SENSE ORGANS. 



99 



all those phenomena which are essential and universal; 

 binocular vision only certain additional phenomena 

 which are the result of the use of two eyes as one instru- 

 ment. We take up, first, the general phenomena char- 

 acterizing all vision — i. e., monocular vision. And as it 

 is impossible to understand these without a knowledge 

 of the structure of the eye, this must be our first subject. 



SECTION II. 

 The Eye of Man — General Structure. 



Shape, Setting, etc. — The eye is a nearly globular 

 organ, about one inch in diameter, a little more pro- 

 tuberant in front. Set in a deep conical socket, it occu- 

 pies only the anterior part ; the posterior is filled with a 

 cushion of fat on which it rests and rolls easily in all 

 directions. The exposed part may be 

 covered by the lids, which protect and 

 at the same time wipe and keep it clean 

 and bright. There is really no inter- 

 ruption of the skin here ; on the con- 

 trary, the skin passes over the lid, then 

 over its edge, then under the lid as mu- 

 cous membrane, then much short of the 

 equator of the ball it is reflected on to 

 the ball (a) (Fig. 70), then over the 

 white of the eye, then over the clear 

 part (but here it is very closely adherent and is trans- 

 parent), and so on to be again reflected on to the lower 

 lid (at a') and out on to the face. By nice dissection it 

 is possible to separate it continuously, so as to leave the 

 ball behind the skin. Now all this tender portion lining 

 the lids and covering the front part of the ball is called 

 the conjunctiva. Ordinary inflammation of the eye is an 

 inflammation of this membrane. Inflammation of the 



