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A MANUAL OF VETERINARY PHYSIOLOGY 



is only sent through the middle piece of the nerve, it gives rise to 

 a current which passes through the whole length of the nerve in 

 the same direction as the exciting current, as shown by the galvano- 

 meters. Turning again to Fig. 126, A, it is evident that between the 

 region of increased and that of reduced irritability there must be a 

 region of neither increased nor diminished irritability. This is known 

 as the neutral point. When the polarising current is weak, this point 

 is nearer the anode ; when it is strong, it is nearer the kathode, 

 so that with very strong currents the whole intrapolar region may 

 be in a condition of anelectrotonus. A clear understanding of this 

 fact will render the subsequent observations intelligible. It will 

 be remembered that the muscle of a nerve-muscle preparation gave, 

 on stimulation, a twitch at ' make,' and another at ' break,' the 

 make contraction always starting from the kathode, and the break 

 contraction from the anode. These contractions arc due to the 

 development of electro tonic currents, and, provided the stimulating 

 current always remains of the same strength, the twitch at ' make * 

 and another at ' break ' will always be obtained. But it is known 

 that an increase or decrease in the strength of the polarising current 

 gives other results in the muscular response ; for instance, if the 

 current be weak (Fig. 126, B), contraction of the muscle only occurs at 

 ' make,' the kathode end of the nerve being the region of increased 

 excitability and conductivity, the anode end being in both these 

 respects decreased, and with a weak current lowered below the point 

 of possible electrical response. If the current be made stronger, 

 contraction occurs both at ' make ' and ' break.' If a very strong 

 current be passed through the nerve, contraction occurs at ' make ' 

 only, owing to the neutral point moving nearer the kathode, and so 

 diminishing the irritability of the anode pole (see Fig. 126, C) . If the 

 muscle be attached to the opposite end of the nerve — viz., nearest 

 the anode pole — and a very strong current sent through, the battery 

 current will be passing up the nerve, as it is termed — i.e., from the 

 muscle to the spinal cord (Fig. 126, D) — under these conditions there 

 is no difficulty in a ' break ' contraction occurring, as the muscle is 

 nearest to the battery current ; but the reduced excitability and con- 

 ductivity of the nerve prevents the current from reaching the kathode, 

 so that no contraction occurs at ' make.' Pfliiger, who investigated 

 these phenomena, formulated a Law of Contraction, which, after the 

 above explanation, may now be stated : 



When a particularly irritable nerve-muscle preparation is stimu- 

 lated with a constant current, it may pass into tetanus at either 

 1 make ' or ' break.' This is known as Ritter's tetanus. The nature 



