Chemoreception 457 



most birds is similar to that of reptiles. The nostrils are narrow, horn encased, 

 and dry. The tongue papillae contain taste-sensitive nerve endings, but they 

 are not well developed. 1 he presence of asafoetida, anise, oil of lavender, or 

 prussic acid has no effect on the selection of food by turkeys. Neither do bit- 

 ter and sweet substances affect selection of food among herring gulls. Ca- 

 naries do not object to bitter aloes, but they do prefer sugar solution to plain 

 water and refuse salt or sour solutions at concentrations above the human 

 threshold. 



Among mammals the sense of taste seems to be well developed, but there 

 is little definite information available. Mice apparently can taste saccharin, 

 quinine, salt, and acid, and, as far as the evidence indicates, seem to have 

 about the same preferences as man, i.e., they react most positively to sweet 

 and most negatively to bitter. Olfaction among lower mammals has been 

 studied extensively only in rodents and dogs. Rats and other rodents either 

 have a very poor sense of smell or else they learn with great diflficulty when 

 olfactory cues are used in training. All evidence indicates that their olfactory 

 acuity is far below that of man. 



In dogs, however, there is a vast amount of excellent evidence of an olfac- 

 tory sense far superior to man's. Dogs are capable of following human and 

 other animal trails and of selecting objects which have been handled by their 

 master. Dogs can also be trained to detect underground fungi (e.g., truffles) 

 with great accuracy. In trailing there are several possible types of olfactory 

 cues. These are: (1) earth odor from the compression and consequent 

 stronger vaporization; (2) plant odor from destroyed vegetation; (3) odor 

 traces from shoes and shoe polish; (4) odor traces from decaying animal or 

 other organic matter; and (5) body odor of a specific individual. 



THE PHYSIOLOGY OF CHEMORECEPTION 



Sensitivity of Chemoreceptors. Olfactory and gustatory chemoreceptors 

 and those mediating the common chemical sense can be differentiated on the 

 basis of their sensitivity. Parker and Stabler^" have demonstrated that in 

 man the threshold concentrations of ethanol required for stimulation of the 

 olfactory, gustatory, and common chemical sense-organs are 0.000125 M, 3 M 

 and 5 to 10 M. Comparable values have been determined as rejection thres- 

 holds in two species of flies. As a gas ethanol is rejected by the housefly at 

 0.005 M and, in solution with 0.1 M sucrose in contact with the tarsal recep- 

 tors of blowflies, it is rejected at 3.2 M concentration.-" These results indi- 

 cate that comparison of the stimulating effectiveness of the same compound 

 on the three types of chemoreceptors of the same or closely related species 

 permits a convenient classification. However, on examination of the rela- 

 tive sensitivities of taste and smell, when the thresholds for the most familiar 

 smell and taste substances are compared and expressed in terms of molecules 

 per cubic millimeter, a continuous spectrum and overlapping stimulating ef- 

 fectiveness is observed.''^^ 



Olfaction. It is generally recognized that the sense of olfaction is much 

 more acute than the sense of taste. Man can detect the odor of methyl mer- 

 captan at an approximate concentration of 9 X 10"^^ M. One milligram of 

 skatol, to which man is even more sensitive, would make a hall, 500 meters 



