808 HEARING, SMELL, AND TASTE. 



Each substance that we smell causes a specific sensation, and we are not 

 only able to recognize a multitude of distinct odors, but also to distinguish 

 individual odors in a mixed smell. 



As in the previous senses, we project our sensation into the external 

 world ; the smell appears to be not in our nose, but somewhere outside us. 

 We can judge of the position of the odor, however, even less definitely that 

 we can of that of a sound. 



The sensation takes some time to develop after the contact of the stim- 

 ulus with the olfactory membrane, and may last very long. When the 

 stimulus is repeated the sensation very soon dies out ; the sensory terminal 

 organs speedily become exhausted. Mental associations cluster more 

 strongly around sensations of smell than around any other impressions 

 we receive from without. And reflex effects are very frequent, many 

 people fainting in consequence of the contact of a few odorous particles 

 with their olfactory cells. 



Apparently the larger the surface the more intense the sensation ; 

 animals with acute scent having a proportionately large area of olfactory 

 membrane. The quantity of material required to produce an olfactory 

 sensation may be, as in the case of musk, almost immeasurably small. 



When two different odors are presented to the two nostrils, an oscilla- 

 tion of sensation similar to that spoken of in binocular vision (p. 788) takes 

 place. 



729. The assertion that the olfactory nerve is the nerve of smell has 

 been disputed. Cases have been recorded of persons who appeared to have 

 possessed the sense of smell, and yet in whom the olfactory lobes were found 

 after death to be absent. Direct experiments on animals, however, show 

 that loss of the olfactory lobes entails loss of smell. On the other hand, it 

 is stated that section or injury of the fifth nerve causes a loss of smell 

 though the olfactory nerve remains intact ; but in these cases it has not 

 been shown that the olfactory membrane remains intact, and it is quite pos- 

 sible that, as in the case of the eye, changes may take place in the nasal mem- 

 brane as the result of the injury to the fifth nerve, sufficient to prevent its 

 performing its usual functions. 



TASTE. 



730. [The peripheral organs concerned in the sense of taste are local- 

 ized in the mucous membrane covering the dorsumof the tongue, the fauces, 

 soft palate, and uvula, and possibly a portion of the upper part of the 

 pharynx. This membrane is analogous in structure to other membranes of 

 its type, except on the dorsum of the tongue, where its structure is similar 

 to that of the integument. At this position it consists of a corium, with a 

 papillary and a superficial epithelial layer. 



The structure of the corium is similar to that of the skin, but is thinner 

 and less compact. It serves as a point of insertion of the muscular fibres of 

 the tongue. 



731. The papillce are thickly distributed over the whole dorsal surface, but 

 more particularly marked in the anterior two-thirds. They project as minute 

 prominences, which give the tongue a roughened, characteristic appearance. 

 The papillae are of two kinds, the simple and compound. The simple papillae 

 are similar to those found in the skin ; they are found scattered over the 

 whole dorsal surface between the compound papillae. They are most nu- 

 merous in the posterior portion of the organ. The compound papillae are 

 of three varieties : the papillae maximae or circumvallatse, the papillae mediae 

 or fungiformes, and the papillae minimse or filiformes. 



