Deadly things come in small packages

Sixteenth century European travellers to South America returned with many stories, one of which was that the native South Americans were using arrows tipped with poisons for hunting and defence. Known locally as 'ourari', 'urari' and 'urali', these poisons, which we now know as curare, were a dried extract from the plant Chondodendrum tomentosum  found in these regions.  

By the mid-1800s, scientists, notably French physiologist Claude Bernard, had confirmed by experiment that curare worked by blocking the transmission of nerve impulses to the muscles.¹ Injection of the arrow poison into the bloodstream caused death by respiratory failure because the chest and abdominal muscles became paralysed. Indeed, these molecules are only effective by injection into the bloodstream and cannot be absorbed from the digestive system, a point which was important to the native Indians who used the poisons for hunting for food. 

We now know that curare acts as an antagonist to the chemical signaling molecule, or neurotransmitter, acetylcholine.