question: Why does our tongue rest on the top of the mouth instead of bottom?
comment: Seems like it’s the natural resting position of the muscle. If I take my tongue off the roof of my mouth, it feels like I’m keeping it down.
passages: The tongue is attached to the floor of the mouth by a ligamentous band called the frenum and this gives it great mobility for the manipulation of food (and speech); the range of manipulation is optimally controlled by the action of several muscles and limited in its external range by the stretch of the frenum. The tongue's two sets of muscles, are four intrinsic muscles that originate in the tongue and are involved with its shaping, and four extrinsic muscles originating in bone that are involved with its movement.
  
  Section::::Components.:Mouth.:Tongue.:Taste.
   The palatoglossus arises from the palatine aponeurosis, and depresses the soft palate, moves the "palatoglossal fold" towards the midline, and elevates the back of the tongue during swallowing.
  
  Section::::In humans.:Structure.:Muscles.:Intrinsic.
   The genioglossus arises from the mandible and protrudes the tongue. It is also known as the tongue's "safety muscle" since it is the only muscle that propels the tongue forward.
  
  The hyoglossus, arises from the hyoid bone and retracts and depresses the tongue. The chondroglossus is often included with this muscle.
  
  The styloglossus arises from the styloid process of the temporal bone and draws the sides of the tongue up to create a trough for swallowing.
   Intercuspal position (ICP), also known as centric occlusion, describes the position of "best fit" between the upper and lower teeth. It is the closest relationship of the mandible to the maxilla as this is where all the teeth fully interlock simultaneously. Therefore, it is a position that is determined by the teeth and to some extent the soft tissues as well. The tongue, cheeks and lips contribute to the development of ICP by guiding the eruption of the upper and lower teeth and stabilising their positions. Though it may seem automatic that an individual goes from a rest position to ICP, this path of closure is actually a learnt behaviour that is conditioned into short term memory.
   The human tongue is divided into two parts, an oral part at the front and a pharyngeal part at the back. The left and right sides are also separated along most of its length by a vertical section of fibrous tissue (the lingual septum) that results in a groove, the median sulcus on the tongue's surface.
  
  There are two groups of muscles of the tongue. The four intrinsic muscles alter the shape of the tongue and are not attached to bone. The four paired extrinsic muscles change the position of the tongue and are anchored to bone.
  
  Section::::Etymology.
   Section::::In humans.:Structure.:Blood supply.
  
  The tongue receives its blood supply primarily from the lingual artery, a branch of the external carotid artery. The lingual veins, drain into the internal jugular vein. The floor of the mouth also receives its blood supply from the lingual artery. There is also a secondary blood supply to the root of tongue from the tonsillar branch of the facial artery and the ascending pharyngeal artery.
   While identifying the causes of tongue thrust, it is important to remember that the resting posture of the tongue, jaw, and lips are crucial to normal development of mouth and its structures. If tongue rests against the upper front teeth, the teeth may protrude forward, and adverse tongue pressure can restrict the development of the oral cavity. The tongue lies low in the mouth or oral cavity and is typically forwarded between upper and lower teeth. If tongue thrust behavior is not corrected, it may affect the normal dental development. The teeth may be pushed around in different directions during the growth of permanent teeth.
   The larynx or voice box is an organ in the neck housing the vocal folds, which are responsible for phonation. In humans, the larynx is "descended," it is positioned lower than in other primates.This is because the evolution of humans to an upright position shifted the head directly above the spinal cord, forcing everything else downward. The repositioning of the larynx resulted in a longer cavity called the pharynx, which is responsible for increasing the range and clarity of the sound being produced. Other primates have almost no pharynx; therefore, their vocal power is significantly lower. Our species is not unique in this respect: goats, dogs, pigs and tamarins lower the larynx temporarily, to emit loud calls. Several deer species have a permanently lowered larynx, which may be lowered still further by males during their roaring displays. Lions, jaguars, cheetahs and domestic cats also do this. However, laryngeal descent in nonhumans (according to Philip Lieberman) is not accompanied by descent of the hyoid; hence the tongue remains horizontal in the oral cavity, preventing it from acting as a pharyngeal articulator.
   Section::::Anatomy.
  
  The thin strip of tissue that runs vertically from the floor of the mouth to the undersurface of the tongue is called the lingual frenulum. It tends to limit the movement of the tongue, and in some people, it is so short that it actually interferes with speaking.
   In bilabial consonants, both lips move so the articulatory gesture brings the lips together, but by convention, the lower lip is said to be active and the upper lip passive. Similarly, in linguolabial consonants the tongue contacts the upper lip with the upper lip actively moving down to meet the tongue; nonetheless, the tongue is conventionally said to be active and the lip passive if for no other reason than that the parts of the mouth below the vocal tract are typically active, and those above the vocal tract are typically passive.
   The tip of tongue drains to the submental nodes. The left and right halves of the anterior two-thirds of the tongue drains to submandibular lymph nodes, while the posterior one-third of the tongue drains to the jugulo-omohyoid nodes. 
  
  Section::::In humans.:Structure.:Microanatomy.
   ICP is a position used by the mandible at the end of a chewing cycle. This position is also used a hundred times a day in function specifically to stabilise the mandible during swallowing. When we swallow, a majority of us will form an anterior oral seal which is when the teeth are in ICP and the lips closed together. However, there is a small number of people who are unable to do that and swallow with their teeth apart instead. The reasons for this could be that there is a discrepancy in the jaw(s), a malposition of the teeth or a deformity in the soft tissues.
   The interdental gingiva takes up the space beneath a tooth contact point, between two adjacent teeth. It is normally triangular or pyramidal in shape and is formed by two interdental papillae (lingual and facial).The middle or centre part of the interdental papilla is made up of attached gingiva, whereas the borders and tip are formed by the free gingiva. The central point between the interdental papillae is called the col. It is a valley-like or concave depression that lies directly beneath the contact point, between the facial and lingual papilla. However, the col may be absent if there is gingival recession or if the teeth are not contacting. The main purpose of the interdental gingiva is to prevent food impaction during routine mastication. 
   Both parts of the tongue develop from different pharyngeal arches.
  
  Section::::In humans.:Structure.:Undersurface of the tongue.
  
  On the undersurface of the tongue is a fold of mucuous membrane called the frenulum that tethers the tongue at the midline to the floor of the mouth. On either side of the frenulum are small prominences called sublingual caruncles that the major salivary submandibular glands drain into.
  
  Section::::In humans.:Structure.:Muscles.
   Section::::Function.
  
  Lingual papillae, particularly filiform papillae, are thought to increase the surface area of the tongue and to increase the area of contact and friction between the tongue and food. This may increase the tongue's ability to manipulate a bolus of food, and also to position food between the teeth during mastication (chewing) and swallowing.
  
  Section::::Clinical significance.
  
  Section::::Clinical significance.:Depapillation.
   Superficial veins run through the base of the frenulum known as varicosities. Their presence is normal, becoming more and more prominent as the patient ages.
  
  Section::::Disorders.
  
  Ankyloglossia, also known as tongue-tie, is a congenital anomaly characterised by an abnormally short lingual frenulum; when severe, the tip of the tongue cannot be protruded beyond the lower incisor teeth.
  
  Additionally, an abnormally short frenulum in infants can be a cause of breastfeeding problems, including sore and damaged nipples and inadequate feedings. The resultant trouble breastfeeding results in slower weight gain in affected infants.
   The genioglossus is innervated by the hypoglossal nerve, as are all muscles of the tongue except for the palatoglossus. Blood is supplied to the sublingual branch of the lingual artery, a branch of the external carotid artery.
  
  The canine genioglossus muscle has been divided into horizontal and oblique compartments.
  
  Section::::Function.
  
  The left and right genioglossus muscles protruding the tongue and deviate it towards the opposite side. When acting together, the muscles depress the centre of the tongue at its back.
  
  Section::::Clinical significance.
   The tongue is an important accessory organ in the digestive system. The tongue is used for crushing food against the hard palate, during mastication and manipulation of food for softening prior to swallowing. The epithelium on the tongue’s upper, or dorsal surface is keratinised. Consequently, the tongue can grind against the hard palate without being itself damaged or irritated.
  
  Section::::In humans.:Function.:Speech.
  
  The intrinsic muscles of the tongue enable the shaping of the tongue which facilitates speech.
  
  Section::::In humans.:Function.:Intimacy.
   Section::::Place of articulation (active).
  
  The articulatory gesture of the active place of articulation involves the more mobile part of the vocal tract, typically some part of the tongue or lips. The following areas are known to be contrastive:
  
  BULLET::::- The lower lip (")
  
  BULLET::::- Various parts of the front of the tongue ("):
  
  BULLET::::- The tip of the tongue ("")
  
  BULLET::::- The upper front surface of the tongue just behind the tip, called the "blade" of the tongue ("")
  
  BULLET::::- The surface of the tongue "under" the tip (")
  
  BULLET::::- The body of the tongue (")
   BULLET::::- The base root of the tongue and the throat ("")
   There are three sets of salivary glands: the parotid, the submandibular and the sublingual glands. The (exocrine) glands secrete saliva for proper mixing of food and provides enzymes to start chemical digestion. Saliva helps to hold together the formed bolus which is swallowed after chewing. Saliva is composed primarily of water, ions, salivary amylase, lysozymes, and trace amounts of urea.
  
  Section::::Structure.:Mouth.:Tongue.
   Loss of muscle tone and skin elasticity due to old age, when most individuals begin to experience edentulism, tend to further exacerbate this condition.
  
  The tongue, which consists of a very dynamic group of muscles, tends to fill the space it is allowed, and in the absence of teeth, will broaden out. This makes it initially difficult to fabricate both complete dentures and removable partial dentures for patients exhibiting complete and partial edentulism, respectively; however, once the space is "taken back" by the prosthetic teeth, the tongue will return to a narrower body.
  
  Section::::Signs and symptoms.:Vertical dimension of occlusion.
   The average length of the human tongue from the oropharynx to the tip is 10 cm. The average weight of the human tongue from adult males is 70g and for adult females 60g.
  
  In phonetics and phonology, a distinction is made between the tip of the tongue and the blade (the portion just behind the tip). Sounds made with the tongue tip are said to be apical, while those made with the tongue blade are said to be laminal.
  
  Section::::In humans.:Structure.:Upper surface of the tongue.
   The elevation of the palatal shelves from a vertical position to a horizontal one occurs during week 8 of embryological development. It is unknown as to what exactly is the direct cause of this movement is, but a number of possibilities have been identified as follows:
  
  BULLET::::- Muscular contraction;
  
  BULLET::::- Hydrostatic forces exerted by Glycosaminoglycans and Hyaluronan;
  
  BULLET::::- Mesenchymal reorganisation;
  
  BULLET::::- Mesenchyme cell contraction;
  
  BULLET::::- Epithelial reorganisation
  
  BULLET::::- Movement of the developing tongue
  
  Section::::Suggested mechanisms for palatal fusion.
   There are two primary articulations of the approximant : "apical" (with the tip of the tongue approaching the alveolar ridge or even curled back slightly) and "domal" (with a centralized bunching of the tongue known as ""molar r"" or sometimes ""bunched r"" or ""braced r""). Peter Ladefoged wrote: "Many BBC English speakers have the tip of the tongue raised towards the roof of the mouth in the general location of the alveolar ridge, but many American English speakers simply bunch the body of the tongue up so that it is hard to say where the articulation is". The extension to the IPA recommends the use of the IPA diacritics for "apical" and "centralized", as in , to distinguish apical and domal articulations in transcription. However, this distinction has little or no perceptual consequence, and may vary idiosyncratically between individuals.
  