"The question of the source of the states of stimulation in the nasal organs now arises. The idea suggests itself that the qualitative organ for olfactory stimuli may be Schneider's membrane and the quantitative organ (dstinct from this) may be the corpora cavernosa. Olfactory substances -- as indeed, you yourself believe, and as we know from flowers -- are breakdown products of the sexual metabolism; they would act as stimuli on both these organs. During menstruation and other sexual processes the body produces an increased Q of these substances and therefore of these stimuli. It would have to be decided whether these act on the nasal organs through the expiratory air or through the blood vessels; probably the latter, since one has no subjective sensation of smellbefore migraine. Thus the nose would, as it were, receive information about internal olfactory stimuli by means of the corpora cavernosa, just as it does about external stimuli by Schneider's membrane: one would come to grief from one's own body."
letter from Sigmund Freud to Wilhelm Fliess, Januaray 1, 1896