A^2 Comparative Animal Physiology 



in structure, are easily differentiated by man and by bees. As far as is 

 known, olfaction among insects is similar to olfaction in man. 



Gustatory Receptors. It is generally agreed that there are four fundamen- 

 tal tastes: sweet, sour, salt, and bitter. The taste buds mediating these four 

 tastes are more or less localized in certain regions of the tongue, as shown in 

 Fig. 150. The taste buds sensitive to sour are on the back lateral surfaces, 

 those for sweet and salt near the tip, and those for bitter toward the back. 

 The distribution is somewhat irregular, and there is a considerable overlap- 

 ping of the areas. However, when this separation of taste buds of different 

 sensitivities is compared with the thorough mixture of the rods and cones of 

 the eye, or of the olfactory cells, the simplicity of the problem of analyzing 

 taste, compared with that of analyzing color vision or olfaction, is astound- 

 ing. Here we have four definite sensations which are more or less localized 

 in four different regions of the tongue, differing in position by as much as 

 several centimeters, and which we can stimulate separately by merely plac- 

 ing the stimulating material on different parts of the tongue. This arrange- 

 ment permits an opportunity for experimentation which has been only par- 

 tially exploited. 



The sour taste which is associated with all acid foods, e. g., vinegar, sour 

 milk, citrus fruits, and apples, is definitely a function of the hydrogen ion 

 concentration. The pH at which an acid becomes detectable by its sour 

 taste varies. For pure mineral acids the threshold pH is between 3.4 and 

 3.5, and for pure solutions of acetic, lactic, citric, and butyric acids, it is be- 

 tween pH 3.7 and 3.9. If a buffer is added, e. g., if Na-acetate is added to 

 acetic acid, the threshold for sour taste is lowered to pH 5.6. With con- 

 centrated buffers even neutral solutions may have a sour taste. Thus the 

 excitation of receptors mediating the sour taste may not depend simply on 

 the pH of the solution bathing the receptors. 



The typical salt taste is that of sodium chloride, but a number of other 

 salts (KCl, NH4CI, LiCl, RbCl, NaBr, NH4Br, LiBr, Nal and Lil) are also 

 predominantly saltv in taste. Other salts are both salty and bitter (KBr, NH4I), 

 and still others (CsCl, RbBr, CsBr, KI, Rbl, and Csl) are predominantly 

 bitter.^" The sweet taste is elicited by many classes of chemicals. Salts of 

 beryllium, lead formate, acetate, propionate, and isovalerionate elicit a sweet 

 taste attributed to the metallic portion of the molecules. Among the organic 

 compounds which elicit a sweet taste are the sugars, dihydroxy and polyhy- 

 droxy alcohols, saccharin, dulcin, amino acids, and many esters. The sweet 

 taste is quite often associated with a bitter taste, as is illustrated in the fol- 

 lowing listing from Cohn.^° 



Sweet: glycol, glycerin, glycine, sugar, phloroglucinol, saccharin, dulcin 



Sweetish: r-climethyl tartrate 



Tasteless then sweet: sodium naphthionate 



Sweet and bitter: glyceraldehydc, p-chlorsaccharin, d-valin 



Sweet then bitter: urotropin, aloin, p-methylsaccharin 



Sweet then disagreeably bitter: m-chlorphenyl alanin 



Sweetish and bitter: guaiacol ester of isovalerianic acid 



Sweetish then bitter: magnesium benzoate, butyramide 



Sweet then bitterish: methylglyceraldehyde 



Sweetish then bitterish: Ethyl butyrate 



