334 MUSCLES AND FASCIAE. 



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 it3 

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

 cervical fascia, the Sterno-hyoid, Sterno-thyroid, Orno-hyoid, the posterior belly 

 of the Digastric, Levator Anguli Scapulae, the Splenius and Scaleni muscles. 

 Below, it is in relation 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 

 cervical 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 shoulder 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 iory neck. One variety of this distortion is produced by spasmodic contraction 

 or rigidity of the Sterno-raastoid ; 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. 220, 221.) 

 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. This, however, should not be done unless the Trapezius has been dism-tcd. 



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 

 muscle is separated, below, from its fellow by a considerable interval ; but they 

 approach one another in the middle of their course, and again diverge as they 

 ascend. It often presents, immediately above its origin, a transverse tendinous 

 intersection, like those in the Rectus Abdominis. 



Varintii'its. This muscle sometimes arises from the inner extremity of the clavicle, and the 

 posterior s^crno-clavicular ligament ; or from the sternum and this ligament; from either bone 

 alone, or from ull these parts ; and occasionally has a fasciculus connected with the cartilage 

 of the first rib. 



Relations. By its superficial surface, below, with the sternum, the sternal end 

 of the clavicle, and the Sterno-mastoid; and above, with the Platysma and deep 

 cervical fascia. By its deep surface, with the Sterno-thyroid, Crico-thyroid, and 



