410 AN AMERICAN TEX1-BOOK OF PHYSIOLOGY. 



of odors is not very definite, and the relation of odors to one another in the 

 way of contrast and harmony is ill understood. No limited number of pri- 

 mary sensations, as in vision, have been discovered out of which other sen- 

 sations can be composed. Certain sensations, as those due to the inhalation 

 of ammonia and other irritant gases, are thought to be due to excitement of 

 the nasal filaments of the fifth nerve, and not of the olfactory. 



Subjective sensations of smell are sometimes experienced, the result of some 

 irritation arising in the olfactory apparatus itself. 



Finally, in man sensations of smell have their most important uses in con- 

 nection with taste; many so-called "tastes" owe their character wholly or 

 partly to the unconscious excitement of the sense of smell. 



G. Taste. 



The peripheral surfaces concerned in taste include, in variable degree, the 

 upper surface and sides of the tongue and the anterior surfaces of the soft 

 palate and of the anterior pillars of the fauces. Other parts of the buccal 

 and pharyngeal cavities are, in most persons, devoid of taste. 1 



The chief peripheral sensory organs of taste are groups of modified epi- 

 thelial cells, known as taster-buds (Fig. 206), seated in certain papillae of the 

 tasting surfaces. According to some authors, only parts provided with taste- 

 buds can give taste-sensations. 2 



The structure of taste-buds is most easily studied in the papilla foliata of 

 the rabbit, a patch of fine, parallel wrinkles found on each side of the 

 back part of the tongue of the animal. The taste-bud is a somewhat globular 

 body seated in the folds of mucous membrane between the furrows of the 

 papilla. It is made up of a sheath of flattened, fusiform cells enclosing a 

 number of rod-like cells each of which terminates in a hair-like process. These 

 cells surround a central pore which opens into a furrow of the papilla. 

 The hair-bearing cells recall the appearance of the olfactory rod-cells, and 

 are probably the true sensory cells of taste, since between them terminate the 

 filaments of the gustatory nerve. In the human tongue taste-buds are con- 

 fined to the fungiform papillae, seen often as red dots scattered over the upper 

 surface; to the circumvallate papillae, the pores of the buds opening into the 

 groove around the papilla; and to an area just in front of the anterior pillar 

 of the fauces, which somewhat resembles the papilla foliata of the rabbit. 



The sensory nerves distributed to the tongue include filaments from the 

 glosso-pharyngeal, the lingual branch of the fifth, and the chorda tympani. 

 The relation of these nerves to the sense of taste has been the occasion of 

 much dispute. The weight of evidence probably favors the belief that the 

 glosso-pharyngeal is the nerve of taste for the posterior third of the tongue, 

 while the lingual and, to some extent, the chorda carry taste-impressions from 

 the anterior two-thirds. Clinical cases have been cited to show that all the 



1 V. Vintechgau : " Geruchsinn," Hermann's Handbnch der Physiologie, iii. 2, 1880. 

 2 Cameror: Zeiischrifl fur Biologie, 1870, vi. 8. 440; Wilczynsky : Hofmann und Schwalbe's 

 Jahresbericht der Physiol., 1875. 



