260 THE SPECIAL SENSES. 



temperature of 40 to 60 C, to a cold spot. In many cases this 

 spot is stimulated and a cold sensation is felt. The same result 

 may be felt at the instant of entering a hot bath. Many efforts 

 have been made to determine whether there is a specific kind of 

 end-organ for each of these senses. Numerous observers have 

 cut out the skin from cold or hot spots and examined the removed 

 part carefully by histological methods. The general result has been 

 that no distinctive end-organs have been found. Von Frey, how- 

 ever, believes that, although the heat spots are supplied simply 

 by a terminal end plexus, the cold spots in some places at least have 

 as a special end-organ the end-bulbs of Krause. This conclusion is 

 based upon the fact that these end-bulbs are found in places, such 

 as the glans penis and conjunctiva, where the cold sense is espe- 

 cially prominent or exclusively present. 



The Sense of Pressure. The pressure points are smaller and 

 more numerous than the cold or warm spots. Von Frey has shown 

 that in those portions of the body that are supplied with hairs 

 the pressure points lie over the hair follicles. The pressure nerve 

 fibers, in fact, terminate in a ring surrounding the hair follicle, 

 this form 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 like 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 delicate a 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 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 



