THYRO-HYOID— DIGASTRIC 449 



Insertion. — The lower border of (l) the outer tliird of the body of the hyoid 

 bone; and of (2) tlie inner half of the greater eornu. 



Structure. — This muscle is a continuation ujiwards of the preceding muscle, 

 many of its fibres being derived from those of the sterno-thyroid without any 

 attachment to the cartilage. It consists of parallel fleshy fibres which run nearly 

 vertically upwards to their insertion into the hyoid bone. 



Nerve-supply. — A special branch of the hypoglossal nerve which enters the 

 deep surface of the muscle near its posterior border. 



Action. — (1) To draw up the thyroid cartilage, as in swallowing, or in the pro- 

 duction of a high note in singing; (2) in association with the sterno-thyroid to draw 

 down the hyoid bone. 



Relations. — Superficially, the sterno-hyoid, omo-hyoid, and sterno-mastoid; 

 deeply, the thyro-hyoid membrane, the thyro-hyoid bursa, the sviperior laryngeal 

 vessels and nerve. 



Variations. — Slips are occasionally seen passing from the cricoid cartilage to the hyoid bone, 

 or between the upper border of the thyroid cartilage in front and the body of the hyoid bone. 



THE SUPRA-HYOID MUSCLES 



These consist of four muscles arranged in three layers: the first containing the 

 digastric and stylo-hyoid, the second the mylo-hyoid, and the third the genio-hyoid. 



1. DIGASTRIC 



The digastric — named from its two bellies (yaffrrj/j, the belly) — is composed of 

 two flattened and somewhat s[)indle-shaped bellies united by a tendon. 



Origin. — (1) Of posterior belly, the digastric fossa of the temporal bone; (2) 

 of (interior belly, the lower border of the body of the mandible just external to the 

 symphysis. 



Insertion. — By its central tendon, which is attached to the outer part of the 

 lower l)order of the body of the hyoid bone and the adjacent part of the great cornu. 



Structure. — The posterior belly at its origin consists of short tendinous fibres, 

 which soon form a laterally compressed muscle. This passes downwards, forwards 

 and inwards, and converges upon a laterally flattened rounded tendon about half 

 an inch (1"2 cm.) above the tip of the great cornu of the hyoid bone. 



The anterior belly arises by short tendinous fibres, and forms a muscle flattened 

 fron^ before backwards and from above downwards, which is shorter and smaller 

 than the posterior belly. Its fibres converge as they pass backwards and slightly 

 downwards and outwards, to end in the flattened tendon a (piarter of an inch (6 

 nun.) above the junction of the body and great cornu of the hyoid bone. The 

 central tendon is bound down to its insertion upon the h3'oid bone, chiefly by a 

 fibrous expansion given off from its lower border, and to a very small degree also 

 by the oblicjue arch over it formed by the division of the stylo-hyoid muscle. Often, 

 a part of this expansion passes inwards across the middle line and, uniting with its 

 felloAV, forms a membranous covering to the deeper structures, and unites the inner 

 borders of the two anterior bellies. 



Nerve-supply. — The posterior belly, which is really a distinct muscle, be- 

 longing to a deeper stratum of the muscular planes, receives a special Ijranch from 

 the facial nerve which enters the upper part of its deej) surface. Perhaps this may 

 be due to the fact that this part of the muscle assists in swallowing; for the facial 

 nerve, by means of the Vidian and the relations which through the lesser super- 

 ficial petrosal nerve it has with the otic ganglion, may l)e considered to take part 

 in this function, as well as in the expression of emotions. 



The anterior lielly receives at the outer part of its deep surface the terminal 

 filaments of the mylo-hyoid twig from the inferior dental (or mandibular) branch 

 of the mandibular division of the fifth nerve. 

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