IIAI-. ix ELECTRICAL EXCITATION OF NERVE 117 



wheii the current is opened. The directly excited muscle of 

 course gives a different reaction, inasmuch as it remains shortened, 

 under certain conditions, during the passage of the current, and 

 even for some time after it has been opened, though the contrac- 

 tion may be only local (persistent closing, or opening contraction). 

 A visible persistent effect of current may also (infra) appear with 

 indirect excitation of the muscle, both during closure and after 

 opening the current. This is expressed in a more or less prolonged 

 contraction of the muscle excited through the nerve, which may 

 be continuous, or interrupted by single twitches, and which, from 

 its resemblance to the tetanic form of contraction due to inter- 

 rupted rhythmical excitation, is known as closure, or openiny, tetanus. 



Bitter (1798) was the first to point out that an indirectly 

 excited muscle may, after prolonged closure of a powerful battery- 

 current, fall, on breaking the circuit, into a state of persistent 

 tetanic excitation a manifestation named, after its discoverer, 

 Hitter's opening tetanus. With this we shall have to deal later : 

 for the moment it is sufficient to point out that nerve 1 , like 

 muscle, may, when a constant current is opened, fall into a pro- 

 tracted state of excitation, so that the opening tetanus and the 

 persistent opening contraction are practically equivalent pheno- 

 mena. A similar persistent excitation (as first remarked by 

 Pfliiger, 2) appears sometimes during closure of the current, 

 and may be perfectly regular. Pfliiger obtained this " tetanic '* 

 effect with quite weak currents ; it increased up to a certain point 

 with increasing intensity of stimulation, and then declined again. 

 Under the most favourable conditions, i.e. with maximal excit- 

 ability of nerve, closure tetanus makes its appearance at any 

 effective strength of current. This is more especially the case 

 with frogs kept in a low temperature for some time before making 

 the preparation (" cooled frogs "). 



The excessive excitability of such preparations is a well-known 

 physiological fact, and we shall frequently have to refer to it. It 

 may be said, as a general rule, that the nerves of all frogs kept 

 at a temperature below 10 C. will sooner or later acquire the 

 property of being tetanically excited by constant currents (v. 

 Frey, 3, Fig. 171). 



Similar excitability is exhibited by nerves at a high temperature 

 (Engelmann, 4), provided they are in a certain stage of dehydration 

 (from evaporation, or treatment with NaCl). In both cases there 



