This section is from the book "A Text-Book Of Pharmacology, Therapeutics And Materia Medica", by T. Lauder Brunton. Also available from Amazon: A text-book of pharmacology, therapeutics and materia medica.
It is not so easy to prove positively that a drug has increased as that it has diminished the excitability of motor nerves. The fact that the nerves of the poisoned leg are found to be more excitable than those of the ligatured one in such experiments as those just described, does not prove it, for it must be borne in mind that the arrest of the circulation in the ligatured leg lessens the excitability of the muscles and the nerves in it. This effect of the ligature strengthens the proof that a drug has produced paralysis when we find that, in spite of the freer circulation, the poisoned leg is less irritable than the ligatured one;. but it prevents our concluding that the drug has increased excitability when we find that the poisoned leg responds more readily to stimuli than the ligatured one.
To try whether a drug increases excitability we treat two muscles with saline solution, and after ascertaining that their excitability is alike we add the drug to be tested to the saline solution in which one muscle is steeped, and after some time test the excitability again. If the muscle in the poisoned saline solution becomes more excitable than the other, we conclude that the increase is due to the action of the drug.