188 DEPRESSORS OF THE OS HYOIDES AND LARYNX. 



Dissection. These muscles are brought into view by removing the 

 Jeep fascia from off the front of the neck between the two sterno-mastoid 

 muscles. The omo-hyoid to be seen in its whole extent requires that the 

 sterno-mastoid muscle should be divided from its origin and turned aside. 



The STERNO-HYOIDEUS is a narrow riband-like muscle, arising from the 

 posterior surface of the first bone of the sternum and inner extremity of 

 the clavicle. It is inserted into the lower border and posterior surface of 

 the body of the os hyoides. The sterno-hyoidei are separated by a con- 

 siderable interval at the root of the neck, but approach each other as they 

 ascend : they are frequently traversed by a tendinous intersection. 



Relations. By its external surface with the deep cervical fascia, the 

 platysma myoides and sterno-mastoid muscle ; by its internal surface with 

 the sterno-thyroid, and thyro-hyoid muscle, and the superior thyroid 

 artery. 



The STERNO-THYROIDEUS, broader than the preceding beneath which it 

 lies, arises from the posterior surface of the upper bone of the sternum, 

 and from the cartilage of the first rib ; it is inserted into the oblique line 

 on the great ala of the thyroid cartilage. The inner borders of these 

 muscles lie in contact along the middle line, and they are generally marked 

 by a tendinous intersection at their lower part. 



Relations. By its external surface with the sterno-hyoid, omo-hyoid, 

 and sterno-mastoid' muscle ; by its internal surface, with the trachea and 

 inferior thyroid veins, with the thyroid gland, the lower part of the larynx, 

 the sheath of the common carotid artery and internal jugular vein, with 

 the subclavian vein and vena innominata, and on the right side with the 

 arteria innominata. The middle thyroid vein lies along its inner border. 



The THYRO-HYOIDEUS is the continuation upwards of the sterno-thyroid 

 muscle. It arises from the oblique line on the thyroid cartilage, and is 

 inserted into the lower border of the body and great cornu of the os 

 hyoides. 



Relations. By its external surface with the sterno-hyoid and omo-hyoid 

 muscle ; by its internal surface with the great ala of the thyroid cartilage, 

 the thyro-hyoidean membrane, and the superior laryngeal artery and 

 nerve. 



The OMO-HYOIDEUS (wfxosr, shoulder) is a double-bellied muscle passing 

 obliquely across the neck from the scapula to the os hyoides : it forms an 

 obtuse angle behind the sterno-mastoid muscle, and is retained in that 

 position by means of a process of the deep cervical fascia which is con- 

 nected to the inner border of its tendon. It arises from the upper border 

 of the scapula, and from the transverse ligament of the supra-scapular 

 notch, and is inserted into the lower border of the body of the os hyoides. 



Relations. By its superficial surface with the trapezius, the subclavius 

 and clavicle, the deep cervical fascia and platysma myoides, the sterno- 

 mastoid, and the integument. By its deep surface with the brachial plexus, 

 the scaleni muscles, the phrenic nerve, the sheath of the common carotid 

 artery and jugular vein, the descendens noni nerve, the sterno-thyroid, 

 and thyro-hyoid muscle, and the sterno-hyoid at its insertion. The sca- 

 pular portion of the muscle divides the great posterior triangle into a su- 

 perior or occipital triangle ; and an inferior or subclavian trivngle, which 

 contains the subclavian artery and brachial plexus of nerves ; the other 

 two boundaries of the latter being the sterno-rnastoid in Iront and the cla- 

 vicle below. The hyoid portion of the muscle divides the great anterior 



