182 DEPRESSORS OF THE OS HYOIDES AND LARYNX. 



mastoid process and 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 pla- 

 tysma 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-hyoid, scaleni, levator anguli scapulae, splenii, and 

 the posterior belly of the digastric muscle ; with the phrenic nerve, and 

 the posterior, and supra-scapular 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 accessory 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 oasophagotomy. 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. 



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 connection between 

 the thorax and the head. Both muscles acting together bow the head 

 directly forwards. The clavicular 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 tJie Os Hyoides and Larynx. 

 Sterno-hyoid, 

 Sterno-thyroid, 

 Thyro-hyoid, 

 Omo-hyoid. 

 Dissection. These muscles are brought into view by removing the 



