The idea that nerves may communicate with other cells by releasing small quantities of chemicals at their junctions may have arisen from observations of the effects of poisons on animals. It was found that some poisons could mimic the effects of stimulating certain nerves, and it must have occurred to somebody that nerves release chemicals in response to stimulation. Histological studies showed that there is always a gap between the nerve ending and the target tissue, and that this gap must be crossed if the signal from the nerve is to reach its target.