MUSCLES OF THE TONGUE 345 



dorsal surface of the inferior margin of the mandible, and (2) into the ventral surface of the hyoid 

 bone. Some of the fibre-bundles may cross the median line. The muscles of the two sides 

 form a sheet with a downward convexity which lies between the inner surface of the bodj' of 

 the mandible and the hyoid bone. On the diaphragm thus formed rests the tongue. 



Nerve-supply. — -From the mylo-hyoid branch of the inferior alveolar (dental) nerve several 

 filaments enter the under surface of the muscle. 



Relations. — The mylo-hyoid muscle is covered externally bj^ the submaxillary gland, the 

 anterior belly of the digastric, and the external cervical fascia. It is crossed by the submental 

 artery. With the genio-hyoid and the genio-glossus muscles it helps to bound a compartment 

 in which are lodged the sublingual gland, the duct of Wharton, and the deep portion of the 

 submaxillary gland. Its deep surface also faces the stylo-glossus and hyo-glossus muscles, the 

 Hngual and hypoglossal nerves, and to a slight extent the buccal mucosa. 



The genio-hyoideus (fig. 349). — Origin. — By short tendinous fibres from the mental spine 

 of the mandible. Structure and Insertion. — The fibre-bundles diverge and are inserted into the 

 ventral surface of the body of the hyoid bone. Usually a special fasciculus goes to the great 

 cornu of the hyoid bone. 



Nerve-supply. — The hypoglossal nerve sends a filament to the middle third of the deep 

 surface of the muscle. The nerve-fibres are thought to be derived chiefly from the first 

 cervical nerve. 



Relations. — It lies between the genio-glossus and mylo-hyoid muscles. It adjoins its 

 feUow of the opposite side and is often fused with it. Lateral to it lie the sublingual and sub- 

 maxillary glands and the hypoglossal nerve. 



Action. — The muscles of this group all elevate the hyoid bone and, through this, the larynx 

 and inferior part of the pharj-nx, and thus play a part in the act of swallowing. The stjdo- 

 hyoid and posterior belly of the digastric serve also to draw the hyoid bone in a dorsal direction; 

 the ventral belly of the digastric and the genio-hyoid, in a ventral direction. The digastric, 

 genio-hyoid, and mylo-hyoid depress the mandible, when the hyoid bone is fixed. The 

 posterior belly of the digastric has a slight power to bend the head backward. 



Variations. — The stylo-hyoid tendon frequent^ passes entirely in front of and less frequently 

 entirely behind the digastric muscle. Its insertion may be of greater extent than usual. A 

 special fasciculus to the lesser cornu is not verj- infrequent; more rarely one extends to the angle 

 of the jaw or to other regions. The muscle may arise from the petrous portion of the tem- 

 poral or from the occipital bone, as in some lower vertebrates. It may be doubled or absent, 

 or fused with the posterior belly of the digastric. The anterior belly of the digastric may be 

 missing; the posterior belly may be inserted into the angle of the jaw. The intermediate ten- 

 dons of the digastric of each side may be connected by a fibrous arch. The anterior bellies of 

 the muscles of each side may be united by a fasciculus or fused. The anterior belly is frequently 

 doubled. The posterior belly maj^ be divided by a tendinous inscription. Fasciculi may pass 

 from either belly to neighbouring structures. The mylo-hyoid may not extend quite to the 

 hyoid bone. It may be more or less fused with neighbouring muscles. Rarely it is absent. 

 The genio-hyoid is frequentlj^ more or less fused with the muscles of the tongue or with the genio- 

 hyoid of the opposite side. A considerable number of infrequently found muscles have been 

 described superficial to the stylo-hyoid and digastric muscles. Most of them are innervated 

 by the glosso-pharyngeal nerve or by the facial nerve. 



4. MUSCLES OF THE TONGUE 



(Fig. 349) 



The tongue is a flexible organ, composed chiefly of various muscles, some of 

 which lie entirely within its substance, while others extend to be attached to 

 neighbouring parts of the skeleton. To the former the term intrinsic, to the 

 latter the term extrinsic, is frequently appHed. In this section the extrinsic 

 muscle will alone be taken up. The intrinsic muscles are described in the 

 section on the Digestre Sy.stem. Certain pharyngeal and palatal muscles 

 which are continued into the tongue are described in connection with the 

 pharynx. The extrinsic musculature of the tongue is concealed below by the 

 suprahyoid musculature and the sublingual gland. It is covered on the free 

 surface of the tongue by the mucosa. 



The musculature of the tongue is supplied by the hj-poglossal nerve, which is 

 in series with the motor roots of the spinal nerves. It is, primitively at least, 

 derived from the ventral portion of myotomes in series with the spinal myotomes. 



Four extrinsic muscles are recognised on each side. The stylo-glossus is a 

 slender muscle, which arises from the styloid process and is inserted into the side 

 of the tongue. It is cylindrical near its origin, flat and triangular near its inser- 

 tion. The thin, quadrilateral hyo-glossus arises from the body and great cornu 

 of the hj^oid bone and is inserted into the dorsum of the tongue. The chondro- 

 glossus arises from the lesser cornu of the hyoid bone and joins the superior and 

 inferior longitudinal muscles of the tongue. The genio-glossus (genio-hyo- 

 glossus), which forms the main part of the bodj^ of the tongue, arises from the 

 mental spine of the mandible, from which the fibre-bundles radiate out toward the 

 whole length of the dorsum of the tongue and to the hyoid bone. 



