The Tongue: One Organ, Four Nerves

The oral cavity: a vestibule, a cavity, and a roof in two halves

Two spaces, one roof, one floor — learn the boundaries before the contents. The mouth is not one space but two, separated by the teeth and gums. The slit OUTSIDE the teeth, between them and the lips and cheeks, is the VESTIBULE — the gutter a dentist's saliva ejector sits in. The larger space enclosed BY the dental arches is the ORAL CAVITY PROPER, occupied almost entirely by the tongue. Its ROOF is the PALATE, and the palate comes in two parts with two different jobs: the HARD PALATE in front — a bony shelf built from the palatine processes of the maxillae and the horizontal plates of the palatine bones, a rigid plate for the tongue to press food against — and the SOFT PALATE behind, a mobile muscular curtain hanging free, ending in the midline pendant of the UVULA. The FLOOR is formed by the tongue and the soft tissues beneath it: raise the tongue tip and you see the midline FRENULUM tethering it, the two SUBLINGUAL FOLDS ridging up over the sublingual glands, and, at the base of the frenulum, the paired SUBLINGUAL CARUNCLES — the tiny openings of the submandibular ducts, whose glands are traced in the salivary glands. The cavity opens forward at the lips and communicates behind, through the oropharyngeal isthmus, with the pharynx set out in the pharynx and tonsils.

The soft palate: five muscles, and one vagal rule with a single exception

The soft palate is a small muscular sail that must swing up to seal the nasopharynx and down to separate mouth from pharynx, and it does this with FIVE paired muscles. The LEVATOR VELI PALATINI is the elevator: it lifts the palate up and back against the posterior pharyngeal wall to shut off the nose during swallowing and during the consonants that would otherwise leak air down the nose. The TENSOR VELI PALATINI tenses and flattens the palate and — its second, examinable job — opens the auditory tube on swallowing, which is why swallowing clears the ears. The PALATOGLOSSUS forms the anterior tonsillar pillar and pulls the palate down towards the tongue, closing the isthmus. The PALATOPHARYNGEUS forms the posterior pillar and pulls the pharynx up. The MUSCULUS UVULAE builds the uvula itself. Here is the rule worth carrying out of this article whole: EVERY muscle of the soft palate is supplied by the VAGUS nerve (CN X) through the pharyngeal plexus — EXCEPT the tensor veli palatini, which is supplied by the mandibular nerve (V3). One exception, and it is because the tensor, alone among them, is a first-arch muscle. The clinical corollary is direct: in a unilateral vagal (or pharyngeal-plexus) lesion the paralysed side of the palate cannot lift, so on saying "aah" the soft palate and the UVULA are pulled AWAY from the weak side, towards the intact one — the uvula points to the good side.

A line drawn across the tongue — and why it is a border, not a fold

Look at the dorsum of the tongue and you can see, or infer, a shallow V-shaped groove pointing backwards: the SULCUS TERMINALIS. At its apex, where the two arms of the V meet, is a small pit, the FORAMEN CAECUM — the remnant of the thyroglossal duct, the track down which the thyroid gland migrated from the tongue's own base into the neck. That innocuous groove is one of the most important boundaries in the head, because it divides the tongue into two territories that were built from different pharyngeal arches and therefore never shared a nerve. In front of it lies the ANTERIOR TWO-THIRDS, the ORAL part, roughened by the papillae you feel with a fingertip. Behind it lies the POSTERIOR ONE-THIRD, the PHARYNGEAL part, smooth-looking but studded with the lymphoid nodules of the lingual tonsil. The anterior surface carries four kinds of PAPILLAE: the countless fine FILIFORM papillae that give the tongue its grip and velvet texture and carry no taste buds; the red, dome-shaped FUNGIFORM papillae scattered among them, each with a few taste buds; the large VALLATE (circumvallate) papillae, eight to twelve of them lined up in a row immediately IN FRONT OF the sulcus terminalis, each sunk in a moat crowded with taste buds; and the FOLIATE papillae, vertical folds at the sides towards the back. The line between the two territories is not cosmetic. It is a nerve boundary, and everything about how the tongue is wired follows from it.

Eight muscles: four to change its shape, four to change its place

Intrinsic muscles fold the tongue; extrinsic muscles move it about the mouth. The tongue is a muscular hydrostat, and its muscles fall into two families. The FOUR INTRINSIC muscles — the SUPERIOR and INFERIOR LONGITUDINAL, the TRANSVERSE and the VERTICAL — have no bony attachment; they run entirely within the tongue and change its SHAPE, curling the tip, narrowing and thickening it, flattening and broadening it, the fine sculpting that lets the tongue explore a tooth or shape a vowel. The FOUR EXTRINSIC muscles anchor to bone and change the tongue's POSITION, and each pulls in a different direction that is worth memorising by its action. GENIOGLOSSUS, a fan arising from the mental spine of the mandible, is the great PROTRUDER — it pokes the tongue out and is the muscle whose failure sends the tip deviating to the weak side. HYOGLOSSUS, from the hyoid bone, DEPRESSES the tongue. STYLOGLOSSUS, from the styloid process, RETRACTS and ELEVATES it, drawing it up and back for swallowing. And PALATOGLOSSUS, descending from the soft palate, ELEVATES the back of the tongue and closes the oropharyngeal isthmus. The pattern to hold is that intrinsic muscles remodel the tongue in place while extrinsic muscles carry it around the mouth — and, as the next section shows, all but one of the eight answer to a single nerve.

Four nerves for one organ — the spine of the whole subject

Movement, ordinary sensation, taste — and a different nerve again for each half. State it with total clarity, because clarity here is the whole battle. MOTOR: every muscle of the tongue, intrinsic and extrinsic, is supplied by the HYPOGLOSSAL NERVE (CN XII) — with a SINGLE exception, palatoglossus, which is really a palatal muscle and is supplied by the VAGUS through the pharyngeal plexus. SENSATION, ANTERIOR TWO-THIRDS: general sensation (touch, pain, temperature) is carried by the LINGUAL NERVE, a branch of the mandibular division (V3) of the trigeminal, laid out in the trigeminal nerve; TASTE from the same anterior two-thirds is carried by the CHORDA TYMPANI, a branch of the FACIAL NERVE (CN VII) traced in the facial nerve, which hitchhikes onto the lingual nerve and rides with it to the tongue. So the front of the tongue has TWO nerves — one for feel, one for flavour — from two different cranial nerves sharing one path. SENSATION, POSTERIOR ONE-THIRD: here both general sensation AND taste are carried by a single nerve, the GLOSSOPHARYNGEAL (CN IX). And the extreme base of the tongue and the epiglottis are supplied by the VAGUS (CN X) via the internal laryngeal nerve. Four cranial nerves — XII, VII, IX, X — plus the trigeminal for touch: the overview that ties them into the whole cranial-nerve scheme is the cranial nerves overview. The reason for the tangle is embryological: the anterior two-thirds develops from the FIRST arch (trigeminal territory, hence V3 for touch), overlaid by the taste fibres of the SECOND arch nerve, VII; the posterior third is THIRD arch, IX territory throughout; and the base is fourth arch, vagus. The sulcus terminalis is simply where first-arch tissue meets third.

The teeth: enamel to root, and the one nerve a dentist silences

Twenty milk teeth, thirty-two adult teeth, and two nerves that own the upper and the lower jaw. A human grows two sets. The DECIDUOUS (milk) dentition has TWENTY teeth — in each half-jaw two incisors, one canine and two molars — erupting between roughly six months and two-and-a-half years. The PERMANENT dentition has THIRTY-TWO — in each quadrant two incisors, one canine, two PREMOLARS and three MOLARS, the last of which is the wisdom tooth — and it replaces and extends the first set from about six years to the late teens. Whatever its shape — the chisel of an INCISOR, the point of a CANINE, the paired cusps of a PREMOLAR, the broad grinding table of a MOLAR — every tooth has the same construction. The crown is capped by ENAMEL, the hardest tissue in the body, acellular and non-living; beneath it and forming the bulk of the tooth is DENTINE, a living, sensitive tissue; the core is the PULP, a chamber of nerves and vessels that enters through the root canal; the root is coated in CEMENTUM, and the cementum is slung into its bony socket by the PERIODONTAL LIGAMENT, a sheet of collagen fibres that suspends each tooth so it can give a fraction of a millimetre under load and register the pressure. The nerve supply divides cleanly by jaw. The UPPER teeth are supplied by the SUPERIOR ALVEOLAR nerves (anterior, middle and posterior), all branches of the maxillary division (V2). The LOWER teeth are supplied by the INFERIOR ALVEOLAR NERVE, a branch of the mandibular division (V3), which enters the mandible at the mandibular foramen, runs the length of the bone in its canal giving twigs to every lower tooth, and emerges at the mental foramen as the MENTAL NERVE to the skin of the chin and lower lip.

The floor of the mouth: vessels, a looping nerve, and where cancer goes

Beneath the tongue lies the sublingual region, and it is busy. The SUBMANDIBULAR DUCT runs forward from its gland along the floor of the mouth to open at the sublingual caruncle beside the frenulum, and along the way the LINGUAL NERVE performs one of the most quoted manoeuvres in the head: it loops UNDER the duct, crossing from lateral to medial and back — "the lingual nerve takes a double cross over the duct," first passing lateral to it, then hooking beneath to lie medial. It is the reason the lingual nerve is at risk in any surgery on the submandibular duct or gland, and in the removal of an impacted lower wisdom tooth, where it lies just deep to the gum on the tongue side. The SUBLINGUAL GLAND sits under the sublingual fold. The BLOOD SUPPLY of the tongue is the LINGUAL ARTERY, the second branch of the external carotid, which runs deep to hyoglossus to reach the tongue, drained by the LINGUAL VEIN. The LYMPHATIC drainage is where the anatomy turns clinical, and it must be learned as three destinations: the TIP of the tongue drains to the SUBMENTAL nodes; the sides of the anterior two-thirds drain to the SUBMANDIBULAR nodes; and the posterior third and the central part of the tongue drain to the DEEP CERVICAL nodes — the whole nodal map set out in the cervical lymph nodes. The single most important fact here is the crossover: the central and posterior tongue drain BILATERALLY, to the deep cervical nodes of BOTH sides. That is why a cancer of the central or posterior tongue can metastasise to lymph nodes on the opposite side of the neck, and why the neck must often be treated on both sides even when the tumour is plainly one-sided.