This section is from the book "Materia Medica And Therapeutics Inorganic Substances", by Charles D. F. Phillips. Also available from Amazon: Materia medica and therapeutics.
Arguing from the conditions already described, and from the fact that poisoning by prussic acid is most rapid if the vapor be inhaled, Preyer concluded that its chief action was exerted on the vagus terminals in the lungs, stimulation, or rather irritation of them being propagated to the respiratory centre, and causing the phenomena of asphyxia. That the stimulation was not, in normal conditions, exerted directly on the respiratory centre he held to be evident from the fact, already noted, that section of the vagi - i.e., interruption of communication between the ends and the centre - delayed the course of the poisoning and the time of death: the occasional occurrence of tetanic spasm of diaphragm he explained by a secondary transfer of irritation from the medulla to the phrenic nerve. In animals with divided vagi, the respiratory changes were somewhat different, and death in such cases was explained either by an action on the terminals of other (unknown) nerves of the lungs, or by direct action on the centre in the medulla, or when occurring under large doses, by direct paralysis of the heart.