The lingual muscles
The muscles of the tongue are divided into two groups: (1) Those which take origin from the skeleton (skull and hyoid bone) and insert into the tongue; (2) those muscles which belong solely to the tongue, both the origin and insertion being situated within the organ. The first group is composed of the genioglossus, the hyoglossus (chondroglossus), and the styloglossus.
The genioglossus is the strongest of all the lingual muscles and arises by a tendon from the mental spine (superior genial tubercle) of the mandible. It is a paired muscle and is situated just to one side of the median line so that the internal surfaces of the two muscles are in opposition. The majority of the fibers terminate in the lingual mucous membrane, or rather in the lingual fascia, but the most inferior fasciculi pass almost horizontally backward immediately above the geniohyoid and insert into the body of the hyoid bone and into the epiglottis (by means of elastic tendinous fasciculi). The adjacent fibers also at first pass backward from their point of origin, but soon curve sharply upward to insert into the mucous membrane of the dorsum, while the most anterior fibers pass almost vertically upward and then curve slightly forward into the tip of the tongue.
The hyoglossus is a flat quadrangular muscle situated at the side of the floor of the mouth, and arises from the body and greater and lesser cornua of the hyoid bone. The portion coming from the lesser cornu, which is known as the chondroglossus, is not always present. The portion of the muscle arising from the body of the hyoid bone is the strongest, that originating from the greater cornu being considerably flatter, and the fasciculi from both origins pass obliquely upward and forward into the tongue, where they pass between the longitudinalis inferior and the styloglossus, partly interlacing with the latter muscle. The fibers of the chondroglossus, concealed by the remaining portions of the hyoglossus, pass from the lesser cornu to the dorsum of the tongue to mix with the fibers of the longitudinalis superior.
The styloglossu is a well-defined muscle which arises from the styloid process of the temporal bone and frequently also from the stylohyoid and stylomandibular ligaments. It becomes markedly flattened as it approaches the tongue and is inserted principally into the lateral margin of that organ as far forward as the tip, lying laterally to the hyoglossus and longitudinalis inferior, some of its fibers also being continuous with the muscular layer designated as the longitudinalis superior. Smaller fasciculi, situated internally and above, penetrate obliquely into the posterior portion of the tongue as far as the median line.
The muscles situated entirely within the tongue are as follows:
1. The longitudinalis inferior, a flat well-defined muscle upon the lower surface of the tongue, situated between the genioglossus and hyoglossus behind and between the styloglossus and genioglossus in front. Its fibers run in the sagittal plane.
2. The longitudinalis superior, a layer of sagittal muscular fasciculi placed immediately beneath the mucous membrane of the dorsum of the tongue and largely made up of prolongations from the other lingual muscles. It is consequently not a separate and distinct muscle.
3. The transversus linguae, composed of a large number of muscular fasciculi which run almost transversely from the median septum to the surface of the lingual mucous membrane, in such a manner that they are intersected by numerous vertical and sagittal fasciculi, and finally insert between the lamellae of the radiating fibers of the genioglossus. The fasciculi of the glossopalatinus are intimately connected with the transversus linguae and some of them originate from it.
4. The verticalis (perpendicular is) linguae includes all the fasciculi which pass vertically through the tongue from the dorsum to the sublingual mucous membrane.
Between the two genioglossi in the median plane there is a connective-tissue partition usually containing fat, the septum, which fades away as it approaches the dorsum and does not reach the mucous membrane. It gives origin to the fasciculi of the transversus.
The fibers of all the lingual muscles interlace abundantly, especially toward their insertions, which are not actually into the mucous membrane proper but rather into the adherent lingual fascia.
All the lingual muscles are supplied by the hypoglossal nerve.
The development of the tongue is intimately connected with that of the oral cavity. The portions of the tongue situated in front of and behind the sulcus terminalis are formed independently, the anterior portion arising partly from the paired mandibular processes and partly from the so-called tuberculum impar which forms the middle of the anterior portion of the tongue, while the root originates from portions of the second and third visceral arches.