DEPRESSORS OP THE OS HYOIDES AND LARYNX. 187 



the neck, and is situated between two layers of the deep cervical fascia 

 It arises^ as implied in its name, from the sternum and clavicle (xXsi&ov), 

 and passes obliquely upwards and backwards to be inserted into the mas- 

 toid process arid into the superior curved line of the occipital bone. The 

 sternal portion arises by a rounded tendon, increases in breadth as it 

 ascends, and spreads out to a considerable extent at its insertion. The 

 clavicular portion is broad and fleshy, and separate from the sternal portion 

 below, but becomes gradually blended with its posterior surface as it 

 ascends. 



Relations. -By its superficial surface with the integument, the platysma 

 myoides, the external jugular vein, superficial branches of the anterior 

 cervical plexus of nerves, and the anterior layer of the deep cervical fascia. 

 By its deep surface with the deep layer of the cervical fascia ; with the 

 sterno-clavicular articulation, the sterno-hyoid, sterno-thyroid, omo-hvoid, 

 scaleni, levator anguli scapula?, splenii, and the posterior belly of the di- 

 gastric muscle ; with the phrenic nerve, and the posterior, and supra-sca- 

 pular artery ; with the deep lymphatic glands, the sheath of the common 

 carotid and internal jugular vein, the descendens noni nerve, the external 

 carotid artery and its posterior branches, the commencement of the internal 

 carotid artery ; with the cervical plexus of nerves, the pneumogastric, the 

 spinal accessory, the hypoglossal, the sympathetic and the facial nerve, 

 and with the parotid gland. It is pierced on this aspect by the spinal ac- 

 cessory nerve and by the branches of the mastoid artery. The anterior 

 border of the muscle is the posterior boundary of the great anterior triangle, 

 the other two boundaries being the middle line of the neck in front, and 

 the lower border of the jaw above. It is the guide to the operations for 

 the ligature of the common carotid artery and arteria innominata, and for 

 cesophagotomy. The posterior border is the anterior boundary of the great 

 posterior triangle ; the other two boundaries being the anterior border of 

 the trapezius behind, and the clavicle below T . 



Actions. The platysma produces a muscular traction on the integu- 

 ment of the neck, which prevents it from falling so flaccid in old persons 

 as would be the case if the extension of the skin were the mere result of 

 elasticity. It draws also upon the angle of the mouth, and is one of the 

 depressors of the lower jaw. The transverse fibres draw the angle of the 

 mouth outwards and slightly upwards. The sterno-mastoid muscles are 

 the great anterior muscles of connexion between the thorax and the head. 

 Both muscles acting together bow the head directly forwards. The cla- 

 vicular portions, acting more forcibly than the sternal, give stability and 

 steadiness to the head in supporting great weights. Either muscle acting 

 singly would draw the head towards the shoulder of the same side, and 

 carry the face towards the opposite side. 



Second Group. Depressors of the Os Hyoides and Larynx. 

 Sterno-hyoid, Thyro-hyoid, 



Sterno-thyroid, Omo-hyoid. 



genio-hyoideus. 7. The tongue. 8. The hyo-glossus. 9. The stylo-glossus. 10. The 

 stylo-pharyngeus. 1 1. The sterno-mastoid muscle. 12. Its sternal origin. 13. Its cla 

 vicular origin. 14. The sterno-hyoid. 15. The sterno-thyroid of the right side. 16. 

 The thyro-hyoid. 17. The hyoid portion of the omo-hyoid. 18, 18. Its scapular portion , 

 on the left side, the tendon of the muscle is seen to be bound down by a portion of the 

 deep cervical fascia. 19. The clavicular portion of the trapezius. 20. The scab"* 

 anticus of the right side. 21. The scalenus posticus. 



