THE SENSE OF SIGHT. 



499 



ling sensation excited in the filaments of the fifth nerve, a 

 smell like that of ammonia was excited by the negative pole, 

 and an acid odor by the positive pole ; whichever of these sen- 

 sations was produced, it remained constant as long as the circle 

 was closed, and changed to the other at the moment of the 

 circle being opened. Frequently a person smells something 

 which is not present, and which other persons cannot smell ; 

 this is very frequent with nervous people, but it occasionally 

 happens to every one. In a man who was constantly conscious 

 of a bad odor, the arachnoid was found after death, by MM. 

 Cullerier and Maignault, to be beset with deposits of bone ; 

 and in the middle of the cerebral hemispheres were scrofulous 

 cysts in a state of suppuration. Dubois was acquainted with 

 a man who, ever after a fall from his horse, which occurred 

 several years before his death, believed that he smelt a bad 

 odor. 



THE SENSE OF SIGHT. 



The eyeball or the organ of vision (Fig. 178) consists of a 

 variety of structures which may be thus enumerated : 



FIG. 178. 



Ciliary muscle. 



Ciliary process. 



Canal of Petit. 



Cornea. 



Anterior chamber. 



Lens. 



Iris. 



Ciliary process. 

 Ciliarv muscle. 



The sclerotic, or outermost coat, envelops about five-sixths 

 of the eyeball : continuous with it, in front, and occupying the 

 remaining sixth, is the cornea . The cornea and front portion 



