THERMAL SENSATIONS 447 



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FIG. 13. Spike potentials from two cold fibers recorded on cooling the tongue of the cat from 

 34° to 32 °C. .4, the temperature drop; B, after i min. , C, after 2 min.; D, after 4 min.; E, after 15 

 min. [From Hensel & Zottcrman (54).] 



FIG. 14. .Spikes recorded from a single cold fiber (cat) at different constant temperatures. 

 Time marks, 50 cps. [From Hensel & Zotterman (54).] 



pulses per sec. from which the three spikes can be 

 seen before the coohng starts (fig. 1 3). The thermode 

 was then quickly cooled down to 32°C. Immediately 

 the frequency of the monophasic fiber rose to 35 

 impulses per sec. and simultaneously there was a 

 discharge of thesecond diphasic fiber which, however, 

 after a few seconds ceased again, while the discharge 

 of the first fiber adjusted itself to a final constant fre- 

 quency which after i min. attained a value of 9.3 

 impulses per sec. After that there is practically no 

 more change. The cold receptor goes on discharging 

 at a fairly regular rhythm for minutes or even hours 

 with remarkable constancy if the temperature of the 

 surface of the tongue is kept constant. 



Figure 14 gives an example of the discharge of a 

 single cold fiber after adjustment to a constant final 

 value of frequency at different constant temperatures. 

 At a temperature of 4i.3°C there is no discharge in 

 this fiber while already at a constant temperature of 

 40.5°C there is a low frequency of about i impulse 



per sec. The upper limit at which a steady discharge 

 of the cold fiber appears, called the steady threshold 

 temperature for this particular cold fiber, lay between 

 41.3° and 40.5°C, i.e. above the ordinary blood 

 temperature. At this temperature (38°C) the fre- 

 quency of the steady discharge was 5 impulses per sec. 

 and the maximum about 3o°C. Below this tempera- 

 ture the steady discharge declines and at lower tem- 

 peratures the discharge generally becomes irregular, 

 occurring in beats of two or three impulses. Between 

 15° and io°C the average discharge increases again 

 (17) to disappear entirely between 12° and io°C. 

 No steady discharge of any cold fibers has been seen 

 at a temperature below 8°C. 



The diagram in figure 1 5 shows the steady dis- 

 charge of a cold fiber as a function of the temperature. 

 The experiiTient was conducted in such a way that 

 the frequency was determined at definite teinperature 

 steps from warm to cold. After reaching the lowest 

 temperature — in about 2 hr. — the impulse frequency 



