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.
An immense step has been made of late years in our knowledge of the relation between chemical constitution and physiological action by the discoveries of Crum-Brown, Fraser, and Schroff, who have shown that by modifying artificially the chemical constitution of a drug it is possible to modify also its physiological action. And not only so, but they have shown that similar modifications in the chemical constitution of various drugs induce similar modifications in the action of their derivatives; thus they have found that by introducing methyl into the molecule of strychnine, brucine, and thebaine, the convulsive action exerted by these substances on the spinal cord was changed into a paralysing one exerted on the ends of the motor nerves. Other alkaloids, also, which do not exhibit a convulsive action, nevertheless exhibit a paralysing one when their constitution is altered by means of methyl; thus methyl-codeine, methyl-morphine, methyl-nicotine, methyl-atropine, methyl-quinine, methyl-veratrine, and several others, all exhibit this paralysing action (p. 150).
As a general rule, most of the compound radicals formed by the union of amidogen with the radicals of the marsh-gas series possess a paralysing action on motor nerves.
The subject of the connection between chemical constitution and physiological action is the most important one in pharmacology, and we shall have to return to it in considering the actions of various groups of organic substances.