280 



THE SPECIAL SENSES. 



of termination serving as an end-organ. On account of their 



position they are stimulated by any pressure exerted upon 

 the hair. The hair, indeed, acts Uke a lever and transmits any pres- 

 sure applied to it with increased intensity, acting, therefore, as re- 

 gards the pressure organ somewhat like the ear-bones in the case 

 of the endings of the auditory nerve. In parts of the body not 

 furnished with hairs the tactile or Meissner corpuscles are found 

 and these structures doubtless function as pressure end-organs. 

 They are particularly abundant in the parts of the hand and feet in 

 which a delicate sense of pressure is present in spite of a much thick- 

 ened epidermis. It has been estimated that for the entire surface 

 of the body, excluding the head region, there are about 500,000 

 of these pressure points. These points are close together on those 

 parts, such as the tongue and fingers, which have a delicate tactile 

 sense and more widely scattered where the sense is less developed. 

 The Threshold Stimulus and the Localizing Power. — The 

 delicacy of the sense of pressure may be measured by determinmg 

 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 — 

 that 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- 



