Exercise XXVI 



SENSORY RECEPTORS 131 



same solutions stimulate the sense of smell, and 

 taste and smell together give us composite sen- 

 sations of flavor. 



Experiment 



Again, work in pairs, one student serving as 

 subject, the other as experimenter. The experi- 

 menter should moisten small rolls of filter paper 

 in each of the following solutions: 



quinine sulphate, 

 5% sugar (sucrose), 

 10% sodium chloride, 

 1% acetic acid, 



and, after shaking off excess liquid, apply each 

 in turn with forceps to different regions of the 

 tongue of the subject, for about 10 seconds. 

 At each application the subject should report the 

 sensation as "bitter," "sweet," "salt," "sour," 

 or "none," and should rinse the mouth with 

 water after each test. 



On a diagram of the tongue mark each region 

 from which sensation is reported, using a dif- 

 ferent symbol (circle, square, triangle, cross) for 

 each primary taste. 



To demonstrate the role of smell in deciding 

 flavor, alternately place a bit of apple and a bit 

 of onion on the subject's tongue, while he keeps 

 his eyes closed and holds his nose shut. Can the 

 subject distinguish them by taste alone? 



Demonstration 



Taste and smell are confined to mucous sur- 

 faces. The stimuli for these chemo-receptors 

 are always substances in aqueous solution. For 

 substances to reach our olfactory areas and 

 stimulate smell sensations they must obviously 

 be in the gaseous state, so that they can be 

 inhaled; but in this case also these substances 

 must dissolve in the layer of mucous that covers 

 the olfactory patch before they can stimulate 

 the smell receptors. 



Animals which have wet skins apparently 

 have such chemo-receptors distributed over 

 large areas or the entire surface. This is true 

 in general of fishes and amphibia. 



Destroy the brain of a frog by pithing or by 

 cutting off the head just at the angle of the jaws. 

 Allow the frog to lie undisturbed for a time to 

 recover from the shock. Now lay a bit of filter 

 paper soaked in dilute acid on its flank, and 

 observe what happens. Touch a similar piece 

 of filter paper soaked in the same acid to your 

 tongue. Note that to you this is a stimulus for 

 taste, not for pain. Presumably it is the same 

 kind of thing for the frog, and unlike us, the 

 frog apparently can taste all over. 



This response in the absence of the brain is 

 an extraordinary demonstration of a complex 

 spinal reflex. Here a mild stimulus, no more 

 than "distasteful," evokes a reasonable and 

 accurate response, all handled at the level of 

 the spinal cord. 



SENSORY JUDGMENTS OF INTENSITY: 

 THE ESTIMATION OF WEIGHT 



All measurements are ultimately sensory 

 judgments of quantity. Yet the response of a 

 receptor to a stimulus varies with duration and 

 state of adaptation, so that sensory reports of 

 intensity of stimulation are at best relative. In 

 general, as regards intensity, our receptors per- 

 mit only three kinds of measurement: (a) the 

 absolute threshold: the strength of stimulus that 

 just excites the sensation; (b) the intensities at 

 which two stimuli seem just equal; (c) the 

 intensities at which two stimuli are just per- 

 ceptibly unequal. This last is the "difference 

 threshold." It plays the same role in our estima- 

 tion of intensity as the two-point threshold does 

 in our estimation of space. 



Beyond these judgments, all measurement 

 ceases. So, for example, I can say accurately 

 the intensity of light that is just visible, its 

 absolute threshold; or how much I need to 

 increase the intensity of a light to make it just 

 perceptibly brighter, the difference threshold; 



