756 



LABORATORY DIRECTIONS FOR EXPERIMENTS ON THE 

 SENSE ORGANS. 



i. Touch. Use the small compasses with rounded tips provided for 

 the purpose, and determine the power of localization of the sense of touch 

 as follows: Have the person observed close his eyes, then touch different 

 parts of the skin, of the hand, arm, face, neck, etc., and let the observed one 

 announce the exact point touched. 



The localization can also be determined by touching two points on the 

 skin with the points of the compasses separated by varying distances. Ex- 

 amine especially the skin on the forearm, on the back of the hand, on the 



FIG. 480. Aristotle's Experiment. 



palm of the hand, the tips of the fingers, and at different points on the face, 

 including the lips and tip of the tongue. Touch these regions of the skin 

 with either one or with two points of the compasses, and allow the person 

 observed to announce results, drawing your conclusions according to the 

 principle of trial and error. Make a table showing the power of local dis- 

 crimination in the different regions. 



2. Aristotle's Touch Experiment. Roll the tips of the middle and 

 index fingers over a marble and note that the sensation from the two fingers 

 is interpreted as that of a single object. Now cross the fingers and repeat 

 the experiment. This time there is the sensation of touching two spheres. 



3. Temperature Sensations. It is a common experience that the 

 hand brought in the neighborhood of a warm or a cold object develops the 

 sensation of warmth or cold. Examine a given small area of the back of 

 the hand, or a square centimeter on the palmar surface of the wrist. Use 

 the thermoesthesiometer, or a pencil with large diameter of the carbon 

 warmed in a sand bath. Certain points will give stronger sensation of 

 heat than others. Map these out carefully. Examine the same area for 

 the cold. A large number of cold spots will be found and they will not 

 coincide with the warm spots, figure 421. 



