THE SENSE OF SIGHT 307 



cells known as olfactory cells, and to them pass the terminal 

 filaments from the olfactory bulbs. These cells are stimulated 

 by contact with odorous substances, and from them go, by way 

 of the nerve fibers, impressions which are recognized as odors 

 of different kinds. The olfactory fibers are the only ones which 

 will convey such impressions. True, the same substance may, 

 at the same time, excite other sensations, as of pain or taste, 

 but the impressions giving rise to these latter sensations are con- 

 veyed by different fibers altogether. The substances which ex- 

 cite olfaction must come in actual contact with the nerve terminals 

 and to do this must be dissolved in the mucus of the nasal 

 mucous membrane; hence dryness of the nasal cavities (as in the 

 first stage of nasal catarrh) interferes with olfaction. It is also 

 said that odorous substances introduced in solution into the 

 nasal cavities will not excite the sense of smell, but that they 

 must be introduced by a current of air. 



Whether an odor is pleasent or unpleasant is largely a relative 

 matter; odors most disgusting to some animals are not offensive 

 to others. This same difference may also hold good among 

 different men. Impairment of the sense of taste, for some reason, 

 follows a loss of the sense of smell. 



3. The Sense of Sight. 



It is not intended to go into a detailed consideration of the 

 sense of sight, but some remarks on the normal eye and its action 

 are in order. 



Protection of the Ball. The orbital cavity has a pyramidal 

 shape with its base forward. It contains the eye-ball, its mus- 

 cles, some adipose tissue and most of the lachrymal apparatus. 

 Above the orbit, the eye-brows prevent a flow of perspiration 

 from the forehead on to the lid, and also shade the eye to some 

 extent. The lids, when closed, entirely obscure the balls and 

 protect them in front. On their free borders are rows of hairs 

 (eye-lashes) curling away from the globe and shading and pro- 



