308 THE SENSES 



to which the olfactory fibers are distributed are delicate 

 spindle-shaped 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 conveyed by different fibers 

 altogether. The substances which excite 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 pleasant 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 pyra- 

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

 its muscles, 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 ob- 



