The reality of the olfactory influences with which we have been concerned, however slight they may sometimes appear, is shown by the fact that odors, both agreeable and disagreeable, are stimulants, obeying the laws which hold good for stimulants generally. They whip up the nervous energies momentarily, but in the end, if the excitation is excessive and prolonged, they produce fatigue and exhaustion. This is clearly shown by Féré's elaborate experiments on the influences of odors, as compared with other sensory stimulants, on the amount of muscular work performed with the ergograph.[79] Commenting on the remark of Bernardin de Saint-Pierre, that "man uses perfumes to impart energy to his passion," Féré remarks: "But perfumes cannot keep up the fires which they light." Their prolonged use involves fatigue, which is not different from that produced by excessive work, and reproduces all the bodily and psychic accompaniments of excessive work.[80] It is well known that workers in perfumes are apt to suffer from the inhalation of the odors amid which they live. Dealers in musk are said to be specially liable to precocious dementia. The symptoms generally experienced by the men and women who work in vanilla factories where the crude fruit is prepared for commerce have often been studied and are well known. They are due to the inhalation of the scent, which has all the properties of the aromatic aldehydes, and include skin eruptions,[81] general excitement, sleeplessness, headache, excessive menstruation, and irritable bladder. There is nearly always sexual excitement, which may be very pronounced.[82] We are here in the presence, it may be insisted, not of a nervous influence only, but of a direct effect of odor on the vital processes. The experiments of Tardif on the influence of perfumes on frogs and rabbits showed that a poisonous effect was exerted;[83] while Féré, by incubating fowls' eggs in the presence of musk, found repeatedly that many abnormalities occurred, and that development was retarded even in the embryos that remained normal; while he obtained somewhat similar results by using essences of lavender, cloves, etc.[84] The influence of odors is thus deeper than is indicated by their nervous effects; they act directly on nutrition. We are led, as Passy remarks, to regard odors as very intimately related to the physiological properties of organic substances, and the sense of smell as a detached fragment of generally sensibility, reacting to the same stimuli as general sensibility, but highly specialized in view of its protective function.