tongue

tongue The tongue plays an essential part in three main processes: in moving food around in the mouth — towards the teeth for mastication, and towards the throat for swallowing; in the special sense of taste; and in normal speech production. It is covered by a mucous membrane that is continuous with the lining of the rest of the mouth and the throat; this is kept moist by the mucous saliva secreted from small glands in its own surface as well as from the main salivary glands. The characteristic roughness of the healthy tongue is due to four types of papillae that cover the top and sides of the front two-thirds. These are outgrowths of the epithelium — the covering layer — and are of various shapes and sizes. Three types of papillae contain taste buds; the largest but least numerous of the papillae, found towards the back of the tongue, have taste buds arrayed in grooves that surround them.

The bulk of the tongue is made up of a set of muscles attached at one end to hard tissues external to the tongue and inserted at the other end into the fibrous tissue of the tongue itself; these are the extrinsic muscles. There are, in addition, vertical and transverse intrinsic muscle fibres that are attached at both ends to fibrous tissue within the tongue; their prime function is to alter the shape of the tongue. The tongue musculature is largely contained within a fibrous sac, so the whole maintains a constant volume irrespective of its shape.

There are three main paired extrinsic mus-cles on each side; their attachments allow the production of the three main components of tongue movement:

The genioglossus muscles take origin from the middle of the back of the lower jaw and have a fan-like insertion into each side of the midline of the tongue. Their contraction protrudes the tongue.

The hyoglossus muscles take origin from each side of the horseshoe-shaped hyoid bone. (This lies below the tongue and is suspended by a sling of muscles from the jaw and the skull.) They extend forward into the tongue at the sides of the genioglossus muscles. Their contraction shortens the tongue towards its base on the hyoid.

The styloglossus muscles take origin from the styloid process on the base of the skull and pass downwards and forwards into side edges of the tongue. Their contraction elevates the sides of the tongue, forming a gutter in the middle.

During feeding, contraction of the styloglossus muscle of one side causes that side of the tongue to tilt upwards. Consequently, solid food is moved to the opposite side of the mouth, placing it between the occluding surfaces of the teeth for chewing. In contrast, all movements involved in the intra-oral transport of food, and in swallowing, are bilaterally symmetrical, so that the bolus moves in the midline.

During mastication, the mechanosensory function of the tongue is essential for the ability to sort the broken particles of food so that the largest remaining particles are always preferentially selected for placement between the occluding teeth. The mechanosensory receptors in the mucosa have an additional role because the control of tongue posture depends mainly upon information supplied by them. The nerves that carry the information to the brain stem for this and for the sense of taste are the cranial nerves V, VII (taste), and IX (the trigeminal, facial, and glossopharyngeal nerves). The motor nerve from the brain stem to most of the tongue muscles is the hypoglossal (XII).

During speech, movements of the tongue take part, along with those of the jaw, the lips, and the cheeks, in the complex configurations of this part of the ‘vocal tract’ that are necessary for articulation.

Looking at the tongue is a diagnostic tradition. It can become coated, glossy, or smoothed in a variety of systemic illnesses, but most changes are non-specific. Because the tongue is essentially a bag of muscles, a drop in the activity of those muscles makes the bag floppy so that its posture is governed primarily by gravity. Failure to maintain the tongue in a forward position, by maintaining contraction of the genioglossus muscles, may therefore restrict or block the airway, especially if a person is lying on their back. Such a blockage can occur in someone who is unconscious from any cause — from fainting to brain damage. Under normal circumstances the converse also applies: a restriction of airflow causes a reflex increase in genioglossus activity.

Allan Thexton

See alimentary system.See also jaw; mouth; speech; swallowing; taste.