THE MUSCLES OF THE TONGUE. 303 



contracts and rotated to the opposite side. This is the attitude in wry neck, which is due to 

 the spasmodic or organic contraction of one sterno-mastoid. Taking their fixed point at the 

 head, they can elevate the upper part of the thorax in forced inspiration. 



While the sterno-hyoid and omo-Tiyoicl muscles act simply as depressors of the hyoid bone, 

 the sterno-thyroid muscle, being a direct depressor of the thyroid cartilage, can also draw 

 down the hyoid bone when it acts in conjunction with the thyro-hyoid, the latter muscle 

 elevating the larynx when the hyoid bone is fixed. When, in the act of swallowing, the 

 hyoid bone and thyroid cartilage have passed suddenly upwards, their original position is 

 restored by the action of the infrahyoid muscles. In the utterance of low notes the larynx 

 and hyoid bone descend below the natural level, in the direction of the sternal muscles ; 

 while in the utterance of high notes there is little elevation of the hyoid bone, but^the larynx 

 is raised by the action of the thyro-hyoid muscles. During deglutition the thyro-hyoid 

 muscles, by drawing the thyroid cartilage up under the hyoid bone, are the principal agents 

 in producing the descent of the epiglottis on the superior aperture of the larynx. The infra- 

 hyoid muscles also act with the sterno-mastoids in forced inspiration. 



The digastric, mylo-liyoid and genio-hyoid muscles are either elevators of the hyoid bone, 

 or depressors of the lower jaw, according as one or other of these bones is fixed by the 

 antagonistic muscles. The stylo-hyoid acts only on the hyoid bone. The mylo-hyoid and 

 genio-hyoid acting alone draw the hyoid bone forwards as well as upwards, while the stylo- 

 hyoid (aided by the middle constrictor of the pharynx) moves it backwards and upwards. 

 The attachment of the digastric to the hyoid bone, however, is not close enough to allow of 

 the independent action of the anterior and posterior bellies, and this muscle therefore, when 

 the jaw is fixed, elevates directly the hyoid bone, as do also the other muscles of this group 

 acting in combination. The mylo-hyoid farther raises the floor of the mouth and presses the 

 tongue against the palate, thus forcing backwards the food in the first stage of deglutition. 



THE MUSCLES OF THE TONGUE. 



The tongue is a muscular organ attached posteriorly to the hyoid bone, and 

 inferiorly to the lower jaw. It is composed partly of fibres peculiar to itself the 

 intrinsic muscles, which will be noticed with the special anatomy of the tongue in 

 Vol. Ill, and partly of muscles arising from neighbouring parts the extrinsic 

 muscles about to be described. 



The genio-glossus or genio-hyo-glossus muscle is fan- shaped, and is placed 

 vertically in contact with its fellow in the median plane. It arises by a short tendon 

 from the superior mental spine behind the symphysis of the jaw : from this its fibres 

 diverge, to be inserted, the inferior, for the most part, into the body of the hyoid 

 bone, and a few into the side of the pharynx ; the superior into the tip of the 

 tongue ; and the intermediate fibres into the whole length of the tongue spreading 

 outwards in its substance. Some of the innermost fibres, as they enter the tongue, 

 decussate across the middle line with those of its fellow muscle. 



Relations. Its external surface is in contact with the inferior lingualis, hyo-glossus and 

 stylo-glossus muscles, the sublingual gland and the ranine vessels, and its lower border with 

 the genio-hyoid muscle. The terminal portion of the hypoglossal nerve enters its posterior 

 part. 



Varieties. Occasional slips of this muscle have been noticed passing to the epiglottis, or 

 to the stylo-hyoid ligament, or more frequently to the small cornu of the hyoid bone. It has 

 also been found united with the genio-hyoid muscle. 



The hyo-glossus is a flat quadrate muscle, arising from the whole length of 

 the great cornu and from the lateral part of the body of the hyoid bone ; it passes 

 upwards to the posterior half of the tongue close to its lateral border, whence the 

 fibres spread forwards and inwards over the dorsum, joining those of the stylo-glossus 

 muscle. 



Relations. The hyo-glossus is concealed by the digastric, stylo-hyoid and mylo-hyoid 

 muscles, except at its posterior inferior angle : the deep part of the submaxillary gland rests 

 on its surface, and it is crossed from below upwards by the hypoglossal nerve, the Whartonian 

 duct, and the lingual nerve. It covers the hinder part of the genio-glossus, the lower end of 

 the stylo-hyoid ligament, and the origin of the middle constrictor of the pharynx, together 

 with the lingual artery and glosso-pharyngeal nerve. 



