284 THE SPECIAL SENSES. 



delicacy of the sense of pressure may be measured by determining 

 the minimal pressure necessary to arouse a sensation, that is, 

 the threshold stimulus, or it may be estimated in terms of the 

 power of discriminating two contiguous stimuli, that is, the mini- 

 mal distance that two points must be apart in order for the sensa- 

 tions to be recognized as distinct. The two methods of measure- 

 ment do not coincide. As determined by the threshold stimulus, 

 the greatest delicacy is exhibited by the skin of the face, the fore- 

 head, and temples. According to the older methods of measure- 

 ment, the forehead will perceive a pressure of 2 mgs., while the skin 

 of the tips of the fingers needs a pressure of from 5 to 15 mgs. to 

 arouse a perceptible sensation. The back of the hand or the arm 

 is more sensitive from this standpoint than the tips of the fingers. 

 When measured by the power of discriminating two points 

 fchat is, the localizing sense the tips of the fingers are far more 

 sensitive than the skin of the face or of the arm. This latter prop- 

 erty, in fact, stands in relation to the closeness of the pressure 

 points to one another. The localizing sense may be determined 

 by Weber's method of using a pair of compasses with blunt points. 

 For any given area of the skin the power of discrimination or local- 

 ization is expressed in terms of the number of millimeters between 

 the two points at which they are just distinguished as two separate 

 sensations when applied simultaneously to the skin. Instruments 

 made for this purpose are designated as esthesiometers. They 

 carry two points the distance of which apart can be readily adjusted 

 and read off on a scale. The most satisfactory form of esthesiom- 

 eter is that devised by von Frey. The two points in this case are 

 made by long, rather stiff hairs whose pressure can be made quite 

 uniform. According to the older measurements, the discriminat- 

 ing sense of different parts of the skin varies greatly, as is shown by 

 the accompanying table : 



Tip of the tongue 1.1 mms. 



Tip of finger, palmar surface 2.3 



Second phalanx finger, palmar surface 4.5 



First phalanx finger, palmar surface 5.5 



Third phalanx finger, dorsal surface 6.8 



Middle of palm 8 to 9 



Second phalanx finger, dorsal surface 11.3 



Forehead 22.6 



Back of the hand 31.6 



Forearm 40.6 



Sternum 45 



Along the spine 54 



Middle of neck or back. . . : 67.7 



The tips of the tongue and the fingers are, therefore, the most 

 delicate surfaces, and that the tongue surpasses the fingers in this 

 respect is easily within the experience of everyone who will recall 

 the ease with which small objects between the teeth are detected by 



