THE EAR, HEARING, TASTE AND SMELL 



203 



tory end organs, and to be connected with the fibers of the ol- 

 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 irregular 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 olfac- 

 tion, have important functions in re- 

 gard to the nourishment of the olfac- 

 tory cells which they surround; they 

 may supply them with needful ma- 

 terial. 



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 sensations they arouse, or arrange 

 them in series: and smells are but 



, . FIG. 76. Cells from the ol- 



mmor sensory factors in our mental factory epithelium, i, from the 

 life, although very powerful* associa- Sf &VK3SF3Z 

 tions of memory are often aroused by process; b, so-called olfactory 



J cell ; c, its narrow outer process ; 



Odors. We Commonly refer them to d, its slender central process, 



i , . . f, , ,1 , ,, 3, gray nerve-fibers of the olfac- 



CXternal Objects, Since We find that the tory nerve, seen dividing into 



Sensation is intensified by "sniffing" fine peripheral branches at a. 



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 



