THE MOLECULAR THEORY OF EXCITATION 



605 



normal, and the resulting deflection of the galvanometer to 

 the right as the normal response. -The direct effect of the 

 coil S on the coil R may be regarded as negligible, when 

 they are separated from each other by a sufficient distance, 

 and this would be even more true if the intervening iron wire 

 were bent at an angle of 90 degrees. 



Employing this mode of obtaining records of response to 

 K- or . A-excitation, we meet with several curious analogies 

 to the responsive effects seen in living tissues, under the 

 electrical mode of stimulation. Electrical excitation of 

 nerve and muscle, for example, is most effective when it is 

 longitudinal, and ineffective when transverse. The same is 

 true of magnetic excitation of iron, where longitudinal 

 excitation is effectively transmitted to a great distance, 

 whereas transverse excitation is relatively ineffective. 



Again, in the case of the electrical excitation of living 

 tissues, it is at the instant of kathode-make, as we have seen, 

 that excitation is induced. Continued action exhibits in 

 general no effect. The same normal excitatory effect seen 

 at kathode-make, is induced again, but at anode-break. 

 Similarly, in the case of an iron wire, the normal galvano- 

 metric response is seen at the moment of K-magnetic 

 excitation, but not during its continuance. The same 

 excitation is also obtained, at break of A, or south polar 

 magnetisation. 



We are led from such close analogies, not only to visualise, 

 but also to obtain some insight into the sequence of 

 molecular events which is the concomitant of excitation. 

 I have already pointed out that excitation and its opposite, 

 depression, being phenomena of molecular distortion, it 

 is to be expected that a particular-directioned distortional 

 movement should be associated with one of these, and the 

 opposite with the other. We also know the further 

 suggestive fact that it is the sudden change of the environ- 

 ment, inducing a sudden responsive molecular disturbance, 

 that is most effective in bringing about excitation. The 

 latent period, and a slowly-rising excitation, correspond to the 



