PAIN SENSATIONS 443 



The cold and hot spots are always distinct, some portions 

 of the skin being sensitive only to cold, some to heat. The 

 cold spots are much more numerous and more sensitive 

 to stimuli than the heat spots. The sensitiveness to 

 stimuli varies for different areas of the body, the order 

 of sensitiveness to temperature being as follows: tip of 

 tongue, eyelids, forehead, cheek, lips, limbs, trunk. Tem- 

 perature sense is always relative to the temperature of 

 the skin, that is, objects colder than the skin stimulate 

 cold spots and feel cold, and vice versa. Hot and cold spots 

 may be stimulated by mechanical and chemical stimuli. 

 Menthol, for example, gives a cold sensation. They may 

 also be stimulated from within, the skin feeling hot in 

 case of fevers being a case in point. No distinctive end 

 organs for these sensations have as yet been discovered. 



Pain sensations. — It used to be thought that pain was 

 the result simply of excessive stimulation of sensorj'^ fibers. 

 Thus, if a pressure nerve was stinmlated by too grea,t a 

 weight the result was a sensation of pain added to that of 

 pressure but conveyed by the same impulse. It is now 

 believed that pain sensations of the skin at least are pro- 

 duced by special impulses which pass over special fibers 

 and that these fibers are distributed in spots as are the 

 other cutaneous nerve ends. The results of careful experi- 

 mentation in locating and determining the peculiarities of 

 the skin pain spots may be summarized as follows: 



Summary. — (a) Pain spots are distinct from and more numerous 

 than other cutaneous sense spots. (6) Pain spots of different regions 

 vary greatly in the amount of stimulus necessary to produce a 

 sensation. Those of the eye being much more sensitive than 

 those of the fingers. All, however, require much greater stimulus 

 than those of temperature or pressure, (c) The nerve fibers 



