On the Vital Principle/Book 2/Chapter 9
It is less easy to define smell and the odorous object, than the subjects which have just been treated of, as the nature of odour is not so clear to us as is that of either sound or colour; and the reason of this is, that our sense of smell is inaccurate, is less delicate, in fact, than it is in many animals. Thus, man has but a coarse smell, and is never sensible of any thing odorous without associating therewith an impression of something painful or grateful; and this seems to indicate an organ imperfectly constituted. It is probable that colours are perceived by creatures which have hard eyes in this same manner, and that shades of colour invariably make upon them an impression of something to be afraid of or otherwise. The human race is circumstanced in a like manner with respect to odours; and there seems to be an analogy between taste and kinds of savours, and smell and kinds of odours, but as taste is a kind of touch, and touch of all man's senses the most perfect, his taste is more delicate than his smell. With respect to other senses, man is far behind many animals, but he is especially distinguished from them all by the accuracy of his Touch; and to this he is indebted for being of all the most intelligent. As proof of which, individuals of the human race are according to the constitution of this sense and nothing else, clever or dull for those with hard flesh are slow, and those, on the contrary, with soft flesh are quick of understanding.
As one savour is sweet and another bitter, so it is with odours; but some bodies impart an analogous savour and odour, impart, I mean, a sweet odour and a sweet savour, while other bodies give out their contraries. Some odours equally with savours are termed pungent, sour, and oily, but, as we have already explained, owing to their not being so distinguishable by us as savours, odours have derived their appellations from these, on account of the similarity of the objects from which they both proceed. Thus, the odour from saffron and honey is called sweet, that from thyme and other herbs of the kind pungent, and so for other bodies and odours.
There is a close analogy between the other senses and the hearing: for as it is sensible of the audible and the inaudible, so is vision of the visible and invisible, and smell of the odorous and the inodorous, and by inodorous is meant whatever is either altogether without odour, or has but a very faint odour; and a sense analogous to this is attached to the term insapid. The smell is perceptive through a medium, such as air or water, for aquatic animals seem to be sensible of odour; and so, likewise, are sanguineous and insanguineous creatures, as well as those which wing the air. Thus, some of these are to be seen proceeding from a distance towards food, of which they have been made sensible by the odour emanating from it. And hence the difficulty of determining why, if other creatures are sensible of odours in a like manner, man alone can smell neither when expiring nor when holding his breath but, only when inspiring; and this whether the odorous object be at a distance from or close to him, or placed immediately within the nostrils. It is common, it is true, to all the sentient organs to be insensible to impressions when objects are placed immediately upon them; but it is peculiar to man (as may be proved experimentally), to be unable to perceive odours without inspiring. So that as insanguineous creatures do not breathe, they ought to have some other sense besides those spoken of, but yet this cannot be, since they do perceive odour; for the perception of odour, whether agreeable or disagreeable, is smell; and as these appear to be destroyed by the same powerful odours as those which destroy man (odours, for instance, from pitch, sulphur, and other like substances), we must conclude that they have smell, although they do not breathe. The olfactory organ in man appears to differ from that in other animals as his eyes differ from those of creatures in which they are hard; for man's eyes are furnished with a rampart, and a kind of sheath in lids, without the elevation and drawing asunder of which he cannot see, while hard eyes, having no such provision, are instantly sensible of whatever may be present in the diaphanous medium. In accordance with this, the olfactory organ is, in such creatures, like the eye, uncovered; but, in such as breathe, it is furnished with a cover, which during inspiration is lifted up, as the veins and pores are then dilated. On which account, creatures which breathe cannot smell while in the water, as in order to smell they must inspire, and while in the water they cannot possibly inspire.
In fine, odour is derived from what is dry, as savour is from what is moist; and the olfactory organ, when in potentiality, is analogous to that from which odour is derived.
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Note 1, p. 109. And there seems to be an analogy, &c.] Aristotle[1] in thus making Touch superior to, and more influential than any other sense, (for it is the most perfect, he observes, of man's senses, although with respect to some others he is inferior to many animals,) is supported by Cuvier[2], who says, "that Touch is the most important of all the senses, and that its several degrees of perfection exercise a surprising influence over the nature of different animals; and that of all the vertebrata man has the most perfect Touch." It is difficult to attach a sense to the term hard or soft applied to flesh, which, by anatomical[3] description, corresponds with the muscular substance of the body; but man is said to have softer flesh than any animal[4], and on this account, through the delicacy of his sense of Touch, to be of all creatures the most intelligent. It is presumable that Aristotle was led to suppose, from this sense being spread, so to say, like the muscular substance, over the surface of the body, that its organ lies somewhere in or beneath the flesh, and thus to have concluded that a relative hardness or density of that substance, by impeding tangible impressions, may be the cause of, or concomitant with dulness of the faculties. The nervous system was then unknown, and Aristotle, so fond of analogies, might readily suppose that the Touch had, like other senses, its appointed organism; and, if there were such an organ, that it is extended over the body, and thus must be in or beneath the flesh. The Taste, as being a modification of Touch, was said to be more delicate in man than animals.
Note 2, p. 110. There is a close analogy.] A similar observation is made in the following chapter, and, besides bringing sentient perceptions under some general law, it was, probably, intended to shew that colour, sound, and odour, although inappreciable by our senses, may still be present. It shews, in fact, that our senses, being limited in their capacity of perception, are not to be relied upon when impressions are very greatly in excess or proportionally faint.
Note 3, p. 111. The smell is perceptive.] "That fishes smell," Aristotle[5] observes, "is shewn in their being taken by baits which have the particular odour, foul or grateful, to which they are attached." But modern science has, of course, determined both the seat and the structure of the olfactory organ in fishes; and shewn "how it is protected from the violent and incessant action of the currents of water required for respiration." Sanguineous[6] creatures are all such as have red blood, and insanguineous, those which, in place of red blood, have a pale bluish fluid circulating in their veins. These last include "insects, molluscs, crustacea, and creatures with more than four feet."
Note 4, p. 111. And hence the difficulty of determining, &c.] If the site and structure of the olfactory sense, in the lower forms of life, are still somewhat conjectural, it may well be supposed that the smell in non-breathing animals was, in that age, although seen to be a fact, inexplicable. But yet, although anatomy could not then determine the seat of the sense, it might have been conjectured that, as such creatures are obviously affected by odours, there must be some other inlet for them than that through which impression is made upon animals; and the detection of this mode of perception, would have been another instance of homologous physiology. Aristotle[7], following Plato, placed the seat of the smell and other senses in the neighbourhood of the heart; but "the organ was said to be located, suitably, between the eyes."
Note 5, p. 112. The olfactory organ in man, appears to differ, &c.] The analogy is obviously faulty, as it seems to imply that the olfactory, like the respiratory organs, are furnished with a cover, by the raising of which odours gain access to the sense; or rather, owing to the intricacy of the parts and imperfect anatomical knowledge, the epiglottis has been associated with the velum and posterior fauces. It could answer no purpose, then, to inquire, as some have, what animals have an operculum for the smell, of that kind? or what mean those veins and pores? As although the operculum, that is, the epiglottis, was known to be protective of the larynx and, therefore, the respiratory organs, the relations of the larynx with the parts associated with it had not been made out; and the veins and pores refer, probably, to the bronchi and vessels within the chest.
Note 6, p. 112. In fine, odour is derived, &c.] Aristotle here differs from Plato, who held that odorous particles are in a state rather of fluidity; and Cuvier[8] says, that "the organ of smell is moistened with abundant viscosity, which arrests the odorous particles contained in air or water; as fishes are sensible of odours. But odour, being regarded as exhalation, was assumed to be of fiery nature and, therefore, like the element, dry, and this required, for the conformity of the hypothesis of like upon like, that the organ of the sense, when in potentiality, should be also dry, and so, in due relation to odour.