PHYSIOLOGY OF GUSTATION 141 



of lithium, which are apparently almost tasteless. Am- 

 monium and calcium ions are also bitter in taste. In 

 picric acid the sour taste of the hydrogen ion is probably 

 completely masked by the bitter taste of the picric anion 

 though the taste of this substance as well as that of ether, 

 dextro-mannose, the glucosides and other such substances 

 appears never to have been fully investigated. 



But the substances that are especially characterized 

 by bitter tastes are the alkaloids. These include such 

 compounds as morphine, cocaine, pilocarpine, quinine, 

 nicotine, and strychnine, the bitterest of all substances. 

 In aqueous solution these substances are the most effec- 

 tive agents in exciting the bitter taste. Gley and Richet 

 (1885) determined that strychnine monochloride could be 

 tasted at 0.0006 gram per liter of water. Of such a 

 solution 5 cubic centimeters, which was the volume used 

 by these investigators in their individual tests, contains 

 only 0.000005 gram of the bitter material and yet this very 

 small amount produces a pronounced taste. Quinine 

 hydrochloride can be tasted in a solution as dilute as 

 0.00004 molar (Parker and Stabler, 1913). Thus bitter 

 substances far exceed hydrogen ions in their capacity to 

 stimulate at high dilution. 



What peculiar chemical feature is characteristic of 

 bitter organic substances whereby they excite this taste 

 is at best, poorly understood. Henry (1895) pointed out 

 that the bitter compounds often included the group 



/CH,OH 

 NO 2 -C- 

 \ 



and this was confirmed by Cohn (1914) whose extensive 

 study of the sapid organic compounds led him to the con- 

 clusion that there were several such groups, the presence 



