294 MUSCLES OF THE HEAD AND NECK. 



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

 hyoid bone, near the place where these unite. 



Belat'wns. — This muscle is concealed by the stemo-hyoid and omo-hyoid, and 

 rests on the ala of the thjToid caiiilage. and on the thyi-o-hyoid membrane. 

 Between that membrane and the muscle, the superior lari^Tigeal nerve and artery 

 are placed before entering- the larj-nx. 



The omo-hyoid is a long ribbon-shaped muscle, consisting of two 

 bellies united by an intervening tendon. It arises from the upper 

 border of the scapula, near the suprascapular notcli, and occasionally 

 from the ligament which crosses the notch. Thence it extends for- 

 wards and only slightly upwards, across the root of the neck, till it 

 passes beneath the sterno-mastoid muscle, and then, curving rapidly, it 

 ascends nearly vertically, to be inserted into the lower border of the 

 body of the hyoid bone. The tendon which divides the muscle is 

 placed beneath the sterno-mastoid muscle, and varies much in length 

 and form in diflFerent bodies. The tendon is enclosed within the deep 

 cervical fascia, which, after forming a sort of sheath for it, is prolonged 

 downwards, and becomes attached to the sternum and the cartilage of 

 the first rib ; and by this means, as also by fascia investing the poste- 

 rior belly and descending to the clavicle, the muscle is maintained in 

 its curved position. 



Hchif'ions. — At its scapular origin the muscle is covered by the trapezius, in 

 the middle of its course by the stemo-mastoid, and at its upper part by tlie 

 platysma ; it crosses the scaleni muscles, the cervical nerves, the sheath of the 

 common carotid artery and jugular vein, and the stemo-thyroid and thj'ro-hyoid 

 muscles. 



Viirht'icx. — The muscle occasionally is attached to the clavicle instead of the 

 scapula, arising from the fonner bone about its middle, and in such cases the 

 posterior belly is absent. One instance has been recorded (R. Quain) in which 

 the posterior belly alone was present, and was connected to the hyoid bone by a 

 band of fascia. The muscle has likewise been observed double, one slip being 

 attached to the clavicle and the other to the normal place of oiigin from the 

 scapula. 



Nerves. — Tlie stemo-mastoid receives its motor nerves from the spinal ac- 

 cessory, which in the substance of the muscle is reinforced by union with 

 branches from the second and third cervical nerves. The infra-hyoid muscles 

 receive their motor nerves mainly from the h,>'i)o-glossal, the tliyro-hyoid by a 

 direct branch from tlie trunk of the nerve, the rest from the descendeus noni, 

 and in part from it in combination with the communicating branches from the 

 second and third cervical nei^ves. 



Actions. — The sterno-cleido-mastoid muscles acting together bend forwards 

 the head and neck towards the sternum. When the muscle of one side only 

 is in action, the head, while it is slightly flexed, is inclined laterally to the 

 side on which the muscle contracts, and rotation is produced, by which the face, 

 and especially the chin, is dii'ected towards the opposite side. This is the 

 attitude in wry-neck, produced by the miequal action of the muscles of the two 

 sides. 



"VNTiile the sterno-hyoid and omo-hyoid muscles act only as depressors of tlie 

 hyoid bone, the stemo-thyroid muscle, being a direct depressor of the thyroid 

 cartilage, may also draw down the hyoid bone when it acts in conjunction with the 

 thyro-hyoid, the latter muscle elevating the lai\>-nx when the hyoid bone is fixed. 

 When, in the act of s^\'allowing. the hyoid bone and thyroid cartilage have 

 passed suddenly upwards and forwards, their original position is restored by the 

 action of the mfra-hyoid muscles. In the utterance of low notes the laiynx and 

 hyoid bone descend below the natural level, in the dii-ection of the sternal 

 muscles ; while in the utterance of high notes there is little elevation of the 



