THE CERVICAL MUSCLES. 545 



Variations. — The omo-hyoid and the sterno-hyoid are derived from a muscular sheet 

 which, in the lower vertebrates, invests the anterior portion of the neck region, lying beneath 

 the platysma. This sheet is represented in man by the two muscles and the middle layer of 

 the deep cervical fascia. The omo-hyoid or one or other of its bellies may be absent, or, on 

 the other hand, an accessory omo-hyoid may be developed. The superior belly not infre- 

 quently fuses more or less completely with the sterno-hyoid and the inferior belly has some- 

 times a clavicular origin. Occasionally the band which binds the intermediate tendon to the 

 clavicle remains muscular, and, uniting at the tendon with the superior belly, produces what 

 has been termed the cleido-hyoideus. 



3. Sterno-Thyroideus (Fig. 541). 



Attachments. — The sterno-thyroid is a band-like muscle which lies immedi- 

 ately beneath the sterno-hyoid. It arises from the posterior surface of the manu- 

 brium sterni and from the cartilages of the first and second ribs, and passes upward 

 to be mserted into the oblique line of the thyroid cartilage. 



Nerve-Supply. — From the first, second, and third cervical nerves, through 

 the ansa hypoglossi. 



Action. — To draw the larynx downward. 



Relations. — Superficially the sterno-thyroid is covered by the sterno-hyoid. 

 Deeply it is in relation with the inferior constrictor of the pharynx, the crico- 

 thyroid muscle, the cricoid cartilage, the lobes of the thyroid gland, the inferior 

 thyroid veins, the trachea, and the common carotid artery, and it crosses the left 

 innominate vein. 



Variations. — The lower portion of the muscle is often crossed by a tendinous intersection, 

 and frequently some of its fibres are continued directly into the thyro-hyoid muscle. The two 

 muscles of opposite sides are frequently united in the median line, sometimes throughout the 

 greater portion of their length, at other times merely by scattered bundles. 



4. Thyro-Hyoideus (Fig. 541). 



Attachments. — The thyro-hyoid lies beneath the upper portion of the omo- 

 hyoid. It arises below from the oblique line of the thyroid cartilage and is 

 inserted above into the lateral portion of the body and into the greater cornu of the 

 hyoid bone. 



Nerve-Supply. — From the first and second cervical nerves, by fibres which 

 run with the h.ypoglossal nerve. 



Action. — To draw down the hyoid bone, or, if that be fixed, to draw the 

 larynx upward. 



Relations. — As the muscle passes across the hyo-thyroid membrane it covers 

 the sup.erior laryngeal nerve and artery. A bursa, the b. musciili thyro-hyoidei, is 

 interposed between the muscle and the upper part of the hyo-thyroid membrane. 



Variations. — The thyro-hyoid is often practically continuous with the sterno-thyroid. A 

 bundle of fibres is sometimes to be found passing either from the lower border of the hyoid or 

 from the thyroid cartilage to the lobe, isthmus, or pyramid of the thyroid gland. It is termed 

 the levator glandiilce thyroidecp, under which name are also comprised fibres which are exten- 

 sions of the inferior constrictor of the pharynx to the thyroid gland. 



5. Genio-Hyoideus (Fig. 497). 



Attachments. — The genio-hyoid is the superior portion of the rectus group 

 of muscles. It is a rather narrow band which arises from the lower genial tubercle 

 of the mandible and extends backward and downward to be inserted into the 

 anterior surface of the body of the hyoid bone. It is situated close to the median 

 line, under cover of the mylo-hyoid and immediately beneath the lower border of 

 the genio-glossus. 



Nerve-Supply. — From the first and second cervical nerves, by fibres which 

 accompany the hypoglossal. 



Action. — If the hyoid bone be fixed, the genio-hyoid depresses the mandible ; 

 if the mandible be fixed, it draws the hyoid bone forward and upward. 



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