THE STRUCTURE AND FUNCTIONS OF THE EYE. 255 



When the nervous structure of the eye is morbidly irritable, 

 as in ophthalmia, the pupil continues small, and sometimes nearly 

 closed, even in a moderate lio;ht. But when the lens becomes 



Fig. 1. 



Fig. 2. 



The right figure represents the eye witli the pupil contracted from exposure to 

 light. Tlie left figure shows the pupil in an expanded state. 



a The pupil. b b The iris. 



c c The corpora nigra, or black bodies, botii at the upper and lower parts of the 

 pupil. 



d The membrana nictitans or haw. 



opaque, the ophthalmia and morbid irritability cease, and the 

 pupil then becomes open and irregular in its form, as in fig. 4. 

 Fig. 3. represents a partial opacity of the lens, or partial 

 cataract, or, as it is more commonly called, specks in the pupil ; 

 here the form of the pupil is not altered, and it remains capable 

 of contracting or opening, according to the degree of light it is 

 exposed to. 



In the horse there are black pendulous bodies attached to the 

 superior margin of the iris. These serve, like the cilium, or 



Fig. 3. 



Fig. 4. 



The riglit figure represents a complete cataract, the crystalline lens being entirely 

 opaque. The left figure represents a partial cataract, the vision being possessed in 

 some degree. 



upper eyelash, to moderate or exclude the direct rays of light 

 which proceed from the sun, without interrupting those which 



