is that of a flat ring. 



Dr. B. It is through the pupil that the rays pass ; 

 if the iris was not perforated in this manner, you see 

 they would be entirely intercepted. The iris has also 

 the power of contracting and dilating, and by thus in- 

 creasing or diminishing the diameter of the pupil, can 

 accommodate the eye to the various changes in the in- 

 tensity of the light. 



Emily. But what fills up the space that is left be- 

 tween the iris and cornea ? I cannot discern anything, 

 and yet I should hardly think it would be left entirely 

 empty. 



Dr. B. It is filled with a watery, transparent fluid, 

 called the aqueous humor. This space is called the an- 

 terior chamber of the eye that behind the iris is called 

 the posterior chamber. The first thing we meet with in 

 this latter chamber, is a small hard body just behind the 

 iris, nearly of a spherioral shape, and is called the crys- 

 talline humor, or lens. The remaining portion of this 

 chamber of the eye, is occupied by a dense, tenacious 

 fluid, called the vitreous humor, from its resemblance to 

 melted glass. This finishes the description of the first 

 division of the parts of the eye, the two others are soon 

 dispatched. Expanded on a portion of the choroid 

 coat, is a very delicate coat, chiefly composed of nerves, 

 and generally considered as an expansion of the optic 

 nerve. It is called the retina, and constitutes the se- 

 cond division of the parts of the eye. The third em- 

 braces only the optic nerve, which issues from the back 

 part of the eye, enters the skull through a hole in the 



