240 THE HUMAN BODY 



factory nerve, which enter the deeper strata of the epithelium 

 and there divide. In Amphibia the corresponding cells have fine 

 filaments on their free ends. The cells of the third kind are irreg- 

 ular in form and lie in several rows in the deeper parts of the 

 epithelium. It may be that the cylindrical cells if not (as is 

 possible) directly concerned in olfaction, have important functions 

 in regard to the nourishment of the olfactory cells which they 

 surround; they may supply them with needful material. 



Odorous substances, the stimuli of the olfactory apparatus, are 

 always gaseous and frequently act powerfully when present in very 

 small amount. We cannot, however, classify them by the sensa- 

 tions they arouse, or arrange them in series; and smells are but 

 minor sensory factors in our mental life, although very powerful 

 associations of memory are often aroused by odors. We com- 

 monly refer them to external objects, since we find that the sen- 

 sation is intensified by "sniffing" air into the nose, and ceases 

 when the nostrils are closed. Their peripheral localization is, 

 however, imperfect, for we confound many smells with tastes (see 

 below); nor can we well judge of the direction of an odorous 

 body through the olfactory sensations which it arouses. 



Although the sense of smell in man is aroused by inconceivably 

 small amounts of odoriferous substance, one part of mercaptan 

 to thirty billion of air being detectible, it is much 'less keen than 

 the sense of smell in many animals, canines in particular. In 

 such animals the sense of smell as a source of information seems 

 to be of the first importance, approaching our eyes in rank. 



A striking thing about the sense of smell is the ease with which 

 it is fatigued. One may notice a bad odor upon entering a room, 

 but in a few minutes ceases to perceive it because his olfactory 

 apparatus has become fatigued. For this reason the sense of 

 smell is wholly untrustworthy as a guide by which to regulate 

 the ventilation of a room. 



Taste. The organ of taste is the mucous membrane on the 

 dorsum of the tongue * and, in some persons, of the soft palate 

 and fauces. The nerves concerned are the glossopharyngeals, 

 distributed over the hind part of the tongue, and the lingual 

 branches of the inferior maxillary division of the trigeminals on 

 its anterior two-thirds. It has been shown that the nerves of 

 * A description of the tongue will be found on page 446. 



