To expand on the esteemed Webster:

Hyoscine is an alkaloid derived from the henbane plant, similar in action to atropine. It is an antispasmodic, a hypnotic, and a mild diuretic in small doses (up to 1/70 grain). Larger amounts can cause dizziness, hallucinations, delerium, convulsions, and death.

Although hyoscine is less well known than other alkaloids, such as belladonna and atropine, it was notorious for a time as the poison used by Hawley Harvey Crippen to murder his wife Cora in 1910.

Also spelled hyocine.

Source: Taylor's Principles and Practice of Medical Jurisprudence