INFLUENCE OF CHEMICAL SUBSTANCES. 



487 



it often happens that the muscular response is increased in intensity 

 when the propagation has traversed such a cooled region of the nerve, 

 and yet the rate of propagation is retarded (Fig. 256, B)} 



The cause of this anomaly may possibly be found in the simul- 

 taneous propagation of the condition produced at the proximal electrode 

 of the ascending exciting current, the condition known as anelectrotonus. 

 This is far more rapidly propagated than the excitatory state, and it 

 appears to be less readily produced in the cold than in the warm. As 

 it is a state of the nerve opposed to the change which constitutes an 

 excitation, any local circumstances which diminish its extent will 

 ceteris paribus augment that of the excitatory state by removing its 

 inhibitory influence. The marked difference between excitation by 

 descending and ascending currents under these circumstances favours 

 this explanation of the anomaly. 



The result opens up a most suggestive line of thought, since it implies that 

 propagation is not necessarily the simple transmission of an excitatory 

 change, but that it may he a complex disturbance, one element of which is 

 derived from a specific alteration produced at the pole of a stimulating 

 current. The transmitted effect thus may bear a specific character, determined 

 by the nature of the external stimulating agent which evoked it. We are thus 

 confronted by the possibility that an excitatory state, constituting by its 



1 / 



11 I I 



LL 



III 



LL 



juJUV-JUi 



INI' ~-~ " TT 



JlJUlJJlJUUUJUUJUUUUUULXJUUL^^ 

 I 3" - jo" 



Fig. 256. 



transmission what is familiarly termed a nervous impulse, may be different in 

 kind when it is evoked by a different form of stimulus. It is scarcely 

 necessary to dwell upon the importance of such variability in the character of 

 the transmitted nerve effect due to variations in the character of the exciting 

 cause. If true, its application to the excitatory phenomena of the sense 

 organs would modify the doctrine of Johannes Midler, which assumes the 

 peripheral sense organ to have been elaborated so as to be able, in response 

 to specific stimuli, to start excitatory states, which are then propagated along 

 nerve fibres having distinct terminal connections. Instead of this, it would 

 be possible on physiological grounds to assume that excitatory states, evoked 

 by different specific stimuli of the sense organ, might be transmitted along the 

 same nerve fibre, and yet be so different as to cause in each instance a 

 specific central response. 2 



Influence of chemical substances on excitability and conduct- 

 ivity. — The use of chemical substances for excitation has already been 

 alluded to, and the response evoked by such means has been described 

 as a series of muscular twitches, passing into a flickering tetanus and 

 culminating in a prolonged continuous contraction. 



If the kaolin paste forming the point of contact of a non-polarisable 

 electrode is the medium for the salt, and arrangements are made such 



1 Gotch. Journ. Ro>/. Inst., London, 1892. 



2 See also Rollett, Arch. f. d. ges. Physiol., Bonn, 1899, Bel. lxxiv. S. 463. 



