256 MUSCLES AND FASCIAE. 



process ; behind, the anterior border of the Sterno-mastoid muscle. The boundaries 

 of the posterior triangle are, in front, the posterior border of the Sterno-mastoid ; 

 below, the upper border of the clavicle; behind, the anterior margin of the 

 Trapezius. 



The anterior edge of the muscle forms a very prominent ridge beneath the 

 skin, which it is important to notice, as it forms a guide to the surgeon in making 

 the necessary incisions for ligature of the common carotid artery, and for cesoph- 

 agotomy. 



Relations. By its superficial surface, with the integument and Platysma, from 

 which it is separated by the external jugular vein, the superficial branches of the 

 cervical plexus, and the anterior layer of the deep cervical fascia. By its deep 

 surface, it rests on the sterno-clavicular articulation, the deep layer of the cervical 

 fascia, the Sterno-hyoid, Sterno-thyroid, Omo-hyoid, the posterior belly of the 

 Digastric, Levator anguli scapulae, the Splenius and Scaleni muscles. Below, with 

 the lower part of the common carotid artery, internal jugular vein, pneumogastric, 

 descendens noni, and communicans noni nerves, and with the deep lymphatic 

 glands \ with the spinal accessory nerve, which pierces its upper third, the cervical 

 plexus, the occipital artery, and a part of the parotid gland. 



Nerves. The Platysma myoides is supplied by the facial and superficial cer- 

 vical nerves, the Sterno-cleido-mastoid by the spinal accessory and deep branches 

 of the cervical plexus. 



Actions. The Platysma myoides produces a slight wrinkling of the surface of 

 the skin of the neck, in a vertical direction, when the entire muscle is brought 

 into action. Its anterior portion, the thickest part of the muscle, depresses the 

 lower jaw ; it also serves to draw down the lower lip and angle of the mouth on 

 each side,, being one of the chief agents in the expression of melancholy. The 

 Sterno-mastoid muscles, when both are brought into action, serve to depress the 

 head upon the neck, and the neck upon the chest. Either muscle, acting singly, 

 flexes the head, and combined with the Splenius draws it towards the sho % ulder 

 of the same side, and rotates it so as to carry the face towards the opposite side. 



Surgical Anatomy. The relations of the sternal and clavicular parts of the Sterno-mastoid 

 should be carefully examined, as the surgeon is sometimes required to divide one or both portions 

 of the muscle in wryneck. One variety of this distortion is produced by spasmodic contraction 

 or rigidity of the Sterno-mastoid ; the head being carried down towards the shoulder of the same 

 side, and the face turned to the opposite side, and fixed in that position. When all other reme- 

 dies for the relief of this disease have failed, subcutaneous division of the muscle is resorted to. 

 This is performed by introducing a long narrow bistoury beneath it, about half an inch above its 

 origin, and dividing it from behind forwards whilst the muscle is put well upon the stretch. 

 There is seldom any difficulty in dividing the sternal portion. In dividing the clavicular portion 

 care must be taken to avoid wounding the external jugular vein, which runs parallel with the 

 posterior border of the muscle in this situation. 



2. INFRA-HYOID KEGioN (figs'. 153 and 154). 

 DEPRESSORS OF THE Os HYOIDES AND LARYNX. 



Sterno-hyoid. Thyro-hyoid. 



Sterno-thyroid. Omo-hyoid. 



Dissection. The muscles in this region may be exposed by removing the deep fascia from the 

 front of the neck. In order to see the entire extent of the Omo-hyoid, it is necessary to divide 

 the Sterno-mastoid at its centre, and turn its ends aside, and to detach the Trapezius from the 

 clavicle and scapula, if this muscle has been previously dissected ; but not otherwise. 



The /Sterno-hyoid is a thin, narrow, riband-like muscle, which arises from 

 the inner extremity of the clavicle, and the upper and posterior part of the first 

 piece of the sternum ; and, passing upwards and inwards, is inserted, by short 

 tendinous fibres, into the lower border of the body of the os hyoides. This 



