In an earlier essay I drew attention to several strange gaps in the Oxford English Dictionary. And I do mean the 20 volume work, with over 600,000 entries. Strange that there could be any gaps in it. But one of the gaps is philtrum. It is the vertical groove from the nose to the upper lip. It is part of the natural topology of shaving, or applying lipstick. Philtrum does not appear as a headword in the OED or in Johnson. However you will find it in Nathaniel Bailey’s English Dictionary (1742) and in the 2nd edition of the Random House Dictionary of the English Language. Other American dictionaries recognise it.
It does however appear in the OED, but only indirectly. In the entry for dysmorphic, a quotation is given from the 1997 Journal of Medical Genetics “Her face appeared mildly dysmorphic with a large forehead, short philtrum, and bushy eyebrows.” Clearly a reference to the thing we are discussing.
But it gets another look in. The entry for philtre includes a passing reference to philtrum, although it does not make much sense. Philtre is defined as “A potion or drug (rarely, a charm of other kind) supposed to be capable of exciting sexual love”, with supporting quotations from 1587 to 1858. But a second meaning is given, supported by two quotations:

“1653 R. Sanders Physiogn. 278 A mole on the philtrum or hollow of the upper lip, under the nostrils.”
“1706 Phillips, Philter or Philtrum.‥ Among some Anatomists, it is taken for the Hollow that divides the upper Lip.”

This meaning is said to be obsolete, but that can’t be right because Bailey recognises it, and it has been used increasingly since the early 1900s in reference to the facial feature, not in reference to love potions.
But more than this striking gap in the OED’s coverage is the quotation from Phillips. How odd to rely on anatomists for reference to the philtrum: the philtrum can be seen plainly on the face without any further examination; but anatomists see things by cutting them.
Anatomy means cutting up, dissection. Its root is Greek tom meaning cut. An atom is something which cannot be cut into smaller parts (that’s what people thought at any rate when the atom was named). The OED puts it well. It defines atom this way: “A hypothetical body, so infinitely small as to be incapable of further division; and thus held to be one of the ultimate particles of matter, by the concourse of which, according to Leucippus and Democritus, the universe was formed.” It was used this way from the 15th century, well before the inner complexities of the atom had been discovered. (JJ Thompson discovered the electron as a component of the atom in 1897; Rutherford found the proton in about 1909, and the neutron was not discovered until 1932, by James Chadwick). Since then, these apparently fundamental, indivisible components of the supposedly indivisible atom have themselves been found to be a fantastic mix of other bits and pieces: quarks, hadrons, gluons, bosons and so-ons.
So the atom is not an atom at all, strictly, but the name has stuck. The root is found in many places:

Anatomy: literally, cutting through.
colostomy: cutting an artificial opening into the colon through the abdominal wall.
dichotomy: division of a whole into two parts.
lobotomy: incision into (especially) the frontal lobe of the brain, in the treatment of mental illness.

And in surgery, countless other –ectomies in which things are cut out. Most familiar is the appendicectomy: cutting out the appendix or, as the OED magnificently has it “Excision of the vermiform appendix of the cæcum” (Note that it is a syllable longer than appendectomy, which is an Americanism not favoured by the Australian medical profession). The familiar CAT scan is Computer Aided Tomography: that is, ‘cutting’ the body by taking computer-processed X-rays to produce tomographic images or ‘slices’ of particular parts of the body.
Similarly, from the same root we have:

epitome: an abridgment of a work, extraction of its principal features.
microtome: in medicine, an instrument for cutting extremely thin sections for microscopic work.
tome: a volume of a (written) work. The original idea was that the whole work was cut into several tomes. And just in case you need it, a hecatontome is a collection of a hundred tomes. Oddly, a monotome is a work comprising a single volume. Although the word has been around since the 19th century, it is rarely used, perhaps because it makes no sense. If it is a tome, it should be a slice of a larger work.

Until I began researching this essay I had not been terribly excited about the absence of philtrum from the OED, but I have become quite worked up about it. On any view it is passing strange that the word which describes a visible thing common to all 7 billion people on earth, which is neither embarrassing nor indecent, should be denied its place in the Oxford sun. Its absence is, as Mark Antony said “the most unkindest cut of all”. (He was not talking about circumcision).
The unkindness is magnified when you have regard to the number of utterly useless words which bask complacently in the OED. For example, words which have the hecato– prefix to describe a hundred utterly pointless things. Hecatologue: a code of a hundred rules; hecatomb: on offering of a hundred oxen (terribly useful these days); hecatomped: an area one hundred feet square; hecatonstylon: a building having one hundred pylons; hecatontarchy: government by a hundred rulers; and hecatophyllous: having leaves consisting each of a hundred leaflets.
And let’s not oblive (= forget) those other space-wasting words which have the prefix sesqui-to signify one and a half of something. How often have you had to resist the temptation to use sesquialter: Proportionate to another object as 1½ is to 1; or sesquiduple to express the meaning ‘two and a half’; or sesquipedal: a foot and a half long (44.1 cm); or sesquiplane: a biplane having one wing of surface area not more than half that of the other; but I suppose we will have to keep sesquiplicate if only because its definition is so wonderfully obscure: “Bearing or involving the ratio of the square roots of the cubes of the terms of a certain ratio”. (Actually, the definition of syzygy when used as an expression in mathematics is better: “A group of rational integral functions so related that, on their being severally multiplied by other rational integral functions, the sum of the products vanishes identically; also, the relation between such functions”).
Philtrum has to go into the OED. If space is a problem, I think there is a case to be argued for dumping some of these words, but if removing any of them to make way for philtrum seems like too great a sacrifice, we might just ditch heptaglottologie, that is, a treatise concerning seven languages.

[A version of this essay also appears in my book about words: Wordwatching (Scribe, 2004, 2006]