CHAPTER XVI. 



THE SENSES. 



Sight. 

 The delicate structures composing the eye receive a very 

 thorough protection by the anatomical arrangement of the 

 parts. The orbital cavity, for example, is nearly entirely 

 surrounded by bony walls, and layers of fat within it 

 assist the muscles in protecting the globe and the optic 

 nerve ; the eyelids sweep the cornea and protect the part 

 from dust ; the tears keep the face of the cornea brilliant ; 

 the membrana nictitans removes particles of solid matter 

 which would otherwise produce injury; the eyeball can 

 also be retracted to a considerable extent to further assist 

 it in withdrawing from injury. The size of the orbit is such 

 that ordinary blows inflicted upon the eye arc expended 

 on the margin of the orbital cavity, and not on the eyeball 

 itself; so that the risk of injury to this delicate part arises 

 less from large than from small bodies. 



The arrangement of the eye corresponds very closely with 

 that of a certain well-known physical apparatus the 

 camera, where into a dark chamber is thrown the inverted 

 image of an object, the inversion being produced by a 

 double convex lens placed in front : the amount of illumi- 

 nation being regulated by a diaphragm, and the focussing 

 power also capable of adjustment. 



The eye is composed of a convex surface in front, forming 

 a chamber containing fluid, immediately behind which is the 

 iris or diaphragm, through which the light passes into a 

 biconvex lens; issuing from the posterior surface of the 



