In its description of the human nose, my 1901 edition of Gray’s Anatomy includes this sentence: “In the septum … a minute orifice may be discerned: it leads into a blind pouch, the rudimentary organ of Jacobson.” (The septum is the interior wall between the two nostrils.) Probably very few people have heard of this organ. Anatomically, it is as obscure as it could possibly be, tucked away up there inside our noses. Gray has nothing further to say about it, and until very recently physiologists seemed to think it was indeed “rudimentary” in the sense Gray probably intended — that is, that it has no function.

Lyall Watson, however, has large claims for Jacobson’s organ (which is also, by the way, called the vomeronasal organ). The gist of his argument is that most land animals, including Homo sap., have two separate olfactory systems: the familiar one, based on the mucous membranes of the upper interior nose, and an older, subtler and more mysterious one whose impressions enter our nervous system via Jacobson’s organ. The former system is the one that delivers signals to the higher faculties, so that we become consciously aware of a smell. By contrast, Watson tells us, “Jacobson’s organs are not receptive to ordinary odors. They respond most often to a range of substances which have large molecules, but no detectable odor.” And they communicate not with the more “modern” part of our brains, but with older, more primitive regions.

But what is it that they communicate? The strictly scientific answer is: we do not know. “Most of the experimental work on Jacobson’s Organ in mammals has been done on small rodents,” Watson admits. The upshot of that work is, that removing Jacobson’s organ from these creatures ruins their social lives, and in particular their sex lives. This is very suggestive, but does not necessarily tell us anything about the human case. In matters of smell and smelling, human beings are peculiar among mammals. As anyone that has walked a dog knows, other animals relish each other’s smells and seek them out. We, however, dislike our own smells, and spend a fair segment of our lives trying to remove them and replace them with others. This is the more extraordinary when one considers that we are, in fact, among the smelliest of all creatures. “There is little in the animal kingdom … to compare with the human armpit for olfactory potency,” notes Watson. (Though he goes on to point out that people of Asian origin often have no armpit sweat glands at all. You can be disqualified from military service in Japan if you have smelly armpits.)

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Being unable to make any firm statements about the function of Jacobson’s organ in human beings, the author indulges in speculation. This is fine with me; so long as the width and breadth of our scientific ignorance have been clearly marked out, I do not see anything wrong with proceeding into speculative territory. In fact, as anyone who has actually done science will confirm, unsupported speculation is an essential part of the process of scientific discovery. It assists and energizes the framing of hypotheses. It also makes for entertaining books; I found Jacobson’s Organ great fun to read. The author does not restrict himself to biology, but brings in witnesses from linguistics, anthropology, history and literature. Proust’s famous cookie is a good data point, of course. Another is the “flow” of adjectives from one sense to another — the linguistic phenomenon that allows us to speak of a “dry” smell and “warm” colors. “Among chemical senses” reports Watson “the flow goes from touch to taste to smell, but never in the other direction.”

To set his material in context, the author digresses into some more general remarks about the sense of smell. His book therefore overlaps somewhat with Piet Vroon’s 1994 survey of the topic, Smell: The Secret Seducer. Some of the curious factoids in that book are recycled here: some others were quite new to me. Did you know that we can smell in stereo? That old people suffer from olfactory hallucinations? That while your dog is sniffing a tree, the tree may be sniffing your dog? Oh, and if you are contemplating rhinoplasty, make quite sure your surgeon knows about Jacobson’s organ and will not inadvertently damage or destroy it. While we do not quite know what it’s for, hints from other parts of the animal kingdom suggest that without it, you may be missing something important.