Curare is a crude extract from certain South American plants in the genera "Strychnos" and "Chondrodendron", originally brought to Europe by explorers such as Walter Raleigh It was known in the 19th century to have a paralysing effect, due in part to the studies of scientists like Claude Bernard. D-tubocurarine a "mono"-quaternary alkaloid was isolated from "Chondrodendron tomentosum" in 1942, and it was shown to be the major constituent in curare responsible for producing the paralysing effect. At that time, it was known that curare and, therefore, d-tubocurarine worked at the neuromuscular junction. The isolation of tubocurarine and its marketing as the drug Intocostrin led to more research in the field of neuromuscular-blocking drugs. Scientists figured out that the potency of tubocurarine was related to the separation distance between the two quaternary ammonium heads.