"The prospect of finding anybody out in anything, would have kept Miss Miggs awake under the influence of henbane. "

Henbane \Hen"bane`\, n. [Hen + bane.] (Bot.)
A plant of the genus Hyoscyamus (H. niger). All parts of
the plant are poisonous, and the leaves are used for the same
purposes as belladonna. It is poisonous to domestic fowls;
whence the name. Called also, stinking nightshade, from the
fetid odor of the plant. See Hyoscyamus.
Here is a URL about henbane. I have always been skeptical that Hamlet's uncle could have killed the King by putting henbane in his ear. I just don't believe it would be absorbed.
http://library.thinkquest.org/C007974/1_1hen.htm?tqskip1=1&tqtime=1002