THE MUSCLES. 135 



186. 5. Sterno-thyreoideus. 



Pos. : behind sterno-hyoid, diverging from below upwards, 

 before the thyroid gland. On the outer border: V. jugular 

 intern.; behind that Carol, comm.; on the inner border: V. thy- 

 reoid media. Or.: broad from Manubrium sterni (posterior 

 surface) and first rib cartilage. Ins. : Linea obliqua of thyroid 

 cartilage. Act. : draws the larynx downwards, shortening the 

 trachea. 



Nerves : (three to five) hypoglossi ram. descendens. 



187. 6. Thyro-hyoideus. 



Pos. : behind the m. sterno-hyoidetis, vertical before the memb. 

 thyreoidea. Or. : os hyoides (the inferior border of the body 

 and root of great cornua). Ins. : Thyroid cartilage (linea 

 obliqua). Act. : approximates hyoid bone and larynx; assists 

 in throwing the epiglottis backwards. 



[Thyreoideus is a prolongation of the thyroid gland upwards: 

 it contains no muscular fibres.] 



Nerves : laryngeus superior and hypoglossus. 



b. Above the Os Hyoides. 



188. 7. Digastricus (s. biventer maxillae inferioris). 



Pos. : in the semicircle between the temporal, hyoid bone and 

 centre of lower jaw, below the angle of the last; covered by M. 

 platysma myoides; behind by the Parotis and Gland, submaxill. 

 Internally to it lie Carotis interna and N. hypogloss. ; the pos- 

 terior belly perforates on the hyoid bone, the M. stylo-hyoideus. 

 Or.: 1. Post, belly: from Incisura mastoidea, covered by M. 

 sterno-cleido-mastoid. 2. Anterior belly : from os hyoides. Ins.: 

 1. Os hyoides. 2. Lower jaw (base). Act.: the anterior belly 

 elevates the hyoid bone towards the lower jaw, the post, draws 

 it backwards and upwards. 



Nerves: post, belly, fadalis ; [ant. belly, trigeminus.'] 



189. 8. Stylo-hyoideus. 



Pos.: between temporal and hyoid bones, covered externally 

 by the posterior belly of the digastric muscle by which it is per- 

 forated shortly before its point of insertion. Or. : Proc. styloideus 

 (the centre of the external surface). Ins.: the Os hyoides^ at the 

 union of its great cornu with the body. Use : to draw the hyoid 



