486 



NERVE. 



more prolonged galvanic currents ; the warmed nerve being in a state 

 of greater molecular mobility, is best excited by energy which is rapidly 

 applied for the purpose. 1 



They also demonstrate that there is a difference between the 

 susceptibility to respond to the action of an external agent (excit- 

 ability), and the susceptibility to respond to the internal agent, that 

 is, the excitatory state in the neighbouring portion of the nerve ; this 

 latter is the basis of nerve transmission (conductivity). 



The propagation of the excitatory state is altered, as already stated, 

 by changes of temperature, as regards both its rate and the magnitude 

 of the transmitted excitatory state. 



The alteration was observed by Helmholtz, who found that, in the 

 case of the motor medullated nerves of the frog, the rate of transmission 

 was lessened by cold and increased by heat up to 25 CV 2 In every 

 class of nerve, and indeed in every excitable tissue through which an 

 excitatory state is propagated by its successive establishment in 



different portions, 

 the same altera- 

 tion in the trans- 

 mission of the 

 excitatory change 

 is produced by 

 variations of tem- 

 perature. On 

 cooling a portion 

 of a nerve, not 

 only is the rate 

 lessened, but the 

 excitatory state 

 diminishes in 

 amount as it tra- 

 verses the cooled 

 region, and on 

 emerging from 

 this it continues 

 with the same 

 diminished inten- 

 sity, so that it 

 evokes a smaller muscular response than it would otherwise do. 



The employment of a series of uniform electrical stimulations 

 recurring at regular intervals, and of minimal or nearly minimal 

 exciting intensity, demonstrates this with great clearness. On cooling 

 to 5 C. the muscular response is diminished or fails, on warming to 35 

 the response increases and becomes maximal (Fig. 256, A, left portion 

 of record). 



A striking discrepancy is sometimes obtained by the use of special 

 modes of stimulation. Thus, if a short galvanic current or a condenser 

 discharge, arranged to ascend the nerve, is used for the excitation of a 

 portion of the frog's sciatic remote from the muscle, and the peripheral 

 portion of the nerve is cooled 30 mm. below the seat of excitation, 



1 See Waller, "On the Characteristic of Nerve," Journ. PhysioL, Cambridge and 

 London, vol. xxiv. , loc. cit., and Proc. Roy. Soc. London, 1899. 



2 Helmholtz, loc. cit. 



FIG. 255. I, Response of gastrocnemius muscle to nerve excitation 

 every 2 seconds by a break induced current. Temperature of 

 nerve at seat of excitation varied from 5 to 30, as indicated 

 by recording thermometer, the rise of the line corresponding 

 to rise of temperature. The stimulus becomes inadequate 

 when the temperature falls. II, Nerve excited every 5 

 seconds by closure of a descending galvanic current. The 

 stimulus becomes inadequate as the temperature rises. 



