50 PHYSIOLOGY CHAP. 



distinct from those of the other modalities of cutaneous sensa- 

 tion. 



Goldscheider succeeded in proving that the cutaneous spots 

 for heat and cold are normally analgesic ; that pressure spots, on 

 the contrary, when excited with strong stimuli give rise to 

 intense pain; and that the area surrounding the pressure spots 

 (and provided, in his opinion, with nerves of common sensibility) 

 reacts to tactile and pain stimuli, but far more feebly than the 

 pressure points. 



Von Frey obtained different results from his wider and more 

 accurate researches. He showed that with suitable mechanical 

 stimuli it is possible to demonstrate the existence of well-circum- 

 scribed spots, which do not usually coincide with the tactile spots, 

 in which sensibility to pain is maximal. To obtain pain sensa- 

 tions unaccompanied with sensations of pressure or contact, it is 

 necessary to use sharp points, to moisten the epidermis previously, 

 and to excite the skin where the touch spots are far apart. Even 

 with chemical stimuli, and under certain conditions with electrical 

 stimuli, it is possible to produce isolated sensations of pain. 



Pain spots are distinguished from tactile spots by a longer 

 latent period, and by being four times as numerous (on an average 

 more than 100 to 1 sq. cm.). 



There is also a marked difference in the minimal value for 

 mechanical stimulation between tactile points and pain spots, 

 according to the area of the excited surfaces. On stimulating 

 surfaces of 3-12 sq. mm. the sensibility of the nerve-endings to 

 pressure is a thousand times greater than that to pain (v. Frey). 

 But as the excited surface diminishes, a given mechanical stimulus 

 becomes gradually more effective for the pain spots, till with a 

 minimal surface the threshold for pain may be lower than that 

 for pressure. 



It was formerly believed that there could not be pain unless 

 the skin were excited with stimuli strong enough to act directly 

 on the subjacent nerves (Weber). But more careful investigation 

 has proved that it is possible to excite pain with such weak 

 mechanical (v. Frey) and thermal (Thunberg) stimuli that all 

 direct excitation of the nerve -fibres must be excluded. It is 

 further to be noted that when the skin is excited by effective 

 instantaneous stimuli (mechanical or thermal) the sensation of 

 pain has a very long latent period (0'9 sec.). While on the one 

 hand this excludes the hypothesis of any direct action on the 

 nerves, which never have this enormous latent period of excita- 

 tion, it shows on the other hand that the stimulus acts on nerve- 

 endings which are capable of transforming weak stimuli into 

 neural excitation by some physico-chemical process (v. Frey). 



The topography of the pain sensibility of the skin (cutaneous 

 algesimetry) has been the subject of much research, mainly from a 



