THERMAL SENSATIONS 



445 



tinct volley of small spikes. When the current of air is 

 forceful enough to make a noticeable deformation on 

 the tongue, larger spikes appear among an increased 

 number of small ones (fig. 1 1 B'). If the air in the syringe 

 is successively warmed, a point is reached when the 

 air stream does not elicit any small spikes, while the 

 large ones still appear as soon as the pressure is raised 

 sufficiently to occasion a noticeable deformation on 

 the surface of the tongue. 



When one drop of hot water (8o°C) is applied to 

 the tongue, two types of spikes may be observed be- 

 sides the large spikes signalling the impact of the water 

 drop (fig. 11^). A careful examination of the record 

 reveals two types of spikes of which one derives from 

 warm fibers and the other, a somewhat smaller and 

 apparently more slowly conducted spike, derives from 

 a pain fiber. In this way it was possible to show that 

 cold and warmth as well as pain are mediated in 

 specific nerve fibers (96). 



Thermal Receptors in Cold-Blooded Animals 



Electrophysiological investigations by Sand (77) 

 on single fibers from the Lorenzinian ampullae of 

 Raja showed that the receptors when kept at constant 

 temperature were discharging continuously at a 

 steady rate which varied with the prevailing tempera- 

 ture. Cooling caused an immediate increase in the 

 frequency while warming led to the reverse eff"ect. 



Recent investigations by Hensel (49, 50) on Scyllium 

 have confirmed Sand's original discovery in all 

 details. At constant temperature the steady discharge 

 in single fibers reaches a maximum of about 65 im- 

 pulses per sec. at about 20°C. The temperature limits 

 for steady discharge were 2 to 34°C. In this range of 

 temperature, sudden cooling produces in single fibers 

 a rapid increase in frequency up to 180 impulses per 

 sec. followed by rapid adaptation to a low steady rate 

 of discharge. The ampullae react definitely to a 

 change in temperature of o.o5°C. Warming produces 

 an immediate decrease or abolition of the discharge 

 which then slowly attains a new steady value. The 

 ampullae are not sensitive to mechanical stimulation 

 and they thus behave qualitatively in every respect 

 like the cold receptors of mammals. Quantitatively 

 they appear to be even more sensitive. 



The remarkable infrared receptors of the facial pits 

 of the pit viper (Crotalidae) have been extensively 

 studied recently by Bullock & Diecke (13). The nerve 

 fibers from the facial pit usually show a continuous 

 nonrhythmic discharge in the absence of environ- 

 mental change. The adequate stimulus for increa.sing 

 this activity is a relative increase in the influx or a 

 decrease in the efllux of radiant energy in the middle 

 and long infrared bands. Relative increases in efflux 

 or decreases in influx reduce or inhibit the steady 

 discharge. No response is obtained to sound vibration, 

 a number of chemicals or heat-filtered light, but 



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FIG. 1 1. Afferent spike potentials from different sensory fibers of a fine strand of the cat lingual 

 nerve obtained by applying different stimuli to the tongue. A. The effect of a drop of water at I4°C 

 falling on the tongue. B. First, the effect of a faint puff of air, which does not cause any visible de- 

 formation of the surface, followed by the effect of a stronger puff of air which makes a definite defer 

 mation. C. A drop at 8o°C falling upon the tongue. D. The effect of pressing a pointed rod into the 

 tongue. E. Squirting hot water (6o°C) over the tongue. [From Zotterman (96).] 



